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 64 of 481
TM 9-2320-272-10
1-47
1-19. ELECTRICAL SYSTEMS OPERATION
Nearly every component of the models covered in this manual is affected by the
electrical system. These components and their electrical connections are described
as part of the following electrical subsystems:
a. Battery System Operation (page 1-48).
b. Starting System Operation (page 1-49).
c. Ether Starting System Operation (page 1-50).
d. Generating System Operation (page 1-51).
e. Directional Signal System Operation (page 1-52).
f. Heating System Operation (page 1-53).
g. Indicator, Gauge, and Warning System Operation (page 1-54).
h. Trailer and Semitrailer Connection System Operation (page 1-56).
Electrical Terms and Definitions.
The following electrical terms and definitions will be frequently referred to
throughout this section and should be understood before proceeding:
Alternating Current (AC signal)
– Current in a circuit that flows, in one
direction first, then in the other direction.
Circuit
– A complete path for electric current flow between components.
Circuit Breaker
– An automatic switch that interrupts current flow in a circuit
when the current limit is exceeded.
Direct Current (DC signal)
– Current in a circuit that flows in one direction.
Female Connector
– One-half of a connector which fits over the other half.
Ground
–
A common return to complete a path for current flow in a circuit.
Harness
– A group of wires connected between devices that are bundled and routed
together to prevent damage and make repair and replacement easier.
Male Connector
– One-half of a connector which fits the other half.
Polarity
– The direction current flows in a circuit (usually positive to negative).
Relay
– An electromagnetic device that operates like an automatic switch to control
the flow of current in the same or different circuit.
Reverse Polarity
– The condition that exists when circuit polarity is connected
opposite of that which was intended.
Sending Unit
– A device that produces an electrical signal and sends this signal to
the device which will make use of it.
Sensor
– An electrical sensor takes a physical condition (temperature, oil presence
or absence) and converts this into an electrical signal.
Splice
– A permanent physical connection of two or more wires.
Terminal
– Fastener at end of wire used to connect the wire to an electrically-
powered device.
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