HEATING AND AIR CONDITIONING SYSTEM (ALL EXCEPT M915A2 AND M916A1)
The heating and air conditioning unit is mounted under the glove compartment. It is a single unit consisting of a
heater core, air conditioning evaporator coil, blower motor, control valves, and air ducts. The system is turned on by the
mode control lever and the four-speed blower switch, which also controls flow rate. An even cab temperature is
maintained by controlling the coolant flow through the heater core, or refrigerant flow through the evaporator coil.
Major components of the system are:
The expansion valve is the dividing point between the high- and low-
pressure parts of the refrigerant system. High-pressure liquid refrigerant
from the receiver-drier passes through the expansion valve and moves
into the low-pressure area of the evaporator coil.
The evaporator coil, in an area of low pressure in the system, lowers the
boiling point of refrigerant, which causes it to absorb heat from the tubing
walls and fins of the coil. As it absorbs heat, liquid refrigerant quickly
boils and turns into a gas.
The compressor squeezes low-pressure gas from the evaporator coil into
a much smaller space. When gas is compressed, the heat it contains
becomes concentrated. In this way, the gas is made hotter than the
outside air, without adding heat. The compressor also moves refrigerant
through the system.
The condenser turns hot refrigerant gas, coming from the compressor,
into liquid. Because of its location, the condenser transfers heat to air
that is drawn in by the engine fan and by air that is forced into the engine
compartment as the vehicle moves forward.
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