GMC 4000 Series Service Manual page 541

Sec. 12
Page 530
CHASSIS ELECTRICAL A N D INSTRUMENTS
DIRECTIONAL LAMP LENS
SIDE MARKER
LAMP BULB
DIRECTIONAL
LAMP BULB
LAMP
AND GASKET
LENS MOUNTING SCREW
Figure 18— Directional and Side M arker Light
DIMMER SWITCH
Foot-operated dimmer switch is used to select
headlight high or low beam. Switch is mounted on
left side of cab floor panel and is operative when
headlights are illuminated. Switch terminals are
blade type with connector plug attached to harness.
SWITCH REPLACEMENT
Removal
Make sure main light switch is pushed all the
way in or placed in "OFF" position, then remove
dimmer switch as follows:
On Conventional Cab Models - raise floor mat
to gain access to switch. Remove two screw s which
attach switch to floor panel and pull wiring con­
nector plug off switch terminals.
On Cowl and "S" Models - remove two screws
which attach switch and mounting bracket to floor
panel. Pull wiring harness connector plug off switch
term inals, then remove two cap screw s which at­
tach switch to mounting bracket and remove switch.
On Tilt Cab Models - remove two screw s from
inside cab which attach switch to toeboard riser.
Tilt cab and from underneath, pull switch free
from toeboard riser. Pull wiring connector plug
off switch terminal.
Installation
On Conventional Cab Models - install wiring
connector plug on switch terminals. Position switch
on cab floor panel and attach with two mounting
screw s. Place floor mat over switch and wiring
connector plug.
On Cowl and "S" Models - position switch on
mounting bracket and secure with cap screws,
washers, and nuts. Install wiring connector plug on
G M C S E R V I C E M A N U A L
DIRECTIONAL
LAMP LENS
LENS MOUNTING SCREW
switch, then position switch and mounting bracket
on cab floor panel and secure.
On Tilt Cab Models - tilt cab forward and in­
stall wiring connector plug on switch terminals.
Position switch under toeboard riser with button
portion of switch extending through cab floor.
From inside cab, attach switch to toeboard riser.
STOP LIGHT SWITCH
Either a mechanically-operated or air-oper­
ated stop light switch is used on vehicles covered
by this manual.
MECHANICALLY OPERATED TYPE
The mechanically operated type switch used
on models equipped with vacuum-hydraulic brakes,
is a plunger or lever type switch. With brake pedal
released, edge of pedal lever holds switch plunger
in, breaking circuit to the stop light. When brake is
applied and pedal lever moves away from switch
plunger, a spring within the switch moves the
plunger out to complete the stop light circuit.
Switch terminals are blade type with wiring con­
nections made through a connector plug on wiring
harness. Switch cannot be disassembled; therefore,
if switch becomes inoperative, it must be replaced.
When installing a lever type switch, make sure
lever on switch is located above brake pedal lug,
otherwise switch will not be operative. Also, make
sure wiring connections at switch are fully engaged.
After installing switch, it must be adjusted so
that initial movement of brake pedal, measured at
the pedal pad, will permit switch to close and com­
plete stop light circuit. After adjusting and tight­
ening switch mounting nuts or bolts, visually check
operation of stop light to make sure stop light
comes on when brakes are applied and goes out
when brake pedal is released.
AIR OPERATED TYPE
The air operated type stop light switch is used
on vehicles equipped with air brakes or I.C.C.
trailer brake controls. Refer to M AIR BRAKES"
(SEC. 5B) for information pertaining to this switch.
DIRECTIONAL SIG NAL LIGHTS
Directional signal lights are either an integral
part of the stop and taillight assem blies or are
mounted on each side of cab, or in combination.
NOTE: On some models, a combination direc­
tional and side marker lamp (fig. 18) is mounted
on each side of cab.
When ignition or control switch is placed in
"ON" position, 12-volts is supplied through a fuse
to the flasher unit on tilt cab models. On conven­
tional cab models, the flasher is fed through a