Compressor Clutch: Description and Operation

COMPRESSOR CLUTCH

The compressor pulley is driven by a belt from the crankshaft; a compressor clutch is used to engage/disengage the pulley and driveshaft. The clutch is electromagnetic. When power is provided to the clutch, the clutch engages and rotates the compressor drive shaft. When the power is cut off, the clutch disengages and the compressor pulley free-wheels. On BMW A/C systems, the compressor is cycled on and off, according to evaporator temperature; it is also cycled off at full-throttle, standing start acceleration conditions.







A diode is used to prevent induced current and voltage from damaging the control module/relays when the clutch solenoid is disengaged. The diode allows the voltage spike created when the clutch solenoid is disengaged, to flow in a loop back through the solenoid coil, until the energy is dissipated. Different vehicles use different control modules to control the compressor clutch.







The compressor clutch is unit replaceable.


COMPRESSOR SERVICE

- When troubleshooting a noisy compressor complaint, make sure the noise is present only when the clutch is engaged.
- If it is present when the clutch is not engaged, remove the compressor drive belt and check again.
- If the noise continues, it is not related to the compressor.
- It removing the drive belt reduces or eliminates the noise, check the torque of the compressor and bracket mounting bolts.
- Check the belt tension and condition, and tensioner pullies which can produce rattling noises that would sound like a defective compressor.
- A loose/slipping belt can cause noise.
- A belt that is too tight can damage the clutch bearings.
- If the compressor is noisy with the compressor clutch engaged, make sure the system is charged with the correct amount of refrigerant.
- An over-charged system can cause compressor noise.
- If the A/C system is overcharged with refrigerant, the liquid entering the compressor can damage it.
- When troubleshooting a noisy compressor complaint, recover the refrigerant and recharge the system with the correct amount.
- A failed compressor must be returned with the inlet and outlet ports sealed using the plastic caps from the replacement compressor. Otherwise the "failed" compressor will be damaged by moisture, and it will be impossible for Warranty to analyze it.