A/C Compressor in a car: how it works, problems, testing

Updated: May 08, 2022
The A/C compressor is the key part of the air conditioning system in a car. It circulates the refrigerant through the A/C system.
A/C compressor A/C compressor

In most cars the A/C compressor is driven by a drive belt, as in this photo. In some vehicles (e.g. hybrids) it's powered by an electric motor.

The failed A/C compressor is one of the common reasons for the A/C system not to work. Can the A/C compressor be repaired or does it have to be replaced as a unit?

Are there cheaper options than replacing it with a new part? How is the A/C compressor tested? Read more below.

How the A/C compressor is tested, common problems with the A/C compressor

In a working A/C system, the A/C compressor cycles on and off. When you turn the A/C on, the compressor comes on for a while to build the pressure then it turns off. Once the pressure becomes low it comes on again and