Compressor HVAC: Description and Operation

The automatic climate control system controls the radiator fan, the condenser fans and the A/C compressor clutch. Each system is discussed separately.


Automatic Climate Control Unit


The automatic climate control unit receives battery voltage at all times from fuse 33. Fuse 4 supplies battery voltage to the control unit with the ignition switch in RUN. The automatic climate control unit is grounded at G401.


A/C Compressor


Fuse 21 supplies battery voltage at all times to the normally open contacts of the A/C compressor clutch relay. When the ignition is in RUN, battery voltage is applied through the coil of the relay to the engine control module (ECM). When the A/C mode is selected, the climate control unit sends an output to the fan control unit. The fan control unit turns on the condenser fans and sends an A/C request signal to the ECM. The ECM adjusts the idle speed (if the engine is idling) and grounds the relay coil. Battery voltage is supplied from the closed relay contacts to the A/C compressor clutch and the compressor runs. When the relay coil is energized, a signal is also sent to the transmission control module (TCM). The transmission control module (TCM) adjusts the shift conditions for the additional engine load. The fan control unit will turn off the condenser fans and the "A/C request" signal to the ECM if the radiator fan control sensor detects a coolant temperature of 266 deg.F. (130 deg.C.) or above. The middle pressure switch sends additional A/C load information to the ECM. The high/low pressure switch will open if the A/C system pressure becomes too high or too low. The pressure will become too high if there is an obstruction or icing at the evaporator and the pressure will be too low if there is a refrigerant loss.


Engagement of the compressor clutch is delayed several seconds after the ignition is released from the START position. The compressor clutch is disengaged when the throttle is wide open.


Condenser Fans


Battery voltage is supplied at all times from fuse 36 through the left condenser fan motor and fuse 37 through the right condenser fan motor to the normally open contacts of the condenser fan relay. With the ignition in run, battery voltage is supplied from fuse 4 through the condenser fan relay coil to the fan control unit. When the control unit receives the condenser fan and A/C request from the automatic climate control unit, it grounds the ORN/BLU wire and energizes the condenser fan relay. The relay contacts close, providing a ground for the condenser fan motors. The motors run to remove heat from refrigerant passing through the A/C condensers.


Radiator and Engine Compartment Fans


Battery voltage is supplied at all times from fuse 31 through the radiator fan motor to the normally open contacts of the radiator fan high and low relays. Battery voltage is supplied at all times from fuse 24 to the normally open contacts of the engine compartment fan relay. With the ignition switch in RUN, battery voltage is supplied from fuse 4 through the radiator fan relay coils and through the engine compartment fan relay coil to the fan control unit. When the radiator sensor detects a coolant temperature above 183 degrees F. (84 degrees C.) it grounds the GRN/BLK wire to energize the radiator fan low relay. With the relay contacts closed, the radiator fan motor is grounded through the radiator fan resistor. The fan runs at low speed to remove heat from the coolant. When the radiator fan control sensor detects a coolant temperature above 194 degrees F. (90 degrees C.) it grounds the BLU/RED wire to energize the radiator fan high relay and the engine compartment fan relay. With the relay contacts closed, the radiator fan motor is grounded. The radiator fan motor runs at high speed to remove heat from the coolant. The closed contacts of the engine compartment fan relay applies battery voltage to the engine compartment fan. The engine compartment fan runs to remove heat from the engine compartment.