GMC 4000 Series Service Manual page 539

Sec. 12
Page 528
CHASSIS ELECTRICAL A N D INSTRUMENTS
Figure 14— Replacing Instrument or
Tell-Tale Light Bulb (Typical)
STOP A N D TAILLIGHT
The stop and taillight is a combination-type
lamp, having a double-filament bulb. The taillight
filament is of 3 candlepower, and the stop light
filament is of 32 candlepower.
BULB REPLACEMENT
Removal
Remove lens retaining screw s, then remove
lens and gasket from housing. P ress bulb inward
and turn counterclockwise to remove.
Installation
Refer to ''Specifications" for bulb size and
type. Note that J-slots in bulb sockets are of dif­
ferent depths and lugs on bulb base are located at
different distances from end of base. Lugs and
slots must be matched to permit installation of
bulb. Insert bulb in socket, press inward and turn
clockwise to lock in place. Position lens and gasket
on body and attach with screw s.
BACK-UP LIGHTS
The back-up light circuit is fed from the ig­
nition switch control circuit. The back-up lamp
switch, mounted on the transmission, is activated
by the reverse shift rail (meahcnical transmission)
when the ignition or control switch is placed in the
"ON" position and transm ission shift lever is in
reverse. Switch adjustment on vehicles with A lli­
son Transmission is covered in ''TRANSMISSION
CONTROL LINKAGE'' (SEC. 7A) of this manual.
If back-up lights become inoperative, discon­
nect the wiring harness connector plug from the
switch. Connect both terminals in the connector
G M C S E R V I C E M A N U A L
with a jumper wire, then place the ignition or con­
trol switch in the "ON" position. If back-up lights
operate, replace the switch.
BULB REPLACEMENT
and remove lens. P ress bulb inward and turn coun­
terclockwise to remove from socket.
to lock in place. Position lens and gasket on hous­
ing and attach with screw s.
either wedge base or single contact, miniature
base type. On all vehicles, except tilt cab models,
bulbs are installed in pronged bulb sockets which
snap into openings in instrument cluster, gauges,
speedometer, and tachometer case. On tilt cab
models, bulbs are installed in plastic holders which
lock into the instrument case and make contact
with the printed circuit. The instrument panel lamp
circuit is protected by a fuse located on the fuse
block.
BULB REPLACEMENT
tion for bulb size and trade number.
Except Tilt Cab Models
gauge unit, speedometer case, or tachometer case.
P ress bulb in and turn counterclockwise to remove
from socket. Install new bulb in socket, p ress in
and turn clockwise to lock in place. After replace-
ing bulb, p ress socket firmly into place in opening
in cluster, gauge unit, speedometer, or tachometer
case.
Tilt Cab Models
from case (fig. 14). Pull the bulb straight out to
Figure 15— Dom e Light (Tilt Cab M odel) (Typical)
Remove screws from lens to lamp assembly
P ress new bulb into socket and turn clockwise
INSTRUMENT A N D TELL-TALE
LIGHTS
The instrument and tell-tale light bulbs are
Refer to "Light Bulb Data" at end of this s e c ­
Pull bulb socket out of instrument cluster,
Turn bulb holder counterclockwise to remove