Multiplex System

Multiplex System
M u lt i p l e x S y s t e m
Overview
Many of the Blue Bird Vision’s electrical circuits communicate and interact with each
other through an advanced Multiplex control system. This solid-state system provides significant advantages over traditional wiring:
• Multiplex simplifies the system by replacing multiple mechanical relays and
switches with reliable solid-state components.
• Multiplex provides intelligent programmed (rather than mechanical) interaction between components. Multiple component signals are transmitted
simultaneously along a common data bus, and are monitored and controlled
by a digital microprocessor.
• The Multiplex module also serves as a centralized troubleshooting tool which
simplifies the process of isolating causes of electrical malfunctions, without
requiring special diagnostic hardware/software interfaces.
The Multiplex system may be thought of as similar in purpose and function to alreadyfamiliar electronic control devices such as the engine ECU, or the Weldon warning
light control module. The main differences are that the programmable module(s) of
a multiplex system are programmed for specific vehicles, and provides information
to the technician by means of a series of visible LED indicators rather than by means
of special diagnostic interface ports.
The Multiplex systems installed on the Blue BIrd Vision is relatively simple. It
primarily controls chassis-related on/off state devices. Most body circuits are wired
conventionally. (Some chassis circuits receive Inputs from body components such
as vandal locks and door signal switches and some body circuits are controlled by
outputs from the MPX system such as turn signals.) Although the modular nature of
Multiplex components allows it to be configured for complex systems using multiple
modules located in several control “zones”, the Vision’s Multiplex system consists of
one zone with a single control module.
The central component of the Multiplex system is the MPX Module, the microprocessor which handles communication between various circuits. The role of the
MPX Module is similar to that of a telephone switchboard which makes and breaks
connections for multiple individual “calls” ( between two components) and/or “conference calls” (between several components), simultaneously. Input signals from various circuits arrive at the MPX Module. Inside the Module, the Inputs trigger Output
signals which activate/deactivate other components or circuits. The programming of
the MPX Module determines which Input (or combination of Inputs) results in any
particular Output. Specific programming of each module is identified by a program
control number on the front of the module. The Module also internally generates a
Feedback signal for each load circuit, and continually monitors the Feedback signals
to verify the integrity of the circuits, much as a technician would use an Ohm meter
to test continuity.
Speed Sensor
Door/Lift Controls
Ignition
Brakes / Throttle
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Multiplex System
The Brake & Throttle Interlock system is an example
of how multiplexing handles simultaneous
communications between seemingly unrelated
components.
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An array of Light Emitting Diodes (LEDs) on the MPX Module provides the means
by which a technician can visually monitor the status of individual Inputs, Outputs,
and Feedbacks. The technician uses a dash-mounted Diagnostic Switch to select
which circuits the LEDs display, and whether they indicate Inputs, Outputs, or Feedback.
To interpret the LEDs, the technician refers primarily to two printed references presented
later in this chapter: the Input/Output charts and the Ladder Logic diagrams.
As with any new technology, troubleshooting and servicing a Multiplex system
requires an initial familiarization, a firm conceptual understanding, and the acquisition of new skills (such as reading Ladder Logic diagrams). However, Multiplexing
does not supersede or invalidate what a qualified electrical technician already knows
about troubleshooting and diagnosis. Rather, it provides a new and valuable tool to
assist the qualified technician in narrowing down the source of a problem, thereby
minimizing downtime and reducing operating costs.
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multiplex system
MPX Module
The CV-CCM-C Main Bus Controller (MPX Module) is mounted in the
Power Distribution Unit to the right of the driver’s area. The Module electronics are powered by “clean” 12 volt power from the battery bus bar,
which isolates it from the normal current fluctuation or “line noise” of
power circuits affected by the alternator. Five connectors on the module
labeled A, B, C, D, and E, receive Inputs from sensors and switches, and
convey Outputs to actuators, lamp loads, and other devices.
Four rows of LEDs indicate the status of the controlled circuits:
Zone: The top row consists of six LEDs, labeled S, A, B, C, D, and
L. The Blue Bird Vision’s single-zone Multiplex System is
contained within Zone A. (Note that all of the addresses
shown in the Input and Output charts begin with “A”.) The
Zone L LED goes off when the Multiplex System enters
Sleep Mode. S and A are the only functionally active LEDs,
B, C, D, and L are not active at this time. When power is first
applied to the system, all the LEDs come on momentarily,
and then go off. The Zone A LED then blinks a number of
times, corresponding to the revision version of the software
installed. Finally, the Zone A LED goes off and the system is in
its normal operating mode.
Port: The three LEDs in this row correspond to the three input/
output connector Ports. When the MPX Module has been put
in diagnostic mode by pressing the Diagnostic Switch, these
LEDs indicate which Port the rows of I/O and CH# LEDs are
presently indicating.
I/O: When in diagnostic mode, the three LEDs in this row, labeled
IN, OUT, and FB, indicate whether the row of CH# LEDs
immediately below are presently monitoring Inputs, Outputs,
or Feedback.
CH#: When in diagnostic mode, the bottom row of eight LEDs
indicate the current state (active on or inactive of) of each of
the Inputs, Outputs, or Feedback being monitored (depending
upon which I/O LED is on) of the Port presently indicated by the
Port LEDs.
Connector E
Connector B
Connector A
Multiplex (MPX) Module
Diagnostic LEDs
Display either Inputs, Outputs, or Feedback, according
to setting of the DIagnostic Switch.
Connector D
Connector C
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The LEDs serve as visual indicators. However, they also represent the type of mechanism by which Output circuits are “notified” of the presence of particular Inputs.
Inside the MPX Module, an LED in a given Input circuit turns on when the circuit is
Active. Nearby, a solid state photo sensor in the Output circuit detects the internal
light source and either closes or opens an Output Circuit according to the programming of the Module. In turn, the status of the Output Circuit is similarly sensed by a
photo sensor in the load circuit. In this sense, the LEDs can be thought of as actually
performing the function of electrical relays.
Rather than having fuses, the MBC provides solid state circuit protection. If an
Output is turned on and is drawing too much current (as in the case of a short circuit), the Module turns the Output off before there can be any damage to the Output
transistor and circuit.
LEDs As Circuit Relays…
The CPU “watches” Input LEDs with photosensors…
… and activates the appropriate Output(s).
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Typical Multiplex Circuit
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Feedback Circuits
Internal Feedback circuits in the MPX Module are connected in parallel to the load
side of Output circuits. A Feedback circuit conducts a faint current through the load,
a resistor, and the Feedback LED. Thus, when the module is set to display Feedbacks,
each of the bottom row LEDs acts much like a conventional ohm meter, indicating
the integrity (continuity) of the load side of the circuit it monitors.
When the load circuit is switched on, full working current is able to bypass the
Feedback LED circuit. This removes the voltage differential across the LED and resistor, and the LED goes off.
Thus, in a properly performing load circuit, the Feedback LED should be on (indicating that the circuit is ready) when the circuit is Inactive; and should be off when
the circuit is Active. If the Feedback LED is on while the circuit is Active, this is an
indication of a short in the load side of the circuit.
LEDs As Circuit Testers
A signal current passing through the load lights the
Feedback LED whenever the circuit is Inactive off.
When the Output becomes active on, a photosensor
detects it, and closes the load circuit, providing a path
for current to bypass the Feedback LED and its resistor.
The Feedback LED goes off.
707
Voltmeter
Typical Multiplex Circuit
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Sleep Mode
The normal monitoring functions of the MPX Module draw a small current from the
vehicle batteries. To minimize battery drain while the bus is parked, the Multiplex
system automatically goes into a low current-draw Sleep Mode one hour after ignition is turned off. The Module awakens from Sleep Mode when either the ignition
switch is turned on or one of a number of other circuits is used (headlights, stoplights, directional lights, horn, accessory switch). There is a slight delay while the system is awakening from Sleep Mode when the system is turned on.
The Sleep Mode delay may differ on some units, in accordance with specific state
requirements. For example, in a state which requires the lift to be operable only with
the ignition switch removed, a longer delay time may be programmed to allow time
for the lift to be used.
