GMC Motor-Home
House Electrical AC SystemsGeneral Maintenance, Troubleshooting &
SAFETY
Author- Bob Hendrickson
For GMCES Rally
May 2006
Thanks to: Ken Henderson
DISCLAIMER: Information presented by GMCES is intended
only to communicate thoughts, ideas, opinions and
procedures to and from GMC owners; there is no attempt to
replace or supersede recommendations from General Motors
Corporation or any other component manufacturer. All
opinions expressed are those of the authors and not GMCES.
Mention of any product does not constitute endorsement by
GMCES. Neither the authors nor GMC Eastern States assume
any responsibility for what you choose to do to your coach.
1
Table of Contents:
Subject:
Page:
Electrical Safety
Rules and safe work practices
3
3
Electrical Visual Inspections
1. Primary power connector
2. Cable inspection
3. Strain relief inspection
4. Outlets/ cover plates
5. Converter Cables
3-4
3
3
3
4
4
Electrical Visual Test Trouble–shooting
5
Electrical Continuity Tests
Tools Needed
1. Ground to Body test
2. Ground to Outlet Ground Test
3. Ground to Neutral Test
4. Neutral to Hot(s) Test (Breakers OFF)
5. Ground to Hot(s) Test (Breakers OFF)
6. Turn ON breakers
7. Neutral to Hot(s) Test (Breakers ON)
8. Ground to Hot(s) Test (Breakers ON)
9. Generator Neutral to Ground bond test
6-9
6
7
7
7
8
8
8
9
9
9
Powered Outlet Test
10
Handy Formulas
10
Meter Test Trouble- shooting
(Note: Steps are numbered in the same order as the electrical
continuity tests from pages 6-9. If a test fails, see the corrective
action in this section.)
30A Power Equipped Measurement Supplement
2
11-12
13
Electrical Safety:
First and foremost, we should all consider electrical safety to be of paramount concern and the
reason to perform a few inspections on a regular basis. We should first establish a few safety
rules before working on equipment:
1. Do not work on “LIVE” electrical equipment unless absolutely necessary and then, only
with the proper safety equipment and a helper.
2. Do not use damaged electrical equipment, connectors, cords etc.
3. Do not use ungrounded power sources.
4. Do not remove protective coverings from circuit panels.
5. ALWAYS measure before working on any electrical equipment.
6. Treat all equipment as if it were energized.
Electrical Visual Inspections:
A few simple visual and meter reading inspections will ensure that your GMC’s AC electrical
system will perform at its optimum level:
Note: We will investigate problems later in this section
1. Unplug your coach and check your
primary power cable for signs of
cracking or separation of the insulating
boot on the plug end. You should see
no cracking of the rubber boot or any of
the internal wiring visible. If you see
any of the internal conductors or have
broken/ damaged connector pins,
DO NOTUSE YOUR CABLE!!!
YOU COULD BE ELECTROCUTED!
Investigate problem immediately.
Primary Power Connector, Replace!
2. Continue by checking your cable from
the plug end to the strain relief to
ensure there are no visible cuts or
abrasions in the insulation of the cable.
If you find any, inspect for damaged
conductors, if none are damaged, a
TEMPORARY repair can be performed
by taping over the damaged portion of
insulation. You should replace your
primary cable as soon as feasible.
Damaged Cable Insulation
(permissible to repair)
Repaired Cable
3
Do not use or repair this cable
3. Visually inspect your primary cable’s
strain relief. You should see no signs of
insulation fatigue. (located on the back
of the electrical panel)
Strain relief on back of circuit panel
If the insulation is fatigued or cracked,
DO NOT USE YOUR CABLE!!!
YOU COULD BE ELECTROCUTED!
Investigate problem immediately.
4. Visually inspect all outlets and cover
plates for damage. Replace any
needing attention
Look for cracks and missing covers
5. Check Your converter cable for
insulation damage, missing pins etc.
Cable on power converter
4
Visual Test Troubleshooting:
If any of the visual tests fail, look at the following steps:
1. Separated insulation or damaged
connector:
Replace the connector observing
the proper pin orientation and
screw terminal torques.
Available from most home building
centers as dryer and range plugs.
New Connector
2. For simple insulation nicks and cuts:
Two layers of electricians tape
over damaged insulation
For exposed and damaged conductors:
Replace power cable by removing
from back of circuit breaker box.
