TM-9-2320-272-10
TRUCK, 5-TON, 6X6, M939, M939A1, AND M939A2 SERIES TRUCKS (DIESEL)
TECHNICAL MANUAL; OPERATOR'S MANUAL
SEPTEMBER 2004
CHANGE 2
TM-9-2320-272-10 - Page 72 of 481
TM 9-2320-272-10
1-55
1-19. ELECTRICAL SYSTEMS OPERATION (Contd)
A
VOLTMETER
– Indicates system voltage and is connected to the batteries
through circuit 27 and to chassis ground through instrument panel.
B
ENGINE COOLANT TEMPERATURE INDICATOR
– Indicates engine
coolant temperature and receives battery power through circuit 27. Circuit 33
completes the circuit to ground through a coolant temperature sensor that
reacts to changes in engine coolant temperature by increasing or decreasing
the resistance in the ground circuit.
C
ENGINE OIL PRESSURE INDICATOR
– Indicates engine oil pressure
and receives battery power through circuit 27. Circuit 36 completes the
circuit to ground through the oil pressure transmitter located on the engine
block.
D
FUEL INDICATOR
– Indicates fuel level. Receives battery power through
circuit 27. Circuit 28 or 29, depending upon which position the fuel selector
switch is in, completes the circuit to ground through the fuel level sensor.
E
TRANSMISSION OIL TEMPERATURE INDICATOR
– Indicates
transmission oil temperature and receives battery power through circuit 27.
Circuit 324 completes the circuit to ground through a temperature sensor
located in the transmission.
F
FRONT-WHEEL DRIVE ENGAGEMENT LIGHT
– Informs the operator
that the front-wheel drive is engaged. The system consists of a normally open
pressure switch, which is powered through circuit 27A and an indicator lamp
powered through circuit 27A.
G
HORN SYSTEM
– The horn system consists of an air-operated horn that is
controlled by an electric solenoid. The solenoid is powered through circuit 26
and controlled by the horn switch through circuit 25.
H
SPRING BRAKE WARNING SYSTEM
– Warns the operator that the
spring brakes are applied. The system consists of normally open pressure
switch powered through circuit 37 and an indicator lamp which is powered
through circuit 37.
I
FAILSAFE WARNING SYSTEM
– Intended to give the operator an audible
as well as visual signal of a malfunction in one of the primary systems. Power
for the system is supplied from the ignition switch through circuit 564. The
failsafe module causes an indicator lamp to illuminate and an alarm to sound
when the air pressure falls below 60 psi (414 kPa) or when the parking brake
is set.
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