Diagnostic Mode
Except for the Zone S LED (which indicates Sleep Mode), the other LEDs only light
when the MPX Module has been put into Diagnostic mode by pressing the Diagnostic switch. Each subsequent press of the switch cycles through the Ports and I/O
options in the following order:
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1st press: Puts the Module in Diagnostic Mode,
displaying Port 1 Inputs.
2nd press: Port 1 Outputs.
3rd press: Port 1 Feedbacks.
4th press: Port 2 Inputs.
5th press: Port 2 Outputs.
6th press: Port 2 Feedbacks.
7th press: Port 3 Inputs.
8th press: Port 3 Outputs.
9th press: Port 3 Feedbacks.
10th press: Ends Diagnostic Mode. Only the Zone 2 LED
remains on.
ABS Diagnostic Switch
Diagnostic Mode Switch
Cycles the MBC’s display
through Ports, Inputs,
Outputs, and Feedbacks.
(3-position momentary)
In this example, the LEDs are indicating the A2 Feedbacks…
…and here, the A2 Outputs.
LED on
LED off
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Multiplex References
Two printed references are provided to assist the technician in interpreting the Main
Bus Controller’s LEDs:
Input and Output Tables
Generally speaking, the first step in troubleshooting an electrical problem is to determine whether the affected or suspected circuit is part of the Multiplex system.
The Input and Output Tables included in this chapter list the circuits with which the
Multiplex system interacts.
Multiplex “Logical Address”
and/or LED Address
Zone
Multiplex Inputs Table. The Inputs Table lists the circuits from which the Main Bus
Controller receives Input signals.
The second column (Zone/Port-Input#) shows the Multiplex “logical address”
and/or “LED address” of each Input circuit. These addresses are not circuit/pin locations, but relate directly to the Ladder Logic diagrams. For example, the Accessory
Switch address is A2-I03 (Zone A, Port 2-Input number 03).
The third column (connector port) shows the physical pin location of the circuits;
which Pin of which Port Connector on the MPX Module to which the Input circuits are
wired. For example, the Input wire leading from the Accessory Switch is connected to
the Module at AC-03 (Port C, Pin 03).
The third and fourth columns list the wire color and whether the Input is a ground
or a 12 volt connection.
Outputs Table. The Outputs table lists the circuits for which the Main Bus Controller
issues Outputs. This table is organized similarly to the Inputs table:
The second column (Zone/Port-Input#) shows the Multiplex “address” and/or
“LED address” of each Output circuit. These addresses are not circuit/pin locations,
but relate directly to the Ladder Logic diagrams. For example, the Accessory Hot
Output’s address is A3-O08 (Zone A, Port 3-Output number 08).
The third column (connector port) shows the physical pin location of the circuits;
which Pin of which Port Connector on the MPX Module to which the Output circuits
are wired. For example, the Accessory Hot Output signal wire is connected to the
Module at AB-116 (Port B, Pin 116).
The third and fourth columns list the wire color and whether the Output is a
ground or a 12 volt connection.
The Outputs Table has a sixth column labeled Ladder Chart Line. This is the line
of the Ladder Logic which describes the requirements for the specific Output. For
example, to troubleshoot the Accessory Hot Output, you would refer to the Ladder
Logic line 34, which describes the requirements for an Accessory Hot Output. (Note
that the Zone/Port-Output# address of the Accessory Hot Output is the item at the
right end of the Logic Line 34.)
Port
Type
Number
Physical Connector
Pin Location
Zone
Connector Ground if 0
port
12v if 1 Pin Number
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Ladder Logic Diagrams
The Ladder Logic Charts are not wiring schematics. They are diagrams of the “logic,”
or rules which the Multiplex program follows to determine which set of Inputs and/
or other conditions results in each particular Output. Each horizontal “rung” of the
diagrams graphically describes a set of requirements for the Output indicated at the
right end of the line. Most of the requirements are either Inputs or other Outputs.
(There are occasional requirements for signals which are generated internally by the
MPX Module’s internal circuitry or programming, rather than by a physical component on the bus.)
Each symbol on a logic line represents one particular requirement for the Output indicated at the right end of the line. Reading a line from left to right, all of the
requirements encountered along the path must be satisfied in order for the Output
at the right end of the line to occur. Thus, the horizontal lines between requirement
symbols may be thought of as logical “and” statements. (An important basic principle
of Ladder Logic is that a symbol can denote a requirement for either the presence on
or absence off of a condition or signal.)
Some Logic diagrams provide alternate paths by which to reach the Output at
the right end. These alternate paths may be thought of as logical “or” statements. If
a requirement on the uppermost path is not met, the Output may still be achieved if
all of the requirements of an alternate path are met.
Thus, troubleshooting a circuit which interacts with the Multiplex system involves finding the Ladder Logic diagram for the desired Output; and then systematically verifying each of its requirements by comparing the chart to what is actually
happening according to the LEDs on the MPX Module. When a discrepancy is found,
it is an indication that the problem exists “upstream” from that point in the Logic. This
will become clearer as you continue through this chapter.
Reading Ladder Logic
This condition… AND this condition… AND this condition…
…will result in this Output…
…OR…
This condition… AND this condition… AND this condition…AND this condition…regardless of this condition…will also result in this Output.
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Ladder Logic Symbols
Following is a description of the symbols which appear on the Ladder Logic charts:
Inputs. A Logic Ladder diagram may require a particular Input to be either Active
or Inactive. Active Inputs are symbolized by two vertical lines. Inactive Inputs are
indicated by the same symbol, but with a diagonal slash.
Outputs. Outputs do not only occur at the right end of Logic diagrams. A particular
Output (or its absence) can also be a requirement for another Output. There is only
one true functional output per logic line located on the far right. All other outputs are
required conditions. Therefore, symbols exist for both Active and Inactive Outputs.
An Active Output is symbolized by a right-pointing arrow inside a circle. The same
symbol with a diagonal slash indicates an Inactive Output.
Timers. A Timer is an internal signal generated by the Multiplex program. Timers
are represented as rectangles containing abbreviated labels, such as the Turn Signal
Flasher at address B1-I01, labeled FLS.
Flags. A Flag symbol is identical to an Output symbol (right-pointing arrow inside a
circle), but with a letter F next to it. A Flag may be thought of as similar to a function
in programming; a “shorthand” way to refer to another “mini-program”, or set of commands. Thus, a Flags is a kind of Output, and has its own Ladder Logic diagram. Each
flag has its own logic line but does not have an output pin, connector and circuit associated wiht it. When a Flag is encountered somewhere along a Logic Line, its own
Logic diagram must be examined in turn to fully investigate the circuit.
Active Input LED On
Inactive Input LED Off
Active Output On
Inactive Output Off
Active Timer (Flasher Shown)
Active Flag On
Inactive Flag Off
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Logical “Latch Relays”
Occasionally, a particular Output may appear to be a requirement for itself. In such
situations, it is helpful to realize that the Multiplex program performs the horizontal
“rungs” of a Ladder in sequence, not all at once. If an initial set of conditions has made
an Output true, that Output may then become one of a second set of conditions
which will keep the Output true—even if one or all of the initial conditions changes.
The Brake Interlock circuit is an example of this kind of Logic Line.
A Logical “Latch Relay”
Once these conditions have made this Output Active…
…this condition has been met…
NOTE
The “latch in” behavior of Logic Line 31’s
alternate path applies to Vision’s equipped with
hydraulic brakes. It does not affect Visions with
air brakes.
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…the Output will remain Active, even if one
of the initial conditions changes.
…so if this condition then becomes true…
multiplex system
Vision MPX Simulator
Blue Bird has provided another reference tool to help technicians become more
quickly acquainted with the Vision’s Multiplex system. On the CD included with this
manual is a file named VisionMPXSimulator.exe. This is a small self-running Macromedia® Flash®-based application, suitable for use on most current Windows® computers.
