GMC 4000 Series Service Manual page 533

Sec. 12
Page 522
CHASSIS ELECTRICAL A N D INSTRUMENTS
flows directly through relay contacts and out No.
3 terminal to the horn. The relay thus provides a
higher voltage to the horn by avoiding voltage drop
through the long circuit through the horn button.
Relay is non-adjustable, therefore, if it becomes
inoperative, replace.
TRAILER I.C.C. MARKER
LAMP RELAY (WHEN USED)
NOTE: Refer to applicable wiring diagram in
"Wiring Diagrams" booklet for wiring connections
at relay.
LOW V A C U U M ALARM SYSTEM
The low vacuum alarm system , on vehicles so
equipped, consists of a low vacuum switch, a buzz­
er, and a gauge to audibly and visually warn the
driver when vacuum is below a safe limit.
The low vacuum alarm buzzer is mounted on
left side of dash panel inside the cab. When circuit
through buzzer is completed at low vacuum switch,
action of the vibrating armature striking the core
produces a buzzing sound.
Low vacuum alarm system wiring connections
are shown on applicable wiring diagrams in "Wir­
ing Diagrams" booklet. The alarm buzzer and switch
are not repairable units. If unit becomes inoper­
ative, replace.
LOW AIR PRESSURE ALARM SYSTEM
The low air pressure alarm system , on ve­
hicles so equipped, consists of a low air pressure
switch, a buzzer, and a tell-tale and/or gauge to
audibly and visually warn the driver when air p res­
sure in the system is below a safe limit for brake
operation. Refer to "AIR BRAKES" (SEC. 5B) for
information on low air pressure switch.
The low air pressure alarm buzzer is mounted
on left side of dash panel inside the cab on con­
ventional cab models; or on instrument panel to
steering column support bracket on tilt cab models.
When circuit through buzzer is completed at low
air pressure switch, action of the vibrating arm­
ature striking the core produces a buzzing sound.
CONVENTIONAL CAB MODELS
A typical instrument cluster used on these
vehicles is shown in figure 8. Cluster contains in­
strument lights, tell-tale lights, gauges, speed­
ometer, and tachometer.
G M C S E R V I C E M A N U A L
ALARM SYSTEM
INSTRUMENT CLUSTER A N D GAUGES
The marker lamp switch on instrument panel
is connected in series with main light switch and
operating coil of I.C.C. marker lamp relay oper­
ating coil at terminal No. 3.
When the main light switch and the marker
lamp switch are placed in "ON" position, a small
amount of current will flow through the relay coil
winding. With winding energized, armature is
attracted to core and points close. Current from
the battery (No. 1 terminal), then flows directly
through relay contacts and out terminal No. 2 to
complete circuit to rear I.C.C. marker lamps.
Low air pressure alarm system wiring con­
nections are shown on applicable wiring diagrams
in "Wiring Diagrams" booklet. In the event of fail­
ure, the buzzer must be replaced.
ENGINE ALARM SYSTEM
This system , used as optional equipment on
some vehicles, audibly and visually warns the
driver of low air pressure, low oil pressure, or
an overheated engine. The alarm system consists
of a low air pressure switch, low oil pressure
switch, hot engine switch, alarm buzzer and rec­
tifier assembly, and a separate tell-tale light for
each condition. The buzzer and rectifier is mounted
on left side of dash panel inside the cab on con­
ventional cab models and on steel-tilt cab models,
it is mounted on instrument panel to steering col­
umn support bracket. Wiring connections are shown
on applicable wiring diagrams.
Buzzer and rectifier assembly consists of a
vibrating armature type relay which produces a
buzzing sound when the circuit through relay coil
is completed by either of three controlling switches
and a three circuit rectifier. The rectifier permits
current flow in one direction only, preventing a
back flow of current from one alarm circuit from
illuminating the other tell-tale lights. Thus, when
only one abnormal condition exists, the buzzer will
sound but only the tell-tale light connected to that
circuit will illuminate.
If alarm buzzer or rectifier assembly does
not operate properly, replace the complete unit.
NOTE: The instruments, gauges, and tell-tale
lights will vary with truck models, depending upon
the size of vehicle and equipment used.
REMOVAL
I M P O R T A N T : Disconnect negative battery