Understanding System Function
When you turn on the air conditioner at the control panel (1), the thermostat
(2), is supposed to sense a warm temperature at the evaporator. A circuit in the
thermostat should close, allowing current to flow through the thermostat to the
compressor clutch field coil (3). When this happens, the clutch field coil be-
comes an electromagnet and pulls the clutch drive plate (4) tight against the
clutch pulley (5).
The same AC switch (1) may also turn on the fan or
blower motor (2a) to circulate air in the cab. The air
feels warm at first but will cool quickly.
A belt connects the clutch pulley to a drive pulley (6) on the engine. The engine
provides the power to turn the clutch pulley and drive the compressor (7) when
the clutch is engaged. When operating, the compressor compresses and pushes
refrigerant gas to the condenser (8), through the receiver-drier (9), and to the
expansion valve (10) orifice. When it does, it puts a lot of pressure on the gas.
The compressor raises the temperature and pressure of the refrigerant inside
the high side of the system.
At the same time, the compressor is also sucking in low pressure refrigerant
gas from the expansion valve orifice, evaporator and through the low side of the
system. The movement of the refrigerant inside the system transfers heat
energy from the cab to the outside air for occupant comfort.
The automatic functions of the thermostat (or the pressure valve on some
accumulators), and the expansion valve, help maintain pressures and tempera-
tures inside the system at safe and efficient operating levels. Pressure and
temperature are constantly changing due to compressor and expansion valve
action, the amount of heat energy being moved and the environment or
The engine cooling system fan and clutch (11), and the evaporator blower
motor (2a), move a sufficient amount of air through the condenser and evapora-
tor. On the road, vehicle speed provides most of the (ram) air required for the
condenser to work right. In a parked or slow moving vehicle the engine fan (or
roof or remote mounted condenser and fans) moves sufficient air through the
Clean refrigerant and refrigeration oil should be inside
the system in the amount specified by the manufac-
turer. Moisture, sludge (moisture combined with refrig-
erant oil or desiccant), or desiccant particles will pre-
vent the correct performance of the system and may
cause component damage.
A Troubleshooting Example
Remember the story at the beginning of this chapter? The vehicle operator
pulled in off the road and asked you to repair the rig. He was in such a hurry he
didn't tell you anything except that the air conditioner wasn't cooling. Here is
the best way to handle that kind of situation.