[CAUTION] The Vision Multiplex Simulator is provided as a reference tool
to help service technicians become more quickly familiar with the Blue Bird
Vision’s Multiplex system. It is intended for training/learning purposes only
(not for actual troubleshooting). The software is only a SIMULATION of the Blue
Bird Vision’s Multiplex system. Although it imitates the behavior of the Vision’s
Multiplex Module, it does not contain the actual multiplex program. Because
the actual program installed on buses is subject to change at the factory, the
behavior of the Vision MPX Simulator may not exactly match that of your particular Vision unit(s).
The Vision MPX Simulator operates upon a script which imitates the logic of the Ladder Logic diagrams of a 2007 Vision. Interactive on-screen controls represent the
driver controls which generate Multiplex Inputs. Other animated graphic elements
indicate the results of most Multiplex Outputs, as they are generated in response
to the Inputs. An on-screen Diagnostic Switch allows the user to cycle the bank of
“LEDs” to display Inputs, Outputs, or Feedbacks, simulating the behavior of the actual
MPX Module. For reference while observing the behaviors of the LEDs, at the top
of the screen is a scrolling list of the same set of Ladder Logic charts printed in this
manual.
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Scrollable Logic Charts
Diagnostic Switch
MBC LEDs
VisionMPXSimulator.exe
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Input Tables
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Multiplex Inputs Table
Circuit Description
1
Accessory Switch
2
Brake Light Switch
3
Diagnostic Switch
4
Door Signal
5
Headlight Switch
6
High Beam Switch
7
High/Fast Idle
8
Horn Signal
9
Ignition Switch
10
Interlock Feedback
11
Neutral Signal
12
Park/Tail Light Switch
13
Park Brake Switch
14
Reverse Signal
15
Speed Signal
16 Starter Lockout / Fuel Door
17
Starter Switch
18
Turn Signal, Left
19
Turn Signal, Right
20
Vandal Lock
21
Washer Pump Switch
22 Intermittent Wiper Switch
23
SCC Alarm
Input LED
A2-I03
A1-I05
A3-I07
A3-I03
A2-I06
A2-I07
A2-I01
A3-I02
A2-I04
A3-I05
A1-I03
A1-I07
A1-I06
A1-I02
A1-I01
A2-I02
A2-I05
A2-I08
A3-I01
A3-I08
A1-I04
A3-I04
A3-I06
Connector / Port-Pin
AC-003
AB-111
AC-015
AC-011
AC-006
AC-007
AC-001
AC-010
AC-004
AC-013
AB-107
AA-004
AB-012
AB-004
AB-003
AC-102
AC-005
AC-008
AC-009
AC-016
AB-108
AC-112
AC-014
Multiplex “Logical Address”
Zone
L
Port
Type
Number
Wire Color
YL
RD
GN
BL
OR
OR
GN
GY
PK
OR
RD
BN
PK
BL
RD
RD
PU
YL
GN
TN
PK
BK
GN
Intput
Ground
12 Volts
Ground
Ground
Ground
Ground
Ground
Ground
Ground
Ground
12 Volts
Ground
Ground
Ground
Ground
12 Volts
Ground
Ground
Ground
Ground
12 Volts
12 Volts
Ground
Physical Pin Location
Ground if 0
Zone Connector 12v if 1 Pin Number
multiplex system
Output Tables
Multiplex Outputs Table
Circuit Description
Output LED
1
Accessory Hot
A3-O08
2
Back up Lights
A2-O04
3
Brake Interlock
A3-O05
4
High/Fast Idle
A3-O01
5
Headlight, High Left
A1-O03
6
Headlight, High Right
A1-O04
7
Headlight, Low Left
A1-O01
8
Headlight, Low Right
A1-O02
9
Horn
A1-O08
10
Ignition
A2-O06
11
Ignition Dropout
A2-O07
12
Park Brake
A2-O03
13
Park Lights
A1-O06
14
Shift Inhibit Signal
A3-O02
15
Lift Enable
A3-O06
16
Starter Relay
A2-O08
17
Stop Lights Relay
A2-O05
18
Turn Signal, Left
A1-O05
19
Turn Signal, Right
A1-O07
20
Wiper Washer Pump
A3-O07
21 Intermittent Wiper Motor
A2-O01
Connector / Port-Pin
AB-116
AA-107
AB-015
AB-001
AA-110
AA-112
AA-102
AA-106
AB-114X
AA-111
AA-113
AA-105
AB-106
AB-005
AB-021
AA-114
AA-109
AB-102X
AB-110X
AB-122
AA-101
Wire Color
GN
BL
WH
OR
RD
PU
BN
OR
BK
OR
PK
PK
BN
GN
RD
RD
RD
YL
GN
TN
BN
Output
12 Volts
12 Volts
Ground
Ground
12 Volts
12 Volts
12 Volts
12 Volts
12 Volts
12 Volts
12 Volts
12 Volts
12 Volts
Ground
Ground
12 Volts
12 Volts
12 Volts
12 Volts
12 Volts
12 Volts
Ladder Chart Line - Std. / CA / FL / WY
32 / 53 / 74 / 95
22 / 43 / 64 / 85
29 / 50 / 71 / 92
27 / 48 / 69 / 90
14 / 35 / 56 / 77
15 / 36 / 57 / 78
12 / 33 / 54 / 75
13 / 34 / 55 / 76
19 / 40 / 61 / 82
24 / 45 / 66 / 87
25 / 46 / 67 / 88
21 / 42 / 63 / 84
17 / 38 / 59 / 80
28 / 49 / 70 / 91
30 / 51 / 72 / 93
26 / 47 / 68 / 89
23 / 44 / 65 / 86
16 / 37 / 58 / 79
18 / 39 / 60 / 81
31 / 52 / 73 / 94
20 / 41 / 62 / 83
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*Note: Flags are common for all states. Outputs are standard except as noted for California, Florida and Wyoming.
