Diagnosis Of The Stoplamp System; Diagnosis Of The Backup Lamp System - GMC G Service Manual

8B-6 CHASSIS ELECTRICAL

DIAGNOSIS OF THE STOPLAMP SYSTEM

PROBLEM
One Bulb Not
Working
1.
One Side Not
Working (Multi-Bulb
Design)
2.
1.
All Stoplamps
Inoperative
2.
3.
Will Not lUrn Off

DIAGNOSIS OF THE BACKUP LAMP SYSTEM

Problem
One Lamp Not 1.
2.
Working or
3.
Intermittent
1.
Both Lamps Not
Working or
2.
Intermittent
3.
4.
5.
Lamp Will Not Turn
Off
POSSIBLE CAUSE
Bulb burned out.
Loose connection, open wiring or faulty
bulbs.
Faulty directional signal switch or cancelling
cam.
Stop-hazard fuse is blown.
Open in the wire from the fuse to the stop-
switch.
Stop-switch misadjusted or faulty.
Stop switch misadjusted or faulty.
Possible Cause
Loose or burnt out bulb.
Loose connection.
Open ground connections.
Gear selector switch is misadjusted (open
when shifter lever is in reverse position).
Loose connection or open circuit.
Blown fuse.
Faulty gear selector or backup lamp switch.
Faulty ignition switch.
Gear selector switch misadjusted (closed
when the shift lever is not in the reverse
position).
CORRECTION
Replace the bulb.
1. Turn on the directional signal. If lamp does
not operate, check the bulbs. If the bulbs are
OK, check all connections. If the lamp still
does not operate, use the test lamp and
check for open wiring.
2. If the lamp will operate by turning the direc­
tional signal on, the switch is not centering
properly during the cancelling operation. Re­
place faulty cancelling cam or directional sig­
nal switch.
1. Replace the fuse. If the new fuse blows, re­
pair the short to ground in the circuit between
the fuse and the lamps.
2. Check for power at the brown wire at the
stop-switch and at the fuse. If there is power
at the fuse but not at the switch, check for an
open in the brown wire.
3. With the brake pedal pressed, check the
white wire terminal in the steering column
connector with a test lamp. If the bulb does
not light, check the stop switch for proper
adjustment. If the adjustment is OK, jumper
the stop switch. If the stop lamps operate,
replace the stop switch.
Readjust the switch. If the switch still mal­
functions, replace the switch.
Correction
1. Secure or replace the bulb.
2. Tighten the connectors.
3. Repair the bulb ground circuit.
1. Readjust the gear selector switch.
2. Check all connectors. If OK, check the con­
tinuity of the circuit from the fuse to the lamp
on either side of the fuse. Correct the open
circuit from the battery to the fuse.
3. Replace the fuse. If the new fuse blows, re­
pair the short to ground in the circuit from the
fuse through the gear selector or from the
fuse through the gear selector or the backup
lamp switch to the backup lamps.
4. With the ignition "ON," check the switch ter­
minals in the backup position with the test
lamp. If the test bulb lights at the dark blue
wire terminal but not at the light green wire
terminal, replace the switch.
5. If the test bulb lights at the ignition switch
battery terminal, but not at the output termi­
nal, replace the ignition switch.
Readjust the gear selector switch.
GMTB-0851-2L
GMTB-0852-2L