Controls

Heater-A/C Control

With the ignition switch in the ON position, voltage is supplied through the HVAC fuse and circuit 41 (B RN) to the heater-A/C control assembly. The voltage from circuit 1199 (DK BLU) is varied from B+ to approximately 1.5V by the temperature control potentiometer. The heater-A/C control receives ground from G200 through circuit 1450 (BLK).

Air Delivery

Control of air through the system is regulated by vacuum. At various positions on the heater-A/C control assembly, mode control valves mix and direct cooled, heated and outside air through the air ducts.

Blower Controls

The blower motor is a variable speed motor. The higher the voltage applied to the motor, the faster the speed. Voltage is supplied to the blower motor by the heater-A/C control through the blower motor resistor. At low and medium speeds (M1, M2 and M3), the voltage is stepped down by the internal blower motor resistors. Each internal resistor also has a thermal limiter in series with it. At high speed, the internal blower motor relay is energized, removing the internal blower motor resistors from the circuit. Voltage is then applied directly to the blower motor through the relay thus the blower motor runs at maximum speed.

Compressor Controls

The compressor for the air conditioning system is belt-driven by the engine through the A/C compressor clutch. The clutch allows the compressor to perform the following functions:

- To engage for A/C and defroster operation
- To disengage when the air conditioning is not requested
- To disengage when the engine load must be reduced

Battery voltage is supplied to the powertrain control module (PCM) when the A/C control is activated or when the heater- A/C control mode selector is placed in any of the following modes:

- MAX A/C
- NORM A/C
- BI-LEVEL
- DEF0G
- DEFROST

The voltage signal tells the PCM that A/C compressor operation is requested.

Under normal operating conditions, the following actions occur:

1. The PCM supplies a ground to the relay coil of the A/C compressor control.
2. The relay energizes.
3. Voltage is supplied to the A/C compressor clutch through the relay center fuse.
4. The A/C compressor clutch engages.
5. The compressor runs.

If the PCM determines that the engine load should be reduced, such as during full throttle conditions the PCM de-energizes the relay of the A/C compressor control in order to disengage the compressor clutch. The de-energization occurs even though the voltage signal from the heater and A/C control is still present at the PCM.

The diode of the A/C compressor clutch connects across the terminals of the A/C compressor clutch. The diode provides a path for the high current that results from voltage spikes generated from the collapsing magnetic field of the A/C compressor clutch coil. Voltage spikes occur every time the coil de-energizes.