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LADDER LOGIC DIAGRAMS
Symbols Key
Active Input
Inactive Input
Active Output
Inactive Output
( LED ON)
( LED OFF)
(LED ON)
(LED OFF)
Active Timer (Flasher Shown)
Active Flag
Inactive Flag
PORT 1 FLAGS
1
A1F01
FLASHER
A1T01
HALF SECOND FLASHER
2
A2I06
HEADLAMP SWITCH
A2I04
IGNITION SWITCH
A2I02
A1I06
STARTER LOCKOUT PARK BRAKE SWITCH
SIGNAL
A1F03
PWM ENABLE INTERNAL
DAYTIME RUNNING LIGHTS
NON FLORIDA
A2I06
HEADLAMP SWITCH
A2I04
IGNITION SWITCH
A2I02
STARTER LOCKOUT
SIGNAL
A1F03
PWM ENABLE INTERNAL
DAYTIME RUNNING LIGHTS
FLORIDA
3
4
A1F04
HIGH BEAM
MOMENTARY SWITCH
A1F04
A2I07
HIGH BEAM
HIGH BEAM SWITCH
MOMENTARY SWITCH
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A1F05
HIGH BEAM
MEMORY
A1F04
HIGH BEAM
MOMENTARY SWITCH
A1F05
HIGH BEAM
MEMORY
A2I07
HIGH BEAM SWITCH
5
A2I07
HIGH BEAM SWITCH
A1F04
HIGH BEAM
MOMENTARY SWITCH
A1F05
HIGH BEAM
MEMORY
A1F04
HIGH BEAM
MOMENTARY SWITCH
A1F05
HIGH BEAM
MEMORY
0120700
A2I07
HIGH BEAM SWITCH
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PORT 3 FLAGS
6
A3F01
INTERMITTENT
LOW
A3I04
INTERMITTENT
WIPER SWITCH
7
A3I04
INTERMITTENT
WIPER SWITCH
A1I04
WASHER PUMP
A3F04
RESET
A3F02
INTERMITTENT
HI
A3F03
INTERMITTENT SET
HIGH DELAY
A3F02
INTERMITTENT
HI
8
A3I04
INTERMITTENT
WIPER SWITCH
A3F02
INTERMITTENT
HI
A3T04
DELAY ON
A3F01
INTERMITTENT
LOW
A3F03
INTERMITTENT SET
HIGH DELAY
A1I04
WASHER PUMP
SWITCH
A1I05
SERVICE BRAKE
LAMP SWITCH
A2I01
HIGH IDLE SWITCH
9
A3T05
DELAY OFF
A3F04
RESET
10
A3F05
FAST IDLE BRAKE
A3F05
FAST IDLE BRAKE
11
A3F02
INTERMITTENT
HI
A3I04
INTERMITTENT
WIPER SWITCH
A3F02
INTERMITTENT
HI
A3I04
INTERMITTENT
WIPER SWITCH
A3T02
INTERMITTENT
HI
A3O07
WIPER WASHER PUMP
A3T03
WASH TIME
A3T01
INTERMITTENT
LOW
A3F06
WIPER MOTOR
717
0120701
A3F01
INTERMITTENT
LOW
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PORT 1 OUTPUTS (Standard)
12
A1F03
PWM ENABLE INTERNAL
DAYTIME RUNNING LIGHTS
A1O01
LOW BEAM HEADLAMP LEFT
A1F04
A2I06
HIGH BEAM
HEADLAMP SWITCH
MOMENTARY SWITCH
13
A1F03
PWM ENABLE INTERNAL
DAYTIME RUNNING LIGHTS
A1O02
LOW BEAM HEADLAMP RIGHT
A1F04
A2I06
HIGH BEAM
HEADLAMP SWITCH
MOMENTARY SWITCH
14
A2I06
HEADLAMP SWITCH
A1F04
HIGH BEAM
MOMENTARY SWITCH
A1O03
HIGH BEAM HEADLAMP LEFT
A2I06
HEADLAMP SWITCH
A1F04
HIGH BEAM
MOMENTARY SWITCH
A1O04
HIGH BEAM HEADLAMP RIGHT
A1F01
TURN SIGNAL
FLASHER FLAG
A2I08
LEFT TURN SWITCH
15
16
718
A2I04
IGNITION SWITCH
A1O05
LEFT TURN LAMPS
A3I01
RIGHT TURN SWITCH
17
A1O06
PARK LIGHTS
A1I07
PARK/TAIL LIGHTS
SWITCH
A1F03
PWM ENABLE INTERNAL
DAYTIME RUNNING LIGHTS
18
A3I01
RIGHT TURN SWITCH
A1F01
TURN SIGNAL
FLASHER FLAG
A2I04
IGNITION SWITCH
A1O07
RIGHT TURN LAMPS
A2108
LEFT TURN SWITCH
19
A1O08
HORN
0120691
A3I02
HORN BUTTON
L
multiplex system
PORT 2 OUTPUTS (Standard)
20
A3F06
WIPER MOTOR
A2O01
WIPER MOTOR INTERMITTENT
A1I06
PARKING BRAKE
SWITCH
A2O03
PARK BRAKE OUTPUT
21
22
A1I02
REVERSE SIGNAL
A2I04
IGNITION SWITCH
A2O04
BACKUP LIGHTS
23
A2O05
STOP LIGHT RELAY
A1I05
SERVICE BRAKE
LAMP SWITCH
A3I05
INTERLOCK FEEDBACK
24
A2O06
IGNITION
A2I04
IGNITION SWITCH
25
A2I04
IGNITION SWITCH
A2I05
START SWITCH
A1I03
NEUTRAL
A3I08
VANDAL LOCK
A2O07
IGNITION DROPOUT
26
A2I05
START SWITCH
A2O08
STARTER RELAY
719
0120692
A2I02
STARTER LOCKOUT
SIGNAL
FUEL DOOR SWITCH
L
Vision [propane]
service manual
PORT 3 OUTPUTS (Standard)
27
A2I01
HIGH IDLE SWITCH
A2I04
IGNITION SWITCH
A1I03
NEUTRAL
A1O05
SERVICE BRAKE
LAMP SWITCH
A2I04
IGNITION SWITCH
A3O01
HIGH IDLE
A1I01
THREE MPH SIGNAL
A3I03
DOOR SWITCH
A3F05
A1I06
PARK BRAKE SWITCH FAST IDLE BRAKE
A3O01
HIGH IDLE
28
A3O02
SHIFT INHIBIT
29
A3O05
BRAKE INTERLOCK
A3O05
BRAKE INTERLOCK
A1I05
SERVICE BRAKE
LAMP SWITCH
A1I03
NEUTRAL
A1I06
PARK BRAKE SWITCH
A3I03
DOOR SWITCH
A1I04
WASHER PUMP
SWITCH
A3I04
INTERMITTENT
WIPER SWITCH
30
A3O06
LIFT ENABLE
31
A3O07
WIPER WASHER PUMP
32
A2I03
ACCESSORY SWITCH
A3O08
ACCESSORY HOT
0120693
720
L
multiplex system
PORT 1 OUTPUTS (California)
33
A1O01
LOW BEAM HEADLAMP LEFT
A1F03
PWM ENABLE FLAG
DAYTIME RUNNING
A1F04
A2I06
HIGH BEAM
HEADLAMP SWITCH
MOMENTARY SWITCH
34
A1O02
LOW BEAM HEADLAMP RIGHT
A1F03
PWM ENABLE FLAG
DAYTIME RUNNING
A1F04
A2I06
HIGH BEAM
HEADLAMP SWITCH
MOMENTARY SWITCH
35
A2I06
HEADLAMP SWITCH
A1F04
HIGH BEAM
MOMENTARY SWITCH
A1O03
HIGH BEAM HEADLAMP LEFT
A2I06
HEADLAMP SWITCH
A1F04
HIGH BEAM
MOMENTARY SWITCH
A1O04
HIGH BEAM HEADLAMP LEFT
A1F01
TURN SIGNAL
FLASHER FLAG
A2I08
LEFT TURN SWITCH
36
37
A2I04
IGNITION SWITCH
A1O05
LEFT TURN LAMPS
A3I01
RIGHT TURN SWITCH
721
38
A1I07
PARK/TAIL LIGHTS SW
A1O06
PARK LIGHTS
A1F03
PWM ENABLE FLAG
DAYTIME RUNNING
39
A1F01
TURN SIGNAL
FLASHER FLAG
A3I01
RIGHT TURN SWITCH
A2I04
IGNITION SWITCH
A1O07
RIGHT TURN LAMPS
A2I08
LEFT TURN SWITCH
40
A1O08
HORN
0120702
A3I02
HORN BUTTON
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Vision [propane]
service manual
PORT 2 OUTPUTS (California)
41
A3F06
WIPER MOTOR
A2O01
WIPER MOTOR INTERMITTENT
A1I06
PARKING BRAKE
SWITCH
A2O03
PARK BRAKE
42
43
A1I02
REVERSE SIGNAL
A2I04
IGNITION SWITCH
A2O04
BACKUP LIGHTS
44
A2O05
STOP LIGHT RELAY
A1I05
SERVICE BRAKE
LAMP SWITCH
A3I05
INTERLOCK FEEDBACK
45
A2O06
IGNITION
A2I04
IGNITION SWITCH
46
A2I04
IGNITION SWITCH
A2I05
START SWITCH
A1I03
NEUTRAL
A3I08
VANDAL LOCK
A2O07
IGNITION DROPOUT
47
A2I05
START SWITCH
A2O08
STARTER RELAY
0120703
722
A2I02
STARTER LOCKOUT
SIGNAL
FUEL DOOR SWITCH
L
multiplex system
PORT 3 OUTPUTS (California)
48
A2I01
HIGH IDLE SWITCH
A2I04
IGNITION SWITCH
A1I03
NEUTRAL
A1I05
SERVICE BRAKE
LAMP SWITCH
A2I04
IGNITION SWITCH
A3O01
HIGH IDLE
A1I01
THREE MPH SIGNAL
A3I03
DOOR SWITCH
A3F05
A1I06
PARK BRAKE SWITCH FAST IDLE BRAKE
A3O01
HIGH IDLE
49
A3O02
SHIFT INHIBIT
