Automotive Tutorials
Automotive Tutorials
Trigger Signals
The following topic explains the common types of trigger variations along with example waveforms and a
brief explanation as to its operation.
Permanent Magnetic Pick-up's
This particular type of pick-up generates its own signal and therefore does not require a voltage supply to
power it. Recognisable by its two electrical connections, the pick-up is used as a signal to trigger the ignition
amplifier or Electronic Control Module (ECM). As the metal rotor spins, a magnetic field is altered which
induces an Alternating Current (AC) voltage from the pick-up. This type of pick-up could be described as a
small alternator because the output voltage rises as the metal rotor approaches the winding, sharply
dropping through zero volts as the two components are aligned and producing a voltage in the opposite
phase as the rotor passes. The waveform is known as a sine wave.
The voltage produced by the pick-up will be determined by several factors, these being:
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Engine speed ² the voltage produced will rise from as low as 2 to 3 Volts when cranking, to over 50
Volts at higher engine speeds
The proximity of the metal rotor to the pick-up winding. An average air gap will be in the order of 8 to
14 thou, a larger air gap will reduce the strength of the magnetic field seen by the winding and the
output voltage will be subsequently reduced
The strength of the magnetic field offered by the magnet. The strength of this magnetic field
determines the effect it has as it 'cuts' through the windings and the output voltage will be reduced
accordingly
A difference between the positive and the negative sine wave voltages may also be apparent, as the
negative side of the sine wave is sometimes attenuate when connected to the amplifier circuit, but will
produce perfect AC when disconnected and tested under cranking conditions
Back probe the two wire multiplug on the inductive pick-up
fly lead coming out of the distributor's body, as illustrated in
Fig 1.0. If the amplifier is mounted on the distributor body
remove the amplifier and attach two small fly leads to the
pick-up's output connections, and refit the amplifier (if you
wish to test the pick-up whilst the engine is running). Crank
or start the engine and observe the resultant waveform. Use
a similar voltage and time scale to that shown in the example
waveform Fig 1.1. A lower than anticipated voltage could be
due to an incorrect rotor air gap, slow cranking speed or
shorted pick-up windings. If you are using an AC volts
setting on a multimeter the voltage recorded will be
approximately 0.7 of the peak-to-peak voltage. Therefore if
the data requires a 3 V peak-to-peak voltage, the allowable
multimeter can be 0.7 of this reading.
Figure 1.0
Figure 1.1
The above illustration (Fig 1.1) was taken with the engine at idle; the voltage will rise to around 50 V at
cruising speed. The voltage output will vary between different systems manufacturers so it's important to
reference to the appropriate specific data.
There are two types of permanent magnetic pick-ups these are called Annular and Limb. The distinguishing
factor between the two types is that the limb will only have one pick-up point where as the annular will have
a pick-up point per cylinder.
The pick-up will invariably be mounted on the distributor shaft and driven from the camshaft. They can be
tested in several ways using either a multimeter or an oscilloscope.
Hall Effect Pick-up's
This form of trigger device is a simple digital 'on / off' switch which produces a square wave output that is
recognised and processed by the ignition control module. The trigger has a rotating metal disc with
openings; this passes between the electromagnet and the semiconductor. The effect of a magnetic field that
is able to pass through one of the 'windows' will stop the flow of voltage. When the 'window' is closed the
flow is reinstated. This action will produce a digital square wave that is understood by the Electronic Control
Unit (ECM) or amplifier.
The sensor will have its characteristic three connections, which are: a live supply voltage, an earth and the
output signal. The square wave when monitored on an oscilloscope may vary in amplitude; this is not
thought to be a problem as it's the frequency that¶s important, not the height of the voltage. When the
voltage from the Hall effect trigger drops to zero volts, it fires the coil. This occurs when the 'window' on the
metallic rotating vane opens.
Connect the ground connection of the oscilloscope to a
known good earth and attach the voltage probe connection
to the centre terminal. The three connections to the Hall
effect pick-up are: the sensors voltage supply, an earth and
the Hall effect output. The switched signal from the Hall
effect is normally the centre pin (Fig 1.2). The Hall output
has been captured in the example waveform shown in Fig
1.3. This voltage may vary between different manufacturers
systems and the appropriate data should be sought.
Figure 1.2
Figure 1.3
Inductive Crank Angle Sensors
This sensor know as a Crank Angle Sensor (CAS) or sometimes Crankshaft Position Sensor (CPS) can be
mounted in various positions and can be located near the front pulley, to the rear of the engine into the
flywheel, in the side of the engine block or within the distributor. The output signal produced is used by the
Engine Control Module (ECM) to determine the exact position of the engine.
On an inductive CAS a resistance value should be seen between the terminals. This type of sensor is the
most popular but Hall effect and AC excited sensors are also used in some engine management systems. The
inductive sensor is normally a two wire device, however some manufacturers use three wires, the third being
a coaxial braid to keep out any HT interference that may interrupt and corrupt the signal seen by the ECM.
The output voltage produced on this sensor will be vehicle specific and the output will reduced by any of the
following three factors:
1. The sensor air gap will in some cases be fixed and is non adjustable, while on other vehicles the air
gap can be adjusted and measured using feeler blades. A larger air gap will decrease the voltage
output from the sensor
2. A failing sensor with shorted windings will also reduce the voltage output, while a sensor with an open
circuit will have no output at all. The condition of the winding inside the crank angle sensor can be
determined by conducting a resistance test with a multimeter
3. A slower than anticipated cranking speed may also cause the output to be low; the characteristics of
this being that the engine will not start when cranked, but starts if the engine is 'bump started'
causing the engine to rotate faster and producing sufficient voltage to trigger the ECM. A large air gap
can also give the same symptoms.
Crankshaft position sensors tend to fail as they become hot and the windings become open circuit, in this
instance the engine will stop, but restarts if left to cool down.
For test purposes, connect the oscilloscope in exactly the same way as you would when testing an inductive
pick-up located within a distributor. The resulting waveform should look similar to the example shown in Fig
1.4. This particular waveform was captured while the engine was cranking, this is evident from the differing
peak-to-peak voltages seen. This is due to the engine reaching a compression stroke, at which point the
engine slows momentarily and the voltage is lower. As the engine speeds up before the next compression
stroke, the voltage increases.
Figure 1.4
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This Tutorial was first published by The Institute of the Motor Industry http://www.motor.org.uk/
Information supplied courtesy of Auto Solve http://www.auto-solve.com/
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