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 57 of 1015
TM 9-2320-272-24-1
1-22. ELECTRICAL SYSTEMS OPERATION (Contd)
b. Starting System Operation.
The starting system is identical for all models covered in this manual and consists of the following major
components and circuitry:
Key Item and Function
8
9
10
11
12
13
BATTERY SWITCH
- Completes circuit 459, closing a relay in the protective control box to supply
power to ignition switch through circuits 5 and 5B.
PROTECTIVE CONTROL BOX
- Locks out starter circuit, which prevents starter from reengaging
while engine is running.
IGNITION SWITCH
- Provides battery power to fuel solenoid through circuit 54 and the neutral
start safety switch through circuit 498.
NEUTRAL START SAFETY SWITCH
- Prevents starter from energizing when vehicle is not in
neutral, by deenergizing circuit 499 and a relay in the protective control box, which disconnects
power from circuit 74 and the starter solenoid.
STARTER SOLENOID
- A magnetic relay that is powered by circuit 74 to transmit 24-volt battery
power to the starter motor through circuit 6.
STARTER MOTOR
- Cranks engine for starting. Supplied with 24-volt battery power through
circuit 6.
1-47
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