Automatic Climate Controls

DESCRIPTION

The compressor for the air conditioning system belt-driven by the engine through the A/C compressor clutch. The clutch allows the compressor to be cycled on and off during normal operation, disengaged when air conditioning is not requested or disengaged when engine load must be reduced.

OPERATION

The Heating Ventilation and Air Conditioning (HVAC) programmer applies battery voltage to the Powertrain Control Module (PCM) with the system operating in DEFOG, FRONT DEFROST or if the A/C button is selected (Pontiac and Oldsmobile) or VENT mode disabled (Buick). The A/C high-pressure cut-out switch and pressure cycling switch must also be closed. This voltage signal tells the PCM flat A/C compressor operation is requested.

Under normal operating conditions, the PCM then grounds the A/C compressor control relay coil and the relay energizes. Voltage is applied to the A/C compressor clutch through relay center fuse 5 and the A/C compressor control relay contacts. The A/C compressor clutch engages and the compressor runs.

The compressor runs until A/C system low-side pressure drops to the point where the pressure cycling switch contacts open 152 kPa (22 psi). When the contacts open, the voltage signal to the PCM is interrupted and the PCM de-energizes the A/C compressor control relay. The A/C compressor clutch is disengaged and the compressor stops running. When A/C system low-side pressure rises to about 296 kPa (43 psi), the pressure cycling switch contacts close again and the compressor begins running.

The A/C high-pressure cut-out switch contacts are normally closed. If A/C system high-side pressure rises too high, the switch contacts open interrupting the voltage signal to the PCM. The PCM then de-energizes the A/C compressor control relay and the compressor stops running.

If the PCM determines that engine load should be reduced, such as during full throttle conditions, it will de-energize the A/C compressor control relay to disengage the compressor, even though the voltage signal from the HVAC programmer is still present.

The A/C compressor clutch diode, connected across the A/C compressor clutch terminals, provides a path for the current resulting from voltage spikes. The voltage spikes are generated by the collapsing magnetic field of the A/C compressor clutch coil, and occurs every time the coil is de-energized.