50
A3O05
BRAKE INTERLOCK
A3O05
BRAKE INTERLOCK
A1I05
SERVICE BRAKE
LAMP SWITCH
A1I03
NEUTRAL
A1I06
PARK BRAKE SWITCH
A3I03
DOOR SWITCH
A1I04
WASHER PUMP
SWITCH
A3I04
INTERMITTENT
WIPER SWITCH
51
A3O06
LIFT ENABLE
52
A3O07
WIPER WASHER PUMP
53
A2I03
ACCESSORY SWITCH
A3O08
ACCESSORY HOT
0120704
723
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Vision [propane]
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PORT 1 OUTPUTS (Florida)
54
A3I06
A1F03
PWM ENABLE INTERNAL SCC ALARM
DAYTIME RUNNING LIGHTS
A2I06
HEADLAMP SWITCH
A1F04
HIGH BEAM
MOMENTARY SWITCH
A3I06
SCC ALARM
A1F01
FLASHER
A1O01
LOW BEAM HEADLAMP LEFT
A3I06
SCC ALARM
55
A3I06
A1F03
PWM ENABLE INTERNAL SCC ALARM
DAYTIME RUNNING LIGHTS
A1O02
LOW BEAM HEADLAMP RIGHT
A3I06
SCC ALARM
A2I06
HEADLAMP SWITCH
A1F04
HIGH BEAM
MOMENTARY SWITCH
A3I06
SCC ALARM
A1F01
FLASHER
A2I06
HEADLAMP SWITCH
A1F04
HIGH BEAM
MOMENTARY SWITCH
A3I06
SCC ALARM
A1O03
HIGH BEAM HEADLAMP LEFT
A2I06
HEADLAMP SWITCH
A1F04
HIGH BEAM
MOMENTARY SWITCH
A3I06
SCC ALARM
A1O04
HIGH BEAM HEADLAMP RIGHT
A2I08
LEFT TURN SWITCH
A1F01
TURN SIGNAL
FLASHER FLAG
A2I04
IGNITION SWITCH
A1O05
LEFT TURN LAMPS
56
57
724
58
A3I01
RIGHT TURN SWITCH
59
A1O06
PARK LIGHTS
A1I07
PARK/TAIL LIGHTS
SWITCH
60
A3I01
RIGHT TURN SWITCH
A1F01
TURN SIGNAL
FLASHER FLAG
A2I04
IGNITION SWITCH
A1O07
RIGHT TURN LAMPS
A2108
LEFT TURN SWITCH
61
A3I06
SCC ALARM
L
A1O08
HORN
0120694
A3I02
HORN BUTTON
multiplex system
PORT 2 OUTPUTS (Florida)
62
A3F06
WIPER MOTOR
A2O01
WIPER MOTOR INTERMITTENT
A1I06
PARKING BRAKE
SWITCH
A2O03
PARK BRAKE
63
64
A1I02
REVERSE SIGNAL
A2I04
IGNITION SWITCH
A2O04
BACKUP LIGHTS
65
A2O05
STOP LIGHT RELAY
A1I05
SERVICE BRAKE
LAMP SWITCH
A3I05
INTERLOCK FEEDBACK
66
A2O06
IGNITION
A2I04
IGNITION SWITCH
67
A2I04
IGNITION SWITCH
A2I05
START SWITCH
A1I03
NEUTRAL
A3I08
VANDAL LOCK
A2O07
IGNITION DROPOUT
68
A2I05
START SWITCH
A2O08
STARTER RELAY
725
0120695
A2I02
STARTER LOCKOUT
SIGNAL
FUEL DOOR SWITCH
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Vision [propane]
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PORT 3 OUTPUTS (Florida)
69
A1I06
PARK BRAKE SWITCH
A3O01
HIGH IDLE
A2I01
HIGH IDLE SWITCH
A2I04
IGNITION SWITCH
A1I03
NEUTRAL
A1I05
SERVICE BRAKE
LAMP SWITCH
A2I04
IGNITION SWITCH
A3O01
HIGH IDLE
A3O02
SHIFT INHIBIT
A1I01
THREE MPH SIGNAL
A2I04
IGNITION SWITCH
A3I03
DOOR SWITCH
A3O05
BRAKE INTERLOCK
70
71
A1I05
SERVICE BRAKE
LAMP SWITCH
A3O05
BRAKE INTERLOCK
72
A1I03
NEUTRAL
A2I04
A1I06
PARK BRAKE SWITCH IGNITION SWITCH
A3I03
DOOR SWITCH
A3O06
LIFT ENABLE
73
A1I04
WASHER PUMP
SWITCH
A3I04
INTERMITTENT
WIPER SWITCH
A3O07
WIPER WASHER PUMP
74
A2I03
ACCESSORY SWITCH
A3O08
ACCESSORY HOT
0120696
726
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multiplex system
PORT 1 OUTPUTS (Wyoming)
75
A3I06
A1F03
PWM ENABLE INTERNAL SCC ALARM
DAYTIME RUNNING LIGHTS
A2I06
HEADLAMP SWITCH
A1F04
HIGH BEAM
MOMENTARY SWITCH
A3I06
SCC ALARM
A1F01
FLASHER
A1O01
LOW BEAM HEADLAMP LEFT
A3I06
SCC ALARM
76
A3I06
A1F03
PWM ENABLE INTERNAL SCC ALARM
DAYTIME RUNNING LIGHTS
A1O02
LOW BEAM HEADLAMP RIGHT
A3I06
SCC ALARM
A2I06
HEADLAMP SWITCH
A1F04
HIGH BEAM
MOMENTARY SWITCH
A3I06
SCC ALARM
A1F01
FLASHER
A2I06
HEADLAMP SWITCH
A1F04
HIGH BEAM
MOMENTARY SWITCH
A3I06
SCC ALARM
A1O03
HIGH BEAM HEADLAMP LEFT
A2I06
HEADLAMP SWITCH
A1F04
HIGH BEAM
MOMENTARY SWITCH
A3I06
SCC ALARM
A1O04
HIGH BEAM HEADLAMP RIGHT
A2I08
LEFT TURN SWITCH
A1F01
FLASHER
A2I04
IGNITION SWITCH
A1O05
LEFT TURN LAMPS
77
78
727
79
A3I01
RIGHT TURN SWITCH
80
A1O06
PARK LIGHTS
A1I07
PARK/TAIL LIGHTS
SWITCH
A1F03
PWM ENABLE INTERNAL
DAYTIME RUNNING LIGHTS
81
0120697
A3I01
RIGHT TURN SWITCH
A1F01
TURN SIGNAL
FLASHER FLAG
A2I04
IGNITION SWITCH
A1O07
RIGHT TURN LAMPS
A2108
LEFT TURN SWITCH
82
A3I02
HORN BUTTON
A1O08
HORN
A3I06
SCC ALARM
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PORT 2 OUTPUTS (Wyoming)
83
A3F06
WIPER MOTOR
A2O01
WIPER MOTOR INTERMITTENT
A1I06
PARKING BRAKE
SWITCH
A2O03
PARK BRAKE
84
85
A1I02
REVERSE SIGNAL
A2I04
IGNITION SWITCH
A2O04
BACKUP LIGHTS
86
A2O05
STOP LIGHT RELAY
A1I05
SERVICE BRAKE
LAMP SWITCH
A3I05
INTERLOCK FEEDBACK
87
A2O06
IGNITION
A2I04
IGNITION SWITCH
88
A2I04
IGNITION SWITCH
A2I05
START SWITCH
A1I03
NEUTRAL
A3I08
VANDAL LOCK
A2O07
IGNITION DROPOUT
89
A2I05
START SWITCH
A2O08
STARTER RELAY
0120698
728
A2I02
STARTER LOCKOUT
SIGNAL
FUEL DOOR SWITCH
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multiplex system
PORT 3 OUTPUTS (Wyoming)
90
A2I01
HIGH IDLE SWITCH
A2I04
IGNITION SWITCH
A1I03
NEUTRAL
A1I05
SERVICE BRAKE
LAMP SWITCH
A2I04
IGNITION SWITCH
A3O01
HIGH IDLE
A1I01
THREE MPH SIGNAL
A3I03
DOOR SWITCH
A3F05
A1I06
PARK BRAKE SWITCH FAST IDLE BRAKE
A3O01
HIGH IDLE
91
A3O02
SHIFT INHIBIT
92
A3O05
BRAKE INTERLOCK
A1I05
SERVICE BRAKE
LAMP SWITCH
A3O05
BRAKE INTERLOCK
A1I03
NEUTRAL
A1I06
PARK BRAKE SWITCH
A3I03
DOOR SWITCH
A1I04
WASHER PUMP
SWITCH
A3I04
INTERMITTENT
WIPER SWITCH
93
A3O06
LIFT ENABLE
94
A3O07
WIPER WASHER PUMP
95
A2I03
ACCESSORY SWITCH
A3O08
ACCESSORY HOT
0120699
729
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Vision [propane]
service manual
Troubleshooting In Diagnostic Mode
General Approach
A systematic approach to electrical troubleshooting of the Blue Bird Vision follows
these general steps:
1. Check the Inputs and Outputs Tables. First determine if the affected circuit
is one which interacts with or is controlled by the Multiplex system. Note
the circuit’s logical address (Zone-Port-Input/Output #) and its physical location (Connector / Port Pin). (If the issue is not a Multiplex circuit, refer to the
Electrical System chapter and appropriate wiring schematics to troubleshoot
conventionally.)
