GMC 4000 Series Service Manual page 531

Sec. 12
Page 520
CHASSIS ELECTRICAL A N D INSTRUMENTS
Figure 6 — Fuse Block (Tilt Cab M odels) (Typical)
example: a fusible link of 16 gauge wire must have
12 gauge wire or larger in all unprotected circuits
of the link system . To aid in identification, the
gauge size of the wire is labeled on the side of the
link.
IMPORTANT: If an overload or some other
condition destroys a link, locate and correct the
cause of the failure, then replace the link with a
w ire of the same gauge size and length. DO NOT
replace a fusible link with a wire of larger size.
Refer to Master Parts Book for correct replace­
ment part number.
CHASSIS JUNCTIONS
Terminal posts on junction blocks are num­
bered to correspond with symbols shown on wiring
diagrams in applicable "WiringDiagrams" booklet.
NOTE: On some models, wiring junctions are
made by use of mating connectors. Refer to applic­
able wiring diagrams for arrangement of wiring.
G M C S E R V I C E M A N U A L
TILT CAB MODELS
left-hand step riser panel and is accessible when
door is open and cover is removed. One 12-term ­
inal junction block is used on vehicles with stand­
ard equipment and an additional 12-terminal junc­
tion is used when required for additional electrical
equipment. A 6-terminal chassis junction block is
also located on frame side rail at left of cab.
automatic reset type circuit breaker built into the
main light switch. Any condition which causes an
overload on this circuit causes the bimetallic
element to open the circuit. When element cools,
the circuit breaker will close the circuit. This
off-and-on cycle will repeat until light switch is
turned off or until cause of overload has been
located and corrected. Circuit breaker is shown
in main light switch wiring diagram (fig. 17). If
circuit breaker becomes defective, the complete
light switch assembly must be replaced.
breaker is used in the two-speed axle shift switch
circuit on vehicles equipped with the two-speed
axle.
motor is protected by an automatic reset type cir­
cuit breaker. Any condition which causes an over­
load on a circuit, such as a short, will cause cir­
cuit breaker bimetallic element to open the circuit;
when element cools, the circuit breaker will again
close circuit. This off and on cycle will repeat
until the switch controlling the defective circuit is
turned off, or until the cause of overload has been
located and corrected. In the event a circuit break­
er becomes defective (burns open), the defective
circuit breaker must be replaced. Install circuit
breakers so the feed or battery wire is connected
to the "BAT" or short terminal and the wires car­
rying the circuit to the electrical units is connected
to the "AUX." or long terminal.
speedometer adapter by a flexible cable* The miles
per-hour hand is magnetic cup actuated, while the
odometer is direct gear actuated. If speedometer
becomes inoperative, disconnect cable at rear of
speedometer head and adapter and check to make
sure cable and drive gear on transmission are
operating properly. This can be done by driving
the vehicle forward while an assistant checks move­
ment of the drive cable inside the flexible cable.
Chassis junction block (fig. 7) is located under
CIRCUIT BREAKERS
The headlight circuit is protected by a 25-amp
A 15- or 20-amp automatic reset type circuit
Circuit to the two-speed axle electric shift
SPEEDOMETER
Speedometer is mechanically driven from
If the drive cable and drive gear appear to be