Chevrolet 1977 light duty truck, Service Manual

Page: 780 / 861

Upper or lower beam
will not light or
intermittent
1. Open connection or
defective dimmer switch
2. Short circuit to ground
1. Check dimmer switch terminals
with test lamp. If bulb lights
at light blue or tan wire terminals,
repair open wiring between dimmer
switch and headlights. If bulb
will not light at one of these
terminals, replace dimmer switch.
2. Follow diagnosis above (All
headlights inoperative or
intermittent)
SID E M A R K E R L A M P D IA G N O S IS
C o n d itio n P o s s ib le C a u s e C o r r e c tio n
One lamp inoperative
1. Turn signal bulb burnt
out (Front lamp)
2. Side marker bulb
burnt out
1. Switch turn signals on. If signal
bulb does not light, replace bulb.
(Bulb filament provides ground
path for marker lamp bulb
through the light blue or dark
blue/white strip wires).
2. Replace bulb.
3. Loose connection or
open in wiring
3. Usiftg test lamp, check brown
wire terminal at bulb socket.
If test lamp lights, repair open
ground circuit. If lamp
does not light, repair open
brown wire circuit.
Front or rear lamps
inoperative 1. Loose connection or
open ground connection
2. Multiple bulbs burnt
out
1. If associated tail or park lamps
do not operate, secure all
connectors in brown wire
circuit. If park and turn
lamps operate, repair open
ground connections.
2. Replace burnt out bulbs.
All lamps inoperative
1. Blown fuse
2. Loose connection
3. Open in wiring
1. If park and tail lamps do not
operate, replace blown fuse.
If new fuse blows, check for short
to ground between fuse panel
and lamps.
2. Secure connector to light switch.
3. Check tail light fuse with
test lamp. If test lamp lights,
repair open wiring between fuse
and light switch. If not, repair
open wiring between fuse and
battery. (Possible open fusible
link).