TROUBLESHOOTING THE M998 ELECT SYST. - OD1617 - LESSON 1/TASK 1
(a) Heater Core.
The main component of the HMMWV's heater
system is the heater core. The heater core transfers heat from the
engine coolant to the passenger compartment.
Its construction is
similar to that of a radiator, except that it is on a much smaller
scale. The heater core fits into the heater unit and is connected to
The connection to
the engine's coolant passages is made at strategic points that cause
the engine's water pump to force a constant supply of engine-heated
coolant through the heater core. The heater core feed line is fitted
with a control valve used by the operator to open, restrict, or shut
off the coolant flow through the heater core to control the heater
The temperature control valve can either be cable-
operated or operated by a vacuum-actuated diaphragm that receives its
vacuum supply from the engine.
(b) Heater Control Knobs and Valves.
The heater core on the
HMMWV has a mechanically-actuated, mechanically-controlled butterfly
valve that is opened, adjusted, and closed by the operator pulling a
control knob. This control knob is connected to one end of a cable,
the other end to the butterfly valve.
When the operator pulls the
knob, the cable pulls the valve actuating lever, opening the valve.
There is also a cable-operated flapper valve that allows the operator
to direct the air to either the windshield vents for defrosting or to
the cab outlet.
(c) Blower Motor. The blower motor is a simple electric rotor
that drives a circular fan, commonly referred to as a squirrel cage.
The blower motor on the HMMWV is controlled by a three-position
switch which allows the operator to select the fan speed.
speed selections are OFF-LOW-HI.
The switch, in conjunction with a
multiple element resistor, provides the circuitry to make these motor
(13) Trailer Receptacle. The HMMWV trailer receptacle, mounted on
the rear of the vehicle body, is protected by a spring-loaded door
and is compatible with all standard military trailer wiring circuits.