Taped Insulation
3. If the Outer insulation has split or
separated:
Loosen the cable grip and push ½
to ¾ inch of cable into the back of
the circuit breaker box and
retighten cable grip
If the conductors are showing:
Remove the cable from the circuit
breaker box observing proper
connection points.
Shorten cable beyond damaged
area.
Reinstall as removed
4. If any outlets or covers are damaged or
missing:
Replace as necessary
5. If your converter cable or plug is
damaged:
Replace with new cable/plug
assembly
Circuit breaker panel
5
Now, let’s gather a few simple tools for some measurement tests:
•
•
•
•
•
Straight blade screwdriver- med size
Phillips Screwdriver- #1 or #2
Digital VOM- preferably a category 2 or 3
Outlet Tester- from a building supply store
Helper if one is available
Required tools
6
Electrical Continuity Tests:
Safety Grounds Breakers OFF
Note: Troubleshooting starts on Page 11,
steps are in the same order as the test
steps.
1. With your coach unplugged, all circuit
breakers OFF and the generator OFF:
Check continuity between the round
GROUND pin and the metal chassis of
the GMC with your Digital VOM. The
door striker plate works well for this
test.
There should be NO resistance
(0 Ohms) Wiggle the cable while
performing this test to make sure the
plug is not internally broken.
Testing the primary power connector
If you see more than 10 OHMS of resistance,
DO NOT USE YOUR CABLE!!!
YOU COULD BE ELECTROCUTED!
Investigate problem immediately.
2. Continue checking the ground between
the round ground pin of the primary
power cable and each AC outlet
ground. Again, there should be NO
resistance (0 Ohms)
Testing the outlet ground connections
If you see more than 10 OHMS of resistance,
DO NOT USE YOUR CABLE!!!
YOU COULD BE ELECTROCUTED!
Investigate problem immediately.
3. Continue by checking for continuity
between the round Ground pin and the
center flat pin.(Neutral) There should
be a very high resistance (infinite
Ohms)
Testing for Neutral to Ground Short
If your meter does not read a high resistance,
DO NOT USE YOUR CABLE!!!
YOU COULD BE ELECTROCUTED!
Investigate problem immediately.
7
Electrical Continuity Tests:
Safety Grounds Cont Breakers OFF.
4. Continue by checking for continuity
between the center flat pin (Neutral)
and any of the other flat pins.(Hot)
There should be a very high resistance
(infinite Ohms)
Testing for Neutral to Hot Short
If your meter does not read a high resistance,
DO NOT USE YOUR CABLE!!!
YOU COULD BE ELECTROCUTED!
Investigate problem immediately.
5. Continue by checking for continuity
between the round Ground pin and any
of the other flat pins.(Hot) There
should be a very high resistance
(infinite Ohms)
Testing for Ground to Hot Short
If your meter does not read a high resistance,
DO NOT USE YOUR CABLE!!!
YOU COULD BE ELECTROCUTED!
Investigate problem immediately.
6. Continue by unplugging your converter
and turning on all of the circuit
breakers:
Converter Unplugged, Breakers all on
8
Electrical Continuity Tests:
Safety Grounds Continuity -- Breakers ON.
7. Repeat step 4,There should be a very
high resistance (infinite Ohms) on one
Hot leg to Neutral and a low resistance
on the other Hot leg to Neutral. (10 to
100 Ohms) If not, try turning OFF the
water heater circuit breaker and repeat
the test. Both Hot legs to Neutral
should read a high resistance (Infinite
Ohms) with the breaker OFF.
Testing for Neutral to Hot Short
If your meter does not read correctly,
DO NOT USE YOUR CABLE!!!
YOU COULD BE ELECTROCUTED!
Investigate problem immediately.
Meter is reading hot water heater element –
that’s OK!
(Note: if you have a gas water heater, you will
have a high reading on both legs to Neutral)
8. Repeat step 5, There should be a very
high resistance (infinite Ohms)
(Note: If you are reading a low resistance, you
may have a shorted water heater element)
Testing for Ground to Hot Short
If your meter does not read a high resistance,
DO NOT USE YOUR CABLE!!!
YOU COULD BE ELECTROCUTED!
Investigate problem immediately.
9. With generator OFF, Plug primary cord
into generator outlet. Measure
between Ground and Neutral in galley
outlet. You should measure 0 Ohms
Galley outlet Neutral to Ground
If your meter does not read a LOW resistance,
DO NOT USE YOUR GENERATOR!!!
