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 during normal operation, disengaged when air conditioning is not requested or disengaged when engine load must be reduced.

The heater and A/C 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. This voltage signal tells the PCM that 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 the HVAC Relay fuse and the A/C compressor control 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 voltage 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 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.

During the compressor ON cycle, if the PCM detects a system error a diagnostic trouble codes (DTC) will force the PCM to disengage the clutch.
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.

Controlled Compressor Clutch at Idle

To minimize customer perception of engine idle speed or condition changes at idle, the compressor clutch will not be allowed to change state (cycle) if certain conditions are met during slow vehicle speed or idle. If engine speed is below 1000 rpm, and the compressor clutch is currently commanded, the compressor will be made to run continuously for a minimum of 45 seconds and a maximum time allowable by freeze protection rules. A vehicle speed above 29 km/h (18 mph) is required to cancel continuous clutch operation once the routine is enabled.

Compressor Minimum ON Interval

Once the clutch has been engaged, it will stay operational for a minimum of 5.0-6.5 seconds depending on outside temperature. This routine will be commanded provided refrigerant temperature data from the thermistors does not fall within the protection guidelines for very low refrigerant levels. After this minimum ON time, the programmer will remove the request for compressor provided the low side temperature sensor indicates a refrigerant temperature below 0°C (32°F). Once the low side temperature reaches 6°C (43°F) the clutch will cycle ON. This cycle will be repeated as long as no compressor disabling DTCs are set.

During the compressor ON cycle, if the PCM detects a system error a diagnostic trouble codes (DTC) will force the PCM to disengage the clutch.