A/C - Diagnosing Compressor Noise
Group 64Heating and Air Conditioning
Bulletin Number
64 10 90 (3180)
Woodcliff Lake, NJ
August 1991
Service Engineering
This Service Information Bulletin supersedes S.I. 64 10 90 (3180) dated January 1991. It incorporates additional troubleshooting guidelines.
Please remove and discard S.I. 64 10 90 (3180) dated January 1991 from your S.I. binder.
SUBJECT:
Troubleshooting Air Conditioning Compressor Noise
MODELS:
All
Situation:
A high percentage of air conditioning compressors which are returned under warranty for complaints of noisy operation have been found to be non-defective.
General Information:
Several situations can produce noisy operation of the compressor without the compressor being faulty. The work involved replacing a compressor often corrects the situation that caused the noisy operation, misleading the technician into believing that the compressor was faulty.
Before replacing an air conditioning compressor, please consider the following guidelines.
Troubleshooting Guidelines
1. Make sure that the noise is present with the clutch of the compressor engaged. If it is present without the compressor clutch engaged, remove the drive belt and recheck - if it continues, it is not related to the compressor.
2. If removing the drive belt reduces or eliminates the noise, before proceeding further, check the following:
A. Tightening torque of compressor and bracket mounting bolts.
B. Belt tension and condition - A loose or slipping belt can cause noise, and an overly tight belt can damage the clutch bearings.
3. If the compressor is noisy with the compressor clutch engaged, make sure that the system is charged with the correct amount of refrigerant.
If an air conditioning system is overcharged by even a small amount the compressor noise level can increase. This is due to varying amounts of liquid refrigerant entering the compressor suction port, resulting in a symptom known as "liquid shock".
If the compressor is replaced for a complaint of noise and the cause of the noise is an overcharged system, the actual cause will be corrected by the proper recharging of the system, and the compressor will have been unnecessarily replaced. When tested, the compressor will then be returned to the dealer.
Because most currently produced refrigerant recycling equipment does not measure the amount of refrigerant recovered from a system, the only reliable means of ensuring that the amount of refrigerant in the system is correct is to evacuate the system and recharge it with the specified quantity of refrigerant. (Refer to the refrigerant quantity label in the engine compartment or to S.I. 64 01 90 (3042).
This procedure must be carried out in all cases of complaint of noisy compressor prior to replacement of any components.
Important Note:
A certain amount of "liquid shock" is unavoidable in one circumstance: if a vehicle has been stored in a cool environment (e.g., in a air conditioned garage) and is then operated in a high ambient temperature and the air conditioning switched on during high engine rpm, the compressor will be noisy for a brief period. This is a normal situation and should be explained to the customer. Compressor replacement will not correct this complaint.
4. The function of the expansion valve and evaporator temperature sensor or switch should also be checked (refer to repair manual, Group 64). If either malfunctions and allows liquid refrigerant to reach the compressor via the suction port, compressor noise can result.
Important Note:
If compressor noise is a result of liquid refrigerant reaching the suction port of the compressor, the compressor may not have been damaged, depending on the type of compressor. Correct the cause of liquid refrigerant at the suction port (e.g., expansion valve stuck open) and recheck the compressor for noise. A vane-type compressor will exhibit noisy operation when liquid reaches the suction post but will not be damaged by this condition, and therefore should not be replaced until it has been rechecked as described above.
5. A complaint may be received of "Compressor continues to run after engine shut off" or "Compressor continues to turn with A/C shut off".
This is caused by pressure equalization inside the compressor as refrigerant and oil bleed back from the high pressure (output) side to the low pressure (suction) side.
This situation will only be encountered in vane-type compressors (others incorporate a one-way valve in the intake port) and should not be considered a defect. This situation may cause a brief noise, as the clutch plate rotates backwards against the pulley, and is considered a normal condition.
Warranty Information:
Compressors that are found to be non-defective will be returned to the dealership and the corresponding warranty claim will be debited.
Labor for evacuating and recharging the air conditioning system (as part of diagnosis of noise related complaints) is covered under the terms of the applicable BMW New Car Limited Warranty.
Refer to the Flat Rate Manual for the appropriate model.
Any questions regarding warranty coverage should be discussed with your District Service Manager.