HVAC Compressor Controls Circuit

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 no requested
^ To disengage when the engine load must be reduced

Battery voltage is supplied to the Powertrain Control Module (PCM) when the heater and A/C control mode selector knob is in the following modes:
^ MAX A/C
^ NORM A/C
^ BI-LEVEL
^ DEFOG
^ 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.
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 current which results from voltage spikes. Voltage spikes generate from the collapsing magnetic field of the A/C compressor clutch coil. Voltage spikes occur every time the coil de-energizes.