GMC 4000 Series Service Manual page 536

starts and terminates at a terminal post. Each
instrument and tell-tale light bulb is a wedge base
bulb, installed in a plastic holder which locks into
the case and makes contact with the printed circuit.
In the event of a visible break in a printed
circuit, it can be repaired by soldering; however,
this work must be performed by a competent elec­
trician. The case must be disassembled to facil­
itate replacement of gauges or instruments. The
cluster face is attached to the case by eight screws;
the two center screw s at top and bottom also at­
tach the retaining clips. T ell-tale and instrument
light bulbs can be replaced by removing the bulb
holder from the back of the case.
Each cluster is retained in panel by four spring
GENERAL
Headlights are equipped with single or dual
long-life "T-3" Sealed Beam units; all other lights
are the conventional bulb type. Refer to "Specifi­
cations" at the end of this section for bulb size
and number. Refer to applicable Wiring Diagram
in "Wiring Diagrams" booklet for lighting system
wiring circuits and wire identification.
In accordance with the Federal Motor Vehicle
Safety Standards, models with overall width of 80
inches or more have clearance and identification
lights as standard equipment. The main light switch
turns on all lights except back-up, hazard warning,
stop, and tell-tale lights.
HEADLIGHTS
All vehicles covered by this manual use the
7-inch single sealed-beam unit type headlights or
dual headlights, using four 5-%-inch sealed-beam
units.
HEADLIGHT BEAM ADJUSTMENT (Fig. 11)
Both the 5-%-inch and 7-inch sealed-beam
units are "T-3" type units. Each "T-3" sealed-
beam unit lens incorporates three projecting guide
points which are optically ground to provide flat
surfaces at right angles to the light beam. This
design, in addition to providing superior lighting,
perm its adjustment of the light beams without the
use of an aiming screen and without requiring a
large work area. Aiming is accomplished with the
use of an aimer (J-6878-01). Instructions for using
the aimer are supplied by the instrument manu­
facturer.
Headlight beam adjustment requirements will
vary on different vehicles due to size and distrib­
ution of load, type of standard or optional suspen-
G M C S E R V I C E M A N U A L
CHASSIS ELECTRICAL A N D INSTRUMENTS
retaining clips, two at top and two at bottom. Holes
are provided in instrument panel to permit using
hooked tool (J-7900) to depress clips and release
cluster from panel. To remove cluster, maintain
a slight pressure on cluster from back side while
releasing the two lower retaining clips, then re­
lease the two upper clips. Use of tool is shown in
figure 10.
LIGHTING SYSTEM
sion, size of tires, tire p ressures, and other fac­
tors. Average requirements are for the high in­
tensity zone of the high beams of inner lights on
dual units to be straight ahead and 2 inches below
the headlight level at 25 feet. Low beam of outer
lights of dual units and all 7-inch units should be
VERTICAL CENTERLINE
STRAIGHT AHEAD OF
LAMP CENTER-
VERTICAL A IM - TOP EDGE OF
HIGH INTENSITY Z O N E AT (H)
LIMITS H + 2 IN.-
LATERAL A IM - LEFT EDGE
OF HIGH INTENSITY Z O N E
2 IN . RIGHT OF ( V ) -------
LIMITS V TO 6 IN . RIGHT
VERTICAL CENTERLINE
STRAIGHT AHEAD OF
LAMP CENTER
VERTICAL A IM - HIGH
INTENSITY ZO NE
CENTERED 2 IN . BELOW (H)
LIMITS + 2 IN/
LATERAL A IM - HIGH
INTENSITY Z O N E
CENTERED O N (V)
LIMITS + 6 IN
CAUTION: DO NOT try to force bent
end of tool too far into opening; this will
require more leverage to depress the cl ip
and may deform the panel opening. Do not
pry under edges of cluster at front side of
panel; this could damage the bake I ite case.
HORIZONTAL CENTERLINE
LOCATED' AT LEVEL OF
LAMP CENTER -
AIM INSPECTION LIMITS FOR LOWER BEAM OF:
5y 4
in. Type 2 Sealed Beam
7 in. Type 2 Sealed Beam
Caution—Do not aim or inspect these lamps on the upper
beam
HORIZONTAL CENTERLINE
LOCATED AT LEVEL OF
LAMP CENTER-
AIM INSPECTION LIMITS FOR UPPER BEAM OF:
53/i in. Type 1 Sealed Beam
7 in. Sealed Beam, except Type 2
Figure 11 — H ead ligh t Beam Adjustm ent (Typical)
Sec. 12
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