Circuit Check - GMC G Service Manual

DIAGNOSIS OF GLOW PLUG ELECTRICAL SYSTEM

CIRCUIT CHECK

If the system does not operate as described, check the
following:
I f f l Inspect
1. All connectors.
2. Engine harness ground connection to the engine.
E & l Tighten
• Nut to 11 N*m (8 ft. lbs.).
3. Four-wire connector at controller. It must be fully
seated and latched.
4. Both controller copper stud upper nuts.
IC il Tighten
• Nuts to 5 N*m (48 in. lbs.).
• Do not tighten lower nuts.
5. Temperature switch connector in the water cross­
over, near the front of the engine.
6. "GLOW PLUG" lamp on instrument panel for tight
connection and operation.
GLOW PLUG INHIBIT SWITCH
Check the temperature-controlled switch to make sure it
is closed at low tem peratures and open at high
temperatures.
1. Remove the connector from the inhibit switch when
the engine temperature is below 38°C (100°F).
2. Set the ohmmeter on a low range or use a self-pow­
ered test lamp.
3. Test across the terminals.
4. The switch should be closed (test lamp on or a read­
ing of less than 0.1 ohm on the meter).
5. Test terminals to ground with a test lamp or with the
ohmmeter on a high range. The lamp should be
"OFF" ; the meter should show greater than
1.0 megohm.
6. Replace the switch if it tests open across the termi­
nals or if either terminal is closed to ground.
7. Disconnect the plug from the switch terminals when
the engine is above 52°C (125°F).
8. Set the ohmmeter on the highest scale or use a self-
powered test lamp.
9. Test across the terminals.
10. Test from each terminal to ground.
ENGINE ELECTRICAL 6D-17
11. Switch should be open (test lamp "OFF" or a high
ohm reading of greater than 1 megohm on the meter).
12. Replace the switch if it is closed.
13. When installing the switch, use a socket wrench.
E & l Tighten
• Switch to 23 N*m (17 ft. lbs.).
GLOW PLUG SYSTEM
If all connections are intact, but the glow plug system
does not operate as stated, perform the system diagnosis
shown in figure 21. It provides a fast way to find if the glow
plug system is working properly. Use this procedure when­
ever there is doubt about correct system operation. Then
refer to the diagnostic chart in figure 22 to pinpoint the
condition.
NOTICE: Do not manually bypass the relay in the
glow plug controller. Do not jump start with
more than a 12-volt system. The glow plugs
could be damaged.
GLOW PLUG AFTERSTART
The glow plug controller provides glow plug operation
after starting a cold engine. This after-start operation is
initiated when the engine control switch is returned to
"RUN" from the "START" position. While loss of this func­
tion may not cause a cold start complaint, it may result in
excessive white smoking and/or poor idle quality after start.
To check for proper operation of this circuit, proceed as
follows:
1. With the engine cold (27°C or 80°F), turn the engine
control switch to the "RUN" position and let the glow
plugs cycle.
2. After 2 minutes crank the engine for 1 second. (It is
not important that the engine starts.) Return the
engine control switch to "RUN." Glow plugs should
cycle at least once after cranking.
3. If the plugs do not turn on, disconnect the controller
connector, and check the connector harness terminal
B with a grounded 12-volt test lamp. The lamp should
be "OFF" with the engine control switch in "RUN," and
"ON" when the engine is cranked.
4. If the lamp does not operate as just described, repair
a short or open in the engine harness purple wire.
5. If the lamp works right, but the afterstart glow plug
feature does not, replace the controller.