YOU COULD BE ELECTROCUTED!
Investigate problem immediately.
9
If all test performed correctly, let’s test the coach under power and perform some
additional checks:
1. Turn OFF MAIN circuit breaker
2. Plug in coach to known good 50A
125/250VAC power Source
3. Turn ON MAIN circuit Breaker
4. Plug in OUTLET TESTER and observe
lights for each outlet in the GMC. IF all
outlets test OK, you are done! If not,
make necessary corrections.
Outlet tester
5. Plug in DC power converter
Enjoy your RV electrical system!
Perform checks on a regular basis!
Handy Formulas:
Formula for power
P=I*E
Power (Watts)= Current (Amps)*Voltage (Volts)
So
P/E=I
Look at the that big toaster @ 1800W with the
line voltage @ 117V
Look at the TV @ 110W + two lamps @ 60W
each
Ohm’s LAW
1800/117= 15.38A, Too much current for a 15A
circuit breaker!
(110 + 60 + 60)/ 117 = 1.967A, Fine for one
15A outlet.
E=I*R
Voltage (Volts) = Current (Amps)* Resistance (Ohms)
So
I= E/R
And
R = E/I
10
Meter Test Troubleshooting
If any of the electrical meter tests fail from pages 6-9, look at the following steps:
(Note: These steps follow in order the meter tests from pages 6-9)
1. High resistance or more than 10 Ohms:
Look at the ground bond
connection to the chassis and
tighten with flat blade screwdriver.
Look at the ground bond between
the chassis and the circuit panel,
tighten with flat blade screwdriver.
Check the ground pin for
looseness, replace the connector if
necessary
Chassis ground bond
Circuit panel ground bond
Plug ground pin
2. If the meter reading is more than 10
Ohms:
Check the screw connections on
the rear of the outlet.
Check the ground bar connections
inside the circuit breaker panel.
Circuit breaker panel ground bar
3. If the meter reading is low:
Be sure all breakers are OFF
Inspect for shorts inside of the
electrical panel
Look for chafed wiring
Circuit breaker panel
11
4. If the meter reading is low:
Be sure all breakers are OFF
Inspect for shorts inside of the
electrical panel
Look for chafed wiring
5. If the meter reading is low:
Be sure all breakers are OFF
Inspect for shorts inside of the
electrical panel
Look for chafed wiring
6. NO TEST
7. If the reading is High on both legs with
the breaker ON:
Suspect an open water heater
element, replace with new.
Possible broken or loose wire on
water heater
YOU HAVE A GAS WATER
HEATER, IGNORE!
8. If the meter reading is low:
Inspect for shorts inside of the
electrical panel
Look for chafed wiring
Suspect a shorted water heater
element. Verify by turning OFF the
water heater breaker and repeating
test.
9. If the meter reading is high:
Suspect a broken Ground to neutral
bond in the generator.
Suspect a broken or loose connection
in the generator outlet box.
12
(The low resistance of one of the Hot legs
to Neutral with the circuit breaker ON is the
resistance of the water heater element. If
readings are both HIGH, check your water
heater element.)
SUPPLEMENT FOR 30A 3 Wire Coaches
30A Power Cord Equipped Coaches
For those equipped with a 30A 3 pole
electrical system, please don’t feel left
out!
Here is a quick reference section for
troubleshooting.
Measurements from the Ground (round
pin) and the Neutral (lower left on
picture) should yield the same results
as the 50A plugs.
Ground
Measurements from the Ground (round
upper pin) and Hot (Lower right blade
in picture) should yield the same
results as those of the 50A plugs as
well.
The major difference between the 30A
and 50A plugs is the 50A plugs have 2
hot wires and the 30A has only one.
Neutral
You should still perform the same
tests using your VOM as the 50A
plugs.
HOT
30A 120VAC 3 prong
Generator Neutral Bond Test:
One additional test:
With the generator OFF, the Circuit
panel switched to GEN and the line
cord unplugged:
Measure the resistance of any outlet
between Neutral and Ground with your
VOM, You should measure 0 Ohms.
If the reading is High or open:
Suspect a broken Ground to
neutral bond in the generator.
Galley outlet neutral to ground
If your meter does not read a LOW resistance,
DO NOT USE YOUR GENERATOR!!!
YOU COULD BE ELECTROCUTED! Investigate
problem immediately.
Check the Neutral section of the
Generator Circuit breaker in the
electrical panel for continuity.
13
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