2. Check the Output. Operate the appropriate bus controls or switches to set
the conditions which are required for the circuit’s Output to be Active. (Refer
to the Ladder Logic Diagrams to determine the required conditions.) If the
MPX Module is generating an Active Output signal, but the circuit is not operational, at least part of the problem exists “downstream” of the Multiplex
logic; probably in the Output’s own load circuit. That is, since the Output signal is present, there is no reason at this point to suspect the “upstream” circuits of Inputs or other Outputs which are requirements of the desired Output. Troubleshooting efforts should be focussed upon the circuit beginning
from the Output’s Connector / Port Pin, through the load, and to ground.
730
3. Check the Feedback. To further verify that the problem exists in the load
circuit, and to possibly gain additional information about its nature, create
conditions (turn off switches, etc.) which should cause the Output to be Inactive. Then verify that the circuit’s Feedback LED is on. If the Feedback LED
is off when the Output is Inactive, an open is indicated in the load circuit,
because the Feedback signal is not being conducted through the load. If the
Feedback LED of a malfunctioning load circuit is on when the Output is Inactive (as it is in a correctly functioning circuit), the Feedback signal is being
conducted, and you at least have an indication of continuity. If the Feedback
LED is on while the Output is Active, a short in the load circuit is indicated.
4. Check Each Requirement. If the desired Output is shown by the MPX Module to be Inactive when it should be Active, the problem should be assumed
to be “upstream” of the Output signal; at least one of the conditions (Inputs)
required for an Active Output signal is absent. Referring to the Ladder Logic
Diagrams, begin checking each requirement’s LED to verify it is correctly Active (on) or Inactive (off). If a discrepancy is found, a malfunction is indicated
“upstream” of that point in the logic (for example, a failed Input). Thus, comparing the states of the MPX Module’s LEDs against the requirements of the
Ladder Logic Diagrams enables the technician to quickly “narrow down” the
source of the problem.
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multiplex system
As you proceed, remember that:
• The Logic Line for a particular Active Output may require that certain Inputs
are Active (LED on) while other Inputs are Inactive (LED off). It is helpful, therefore, to think of each item encountered on the Logic Line as a “statement”
which is either true or false (This Input active=true; This Input inactive=true;
This Input inactive=false). You seek to find and correct the false “statements.”
• The set of requirements for an Active Output may include requirements that
other Outputs—which have their own Logic Lines—are either Active or Inactive. In such cases, the required Output states must also be investigated according to their own Logic Lines.
Troubleshooting Tree
Refer to Output Tables
Is the malfunctioning circuit part of the
Multiplex System?
No
See Applicable Wiring Schematic
Troubleshoot conventionally.
Yes
Refer to Ladder Logic Line for Desired Output
Set conditions (controls, switches) required
for an Active Output.
731
Does the MPX Module indicate that the
Output is Active?
No
Problem is “upstream” of
the Output signal.
Problem is “downstream” of
the Output signal.
Check first requirement for
the Active Output.
Set conditions required for
an Inactive Output.
Does the MPX Module indicate that the
requirement is True?
No
Troubleshoot and correct
before continuing.
Yes
Yes
Check next requirement.
Does the MPX Module indicate the Feedback
is Active?
No
Check for open in load side
of circuit.
Yes
Check for short in load side
of circuit.
Check for
component failure.
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Vision [propane]
service manual
Multiplex Terms:
Multiplex: A way of transmitting multiple lines of communications (signals) simultaneously on one data line. This is similar in principle to the J1939 wiring harness which
connects the engine, transmission, and ABS electronic modules together, providing
an multiple communications between their respective electronic control modules.
The Vision’s Multiplex system uses a module containing microcomputer technology
and electronic switching devices to provide a modern method of power distribution,
circuit control, and circuit protection using.
MPX: Blue Bird’s abreviation for Multiplex.
Hard-wiring: A conventional method of routing current from an electrical source to
a load and back to the source using copper wire, switches, relays, circuit breakers,
fuseable links, fuses and other non-electronic components.
MPX Input: Typically a ground or 12 volt signal supplied to the MPX module, necessary for the MPX Module to produce an Output. An examle of an Input is the hard
wired ground signal the turn signal switch provides to the MPX module when the
turn signal is switched on. The programming of the MPX Module can require that a
particular Input be either active (on) or inactive (off) in order to generate a particular
Output.
732
Active Input:
An Input which is in an “ON” state when received by the MPX Module. When troubleshooting, a active input symbol in the Logic Diagram indicates the
LED in question should be ON to satisfy the requirement of the Output.
Inactive Input: An Input whic is in an “OFF” state when received by the MPX Module. When troubleshooting, an inactive input symbol in the Logic Diagram indicates
the LED in question should be OFF to satisfy the requirement of the Output.
Output: A 12 volt or ground signal available at a physical output pin on the MPX
Module. The Output is the end result of the functionality of the MPX module after all
the requirements have been satisfied.
Active Output:
An Output that is “ON” or “True”. An Active Output is providing
voltage or ground for a downstream load. When troubleshooting, an active output
symbol at the right end of the Logic Diagram indicates the LED in question should be
ON when all the requirements of the Logic Diagram are satisfied.
Inactive Output:
An Output that is “OFF” or “False.” An inactive Output is not providing voltage or ground for a down streamload. When troubleshooting, an inactive
output symbol at the right end of the Logic Diagram indicates the LED in question
should be OFF when all the requirements of the Logic Diagram are satisfied.
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multiplex system
Flag:
An Output that is internally generated by the MPX Module, but which is not
directly indicated on the troubleshooting LEDs. A Flag may be thought of as a programmatic function of the MPX Module logic. See Troubleshooting Principles, below,
for more information.
Logic Diagram: A diagram describing which requirements are necessary for the MPX
module to provide particular Outputs.
Logic Line: One horizontal path of the Logic Diagram. Each Logic Line leads to a
single true Output located on the far right of the line. Along the path are icons indicating the requirement for that Output to be true. Depending on the programmed
circuit logic for that particular function, a Logic Line may have only one requirement
(Input) or several. Each requirement and Output on the Logic Line is identified by a
description and an LED address.
LED Addresses: An alpha-numerical code which denotes which arrangement of
LEDs on the MPX Module is associated with a particular Input or Output.
Connector/Pin Addresses: An alpha-numerical code which denotes which physical
connector pin on the MPX Module is associated with a particular Input or Output.
Troubleshooting Principles:
About Flags
A Flag is an Output that is internally generated by the MPX Module. Just as with any
other Output, a Flag is generated when a set of requirements (Inputs) are true. Being
an internal signal, however, a Flag has neither a physical Output pin nor a voltage
or ground signal associated with it that can be directly checked by the technician. A
Flag therefore, can be thought of as an internal function, which may in turn be used
as a requirement on other Logic Lines.
The troubleshooting LEDs only indicate the active or inactive state of Outputs
and Inputs. They do not display the state of Flags. Therefore, Flags are verified by a
process of elimination. When troubleshooting a logic diagram that results in a Flag,
the technician uses the troubleshooting LEDs to verify the required state of Inputs
and Outputs required for the Flag to be active. When all of the Flag’s requirements
have been verified, the technician must assume that the Flag has been generated
and that it is active.
If the Flag occurs as one of the requirements for a physical Output that can be
verified by an LED, the troubleshooting LEDs can be used to confirm that all requirements except the Flag are satisfied. If all the verifiable Inputs are true, yet the desired
Output does not occur, then the technician could logically assume the Flag is suspect.
The technician would then look for a Logic Line that ends in the suspect Flag. If there
is one, he would proceed to verify the requirements for that Flag to be active, and so
on. If all the verifiable conditions check out correctly, and yet the desired output is
not achieved, then the functioning of the MPX module would be suspect. Blue BIrd
733
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Vision [propane]
service manual
recommends contacting your Blue Bird service source to confirm your troubleshooting before replacing a Multiples module.
Two of the circuits on the 2008 model Visions have a more complex MPX logic
then on previous models, and serve as examples of the troubleshooting of circuits involving Flags: The high beam headlight logic, and the intermittent wiper logic. Both
of these incorporate multiple logic lines which generate flags that are in turn used
as requirements (Inputs) on other Logic Lines. Only when all the Logic Lines involved
have been satisfied, will a voltage be provided by the MPX module to energize the
circuit.
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Flag Example: Headlights
The high beam switch used on the new steering column for the 2008 Visions is a
momentary switch with a single Output. Thus, the headlights cycle from low beam to
high beam, and then back to low beam, with an identical signal from the high beam
switch. This is where the MPX module logic comes into play. Referring to Logic Line
number 7, note that two requirements are indicated for the MPX module to provide
a physical Output for the high beam headlight left. (The logic for left and right high
beam headlight is identical.)
With the headlight switch on (active), the A2-I06 headlight switch input requirement is satisfied. However, for the MPX module to provide a high beam headlight
Output (A1-O03), the high beam momentary switch Flag (A1-F04) must be active. To
verify the requirements for the active Flag, you would locate the A1-F04 Flag in the
Output column on one of the other Logic Lines. Logic Line number 3 is the one which
describes the requirements for the high beam momentary switch Flag (A1-F04) to be
active. Studying the requirements for this Flag becomes somewhat more involved
because of the use of multiple Flags occuring not only on Logic Line number 3, but
also on Logic Line number 4.
Referring to Logic Lines 3, 4, and 7, note that there are only two physical inputs
that a technician can actually verify with the LEDs: The headlight switch (A2-IO6)
and the high beam switch (A2-I07). If these Inputs are verified, yet the high beam
headlight left is not active, it can therefore be logically assumed that one of the flags
is faulty and the MPX module would be suspect. Blue Bird advises contacting your
Blue Bird service source to verify your troubleshooting before replacing a Multiples
module.
Flag Example: Wiper motor intermittent A2-O01 (Logic Line 13)
This Logic Line is another example of one which involves Flags, and therefore may
require a process of elimination when troubleshooting. Logic diagrams 13, 21, 22, 23,
24, 26, and 33 all have a role in providing an intermittent wiper with two intervals. A
similar troubleshooting methodology would be used.
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multiplex system
Troubleshooting Example
The following example steps through a typical examination of a Ladder Logic
diagram, using the Vision MPX Simulator for illustrative purposes. DoubleClick the
VisionMPXSimulator.exe icon to launch it.
The Problem. This example assumes a problem with starting the bus. It will walk
through an investigation of the Multiplex logic which affects starting.
1. Refer to Input/Output Tables. The tables reveal that the Multiplex system
does indeed contains logic affecting several circuits having to do with starting the bus. The MPX Module receives Inputs named Ignition Switch, Neutral,
Vandal Lock, Starter Lockout Signal, and Starter Switch. Outputs are generated by the MPX Module named Ignition, Ignition Dropout, and Starter Relay. Often, by merely considering these listings we can gain “hints” regarding
how the Multiplex system comes into play: For example, it is easy to “guess”
that the Starter Lockout Signal must be Inactive in order to start the bus.
In this scenario, we’ll assume the starter is not turning, and therefore begin
our investigation with the Starter Signal Output. We note that the physical
pin location for this Output is AA-114 (Connector A, pin number 14). The
Outputs Table lists the logical address of the Starter Relay Output as A2-O08
(Port 2, Ouput number 8), and also shows that that Output is on line #20 of
the Ladder Logic diagrams.
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Vision [propane]
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2. Check the Output. To check
the status of a desired Output, we must first put the
MPX Module in Diagnostic
Mode and cycle it to display
Outputs for the appropriate
Port. Press the Diagnostic
Switch repeatedly until the
Port 2 LED (in the PORT row)
is on, and then until the Output LED (in the I/O row) is on.
Turn the ignition switch to
the start position and note
whether the #8 LED (in the
CH# row) comes on when attempting to start the engine.
By verifying an active output
first, you are in fact, verifying all input requirements are correct. If any one
requirement does not match the logic line the MPX module will not activate
the output.
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3. Check the Feedback. If the A2-O08 LED lights when trying to start the engine, we know that the MPX Module is generating an Active Starter Relay
Output. If the starter failed to operate despite the Output being Active, the
cause would most likely be “downstream” of the multiplex system, in the
load circuit itself (i.e.; Starter Relay, Solenoid, etc.) That is, there would be
no reason at this point to suspect the various circuits which are required for
an Active Starter Relay Output, because the Output is Active. Even in such a
case, though, further insight into the nature of a problem may be gained by
checking the Feedback LED.
Press the Diagnostic Switch once more to cycle the MPX Module to monitor
the A2-O08 Feedback LEDs. (Note the Output and Feedback use the same
LED address.) In our scenario, if the Starter Relay’s Feedback LED were on
while its Output is Active, it would, therefore, be reasonable to suspect a
short to ground in the Output load circuit or a failed MPX Module.
The most likely cause for an Output LED’s being on while the Feedback LED
is on is that a short to ground has been detected by the MPX Module. In
that case, a MOSFET (the electronic switching device and circuit protector
inside the Module) will trip (open the circuit) and deactivate the Output, thus
protecting the circuit from damage. The Output circuit connector should be
removed and a conventional short to ground test with a voltmeter should be
conducted.
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LED on
LED off
multiplex system
If a short to ground is not indicated by the above condition (an Output LED’s
being on while the Feedback is on), the MPX Module is suspect. A physical
test at the output pin could be conducted. It is important to bear in mind
that the MOSFET will not reset simply because the short has been removed
or the connector containing the circuit in question has been removed from
the MPX Module. If a short to ground has occurred, the Output will continue
to be deactivated by the MOSFET in its tripped (open) state. It can be reset
by turning the ignition switch off, or by causing one of the required Inputs
for that circuit to be inactive. One could misdiagnose a correctly functioning
MPX Module or even damage the Module by checking for voltage on an Output that has been shorted to ground without first performing these steps:
1.
turn the ignition switch off.
2.
Isolate the short.
3.
turn the ignition switch on.
4.
verify that all requirements for an active Output are correct (true).
5.
Refer to the output chart to see if the Output should be 12 volts or
ground.
6.
Measure the Output at the correct terminal for the correct condition
with a voltmeter.
If the physical output is not found to be correct by conducting the above test,
the MPX Module is suspect. Confirm proper voltage and grounds at connectors D and E with a voltmeter. Consider consulting with a Blue Bird service
source to confirm your diagnosis before replacing the MPX Module.
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If, as is normal, the Feedback LED is off while the Output is Active, then the
circuit should be operative. If not, there is an open on the Output circuit.
Thus, in cases where the desired Output is Active, but the circuit is still inoperative, the Feedback LED can help indicate whether the probable cause is an
open or a short.
But what if a desired Output is Inactive? Even then it is still good practice to
check the Feedback LED before proceeding to verify the presence of each
requirement for an Active Output. In our example, we know that if the Starter
Relay Output is Inactive, we should see its Feedback LED on, indicating that
continuity exists in the Starter circuit, and that it should become operative
when the Output becomes Active. But if the Feedback LED were off while the
Output is Inactive, then we would already have an indication of an open in
the Starter circuit, and that the Starter will not work even when the requirements for an Active Starter Signal Ouput are met.
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4. Check Each Requirement. At this point in our example, we assume that the
Starter Relay Output is Inactive, and that the Feedback LED is on, as it should
be. It is now safe to assume that any starting problem must be “upstream” of
the load circuit itself. We must begin to systematically ensure that each of the
requirements for the Starter Relay Output to be active is met. It is now time
to refer to the Logic Diagram.
As indicated in the Output Table, Logic Line # 20 is the one which leads to
an Active Starter Signal Output. Click the yellow tab at the upper left of the
Vision MPX Simulator to reveal a scrolling list of the Logic Diagrams. Click the
red down arrow to scroll down to line 20.
Reading from left to right, we see that the requirements for an Active Starter
Signal are:
•
The Neutral Signal Input at logical address A1-I03 must be Active.
•
The Vandal Lock Input at A3-I08 must be Inactive.
•
The Starter Lockout Signal Input at A2-I02 must be Inactive.
•
The Front Start Switch Input at
A2-I05 must be Active.
Our procedure, then, is to systematically set the controls which operate each
of these requirements and verify their Active or Inactive status:
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Logic Line 20 of the Logic Diagrams
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multiplex system
4.1. Verify that the transmission is in Neutral. Press the Diagnostic Switch
enough times to cycle the MPX Module LEDs to display Zone A Port 1
Inputs. If the #3 Input LED were not on, at least part of the problem
would have already been isolated. In this case, you would then refer
to the wiring schematic for the Neutral signal switch and troubleshoot that circuit. Locating a wiring schematic for a specific failure can
be simplified by referring to the Input Tables. Locate the Input that
has failed (in this case, the Neutral Signal, Line 12). The third column
shows AB-107, indicating the Neutral Signal circuit connects to the
MPX Module through the B connector pin number 7. Connectors A, B,
and C of the MPX Module have dedicated wiring schematics. Referring
to the wiring schematic index page, find that the AB Multiplex Module
(Zone A Connector B) is located on sheet 11. The circuit in question is
located at pin 7 of the AB connector. Since the A1-I03 LED is on, the
Input is Active as it should be, and we proceed to step 4.2.
4.2 Make sure the Vandal Lock is not on. This is an A3 Input, so the MPX
Module needs to be cycled to indicate addresses for A3 Inputs. However, now we are looking for an Inactive Input, not an Active one. Verify that the #8 Input LED is off. If the A3-I08 Input LED were to stay on
while the Vandal Lock was unlocked, it might be suggestive of a stuck
or shorted Vandal Lock signal switch. Again, you would refer to the
appropriate wiring schematic and troubleshoot that circuit.
4.3 The fourth requirement is also for an Inactive Input; the Starter Lockout Signal. The Starter Lockout Signal is generated by the engine ECU
to prevent the starter from engaging if the engine is already running.
Therefore, it is not an Input over which the Driver has direct control.
He controls it indirectly by starting the bus. Put the MPX Module in its
display mode for A2 Inputs and verify that the #2 Input LED is off. If
it were on when the engine is not running, a diagnostic check of the
engine ECU would be in order.
The MPX Module is displaying A1 Inputs. Input #3 (Neutral
Signal) is Active.
The MPX Module is displaying A3 Inputs. LED #8 (Vandal Lock
Signal) is off—satisfying a requirement for an Inactive Input.
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The MPX Module is displaying A2 Inputs. LED #2 is off, so the
Starter Lockout Signal is Inactive.
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Vision [propane]
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service manual
4.4 Finally, the A2-I05 Start Switch Input must be Active. This is a signal
from the ignition switch when it is turned to the Start position. The
MPX Module is already displaying A2 Inputs, and the #5 Input LED is
off. Press and hold the ignition switch and you will see that the #5 LED
comes on while the switch is in the Start Position. If this did not occur,
the ignition switch or its wiring would be suspect.
Notice also that as soon as the engine starts, the #2 LED (Starter Lockout Signal, described above in Step 4.3) comes on and remains on until you click the ignition switch again to turn off the engine.
As the #2 Starter Lock Out Signal becomes Active, it no longer provides a correct requirement for the MPX Module to provide a Starter
Relay Output. Therefore, when the engines starts, the MPX Module
will deactivate the Starter Relay Output and the starter will disengage
even if the driver keeps the ignition switch in the Start position.
At this point, you have verified the presence of a full set of conditions (Inputs) required for an Active Starter Signal Output, and the starting circuit should be functional. If you experiment with the MPX Simulator by changing any of the required
conditions, you will see that when any of the requirements are not met, the A2-O08
Output will not be Active, and the engine will not start. This example has shown
that the Multiplex Diagnostic Mode can be used to quickly isolate the location of an
electrical system malfunction in a circuit with which the MPX Module interacts. (For
example, consider how much time might be spent in determining that the cause of
a starting problem is a damaged parking brake switch, without the benefit of multiplex.) Note that the use of Multiplex does not negate or invalidate traditional means
of tracing and troubleshooting circuits, but serves to eliminate much of the preliminary guesswork in determining which circuit(s) to investigate.
In a real-case scenario, only after verifying all the required conditions, yet still finding
the Output to be Inactive and/or the circuit inoperative, should there be reason to
suspect a failure of the MPX Module itself. Remember that all Input and Outputs can
be verified using a multimeter at the associated Connector/ Port-Pin locations. In
the unlikely case that you have thorougly investigated the Multiplex Logic and have
come to suspect a defective MPX Module, you may wish to contact your Blue Bird
Distributor’s service technicians for a second opinion verification of your troubleshooting.
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Still displaying A2 Inputs, LED #5 is on only when the ignition
switch is in its momentary Start position. As soon as the
engine starts, LED #2 (Starter Lockout Signal) comes on and
remains on until the engine is switched off.
multiplex system
Shared Logic
To avoid confusion when reading Ladder Logic and trobleshooting, remember that
the requirements shown on a Ladder Logic line are only requirements for the MPX
Module to make the Output indicated at the end of the line Active. It is possible for
other conditions to be required beyond the scope of the Multiplex system. In the
present implementation of Multiplex one such “special case” is the Fast Idle logic,
shown on line 27 of the Ladder Logic chart.
The Active Fast Idle Output from the MPX Module is one of two signals required
by the ECM before it engages Fast Idle. The other is a signal required by the engine
ECU logic. The ECU, much like the MPX Module, also requires Two Active Inputs: An
Active Neutral Input, and an Active High Idle Input from the MPX Module.
In other words, Logic Line 27 is correct for the MPX Module to generate an Active Fast Idle Output; but Neutral is also required by the engine ECM for it to cause
High Idle to actually occur. In this sense, even though the MPX Module and engine
ECM are separate processors which do not share programming, they work together
to “share” the logical requirements for a particular function. The Active Fast Idle Output from the MPX Module alone does not cause the actual occurance of High Idle; it
serves as but one of the ECM’s requirements.
27
A2-I01
HIGH IDLE SWITCH
A2-I04
IIGNITION SWITCH
A1-I03
NEUTRAL
A1-I06
A3-F05
PARK BRAKE SWITCH FAST IDLE BRAKE
A3-O01
HIGH IDLE
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This Input is required by the MPX Module for
an active High Idle Output.
However, it is a “shared” signal because it is
also rececived directly by the engine ECU.
This Output from the MPX Module is also
required by the engine ECU in order for High
Idle to actually occur.
High Idle actually occurs only when
the engine ECU receives BOTH the
Neutral Signal directly from the
transmission AND the High Idle Output
from the MPX Module.
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