Climate Control
How the Climate Control Circuit WorksClimate Control Unit
The climate control unit receives battery voltage at all times through fuse 33. Battery voltage is also supplied through fuse 4 to the control unit when the ignition switch is ON (II). The 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 you turn the ignition ON (II), battery voltage is applied through the coil of the relay to the engine control module (ECM). When you turn the A/C on, the climate control unit sends a signal 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, supplying battery voltage through the closed relay contacts to the compressor clutch, and the compressor runs. When the relay coil is energized, a signal is also sent to the transmission control module (TCM) which 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 °F (130 °C) or higher. 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 gets too high or too low. High pressure could be caused by an obstruction or icing blocking the evaporator. Low pressure could be caused by a refrigerant leak.
Engagement of the compressor clutch is delayed for several seconds after you release the key from the START (III) position. The compressor clutch is also disengaged at wide open throttle.
Air Delivery
The air delivery system directs the flow of air used by the climate control system.
Each of the plenum door motors is driven and controlled by the climate control unit.
The climate control unit applies a reference voltage to each of the four door position switches in the mode control motor. The control unit determines the position of the door by grounding various combinations of position switches. For example, if the BLU, PNK/BLK and GRY wires are grounded, the door and motor move to the HD2 position.
The air mix control motor limit switches and diodes interrupt voltage to the motor when the air-mix door reaches the end of its travel. During start-up, the motor is turned on by the control unit to learn the position sensor readings for its range of travel. The position sensor varies resistance to provide door position feedback through the GRN wire.
The limit switches and diodes for the vent door control motor interrupt voltage to the motor when the vent door reaches the end of its travel. During start-up, the motor is turned on by the control unit to learn the position sensor readings for its range of travel. The position sensor varies resistance to provide door position feedback through the YEL/WHT wire.
The limit switches for the recirculation control motor interrupt voltage to the motor when the door reaches the end of its travel. A position sensor is not required for this door since it only has two positions: full fresh or full recirculation.