For 1990-2009 cars

## HVAC Compressor Controls Circuit Description Buick

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 the 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.

## HVAC Compressor Controls Circuit Description Oldsmobile

The powertrain control module (PCM) controls the compressor clutch in response to requests from the heater and A/C programmer. When the programmer requests compressor clutch engagement, and all other enable criteria are met, the PCM will energize the control relay after a delay period. The PCM engages the clutch during crank and at engine speed below the threshold. The following elements are inputs to the PCM:

• | The heater and A/C programmer request for compressor

• | The high side refrigerant pressure switch

• | The low side refrigerant pressure switch

• | The high side temperature switch

• | The low side temperature switch

• | The ambient air temperature sensor

• | The vehicle speed

• | The heating, ventilation, and air conditioning (HVAC) system mode

The voltage source of the compressor clutch is from the 10 amp HVAC relay fuse through the clutch relay. The PCM controls the clutch relay. The programmer sends a signal to the PCM that compressor operation is possible if the following conditions exist:

• | The heater setting is not OFF.

• | The A/C control setting is not OFF.

• | The outside temperature is above 5 °C (41 °F).

• | No compressor disabling codes are set.