Compressor Clutch Control

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 be cycled on and off. The clutch is disengaged when air conditioning is not requested or when engine load must be reduced.

The heater and A/C programmer sends a Class 2 serial data message to the PCM with the system operating in any mode but OFF or VENT.

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 a fuse and the A/C compressor clutch relay contacts. The A/C compressor clutch engages and the compressor runs.

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 message signal from the heater and A/C programmer is still present.

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

Stall Prevention
The PCM may disengage the compressor clutch due to high reference power steering system pressure caused by quick steering input voltage at low engine speed. The clutch will remain OFF for as long as the high pressure condition exists.

Slugging Prevention
The PCM may engage the compressor clutch during crank to help rid the compressor of liquid that may have migrated during the system OFF period. This process is important so that the harshness of the initial engagement is lessened to the greatest extent possible. The anti-slugging routine will not be applied on each crank as several criteria must be met for it to be enabled.

Wide Open Throttle Disengagement
Maximum Wide Open Throttle (WOT) performance with A/C ON requires that the clutch be disengaged, as soon as possible after the WOT criteria is detected. The clutch will be engaged during extended WOT after a time period has elapsed provided that the upper compressor speed threshold has not been exceeded. This disengagement will last for six seconds for each WOT application, however, the compressor will not be disengaged for additional WOT applications within a 15 second period.

High Engine Speed
If the engine speed exceeds 5985 RPM, the compressor clutch will be disengaged if currently commanded, to protect the compressor from overspeed damage. After engine speed is reduced from this level, a time period of 10 seconds will pass before engagement is commanded to protect against high speed, high pressure clutch application. If the engine speed is above 5325 RPM, the compressor clutch will not be allowed to engage if it is currently commanded OFF.