TM-9-2320-272-24-1
TRUCK, 5-TON, 6X6, M939, M939A1, AND M939A2 SERIES TRUCKS (DIESEL)
TECHNICAL MANUAL; UNIT, DIRECT SUPPORT, AND GENERAL SUPPORT MAINTENANCE MANUAL; VOLUME 1 OF 4
JUNE 1998
TM-9-2320-272-24-1 - Page 61 of 1015
TM 9-2320-272-24-1
1-22. ELECTRICAL SYSTEMS OPERATION (Contd)
f. Heating System Operation.
The electrical portion of the heating system is identical for all models covered in this manual and
consists of the following major components and circuitry:
Key Item and Function
7
8
9
10
11
12
PROTECTIVE CONTROL BOX
- Provides 24-volt power to circuit breaker through circuits 5 and
5A and to heater switch through circuits 27 and 5C.
CIRCUIT BREAKER
- Provides overload protection for 24-volt circuits 5, 5A, 27, and 5C leading to
the heater switch.
BATTERY SWITCH
- Provides 12-volt battery power from circuit 569 through 569A to the heater.
CIRCUIT BREAKER
- Provides overload protection for 12-volt circuit 569A leading to heater
switch.
HEATER SWITCH
- Controls low and high blower motor speed and has two sources of power: 12-
volt power is supplied through circuit 569A from battery switch and is used to provide low speed;
24-volt power is supplied through circuit 5C from protective control box and is used to provide high
speed.
HEATER BLOWER MOTOR
-A direct current motor controlled by heater switch through
circuit 400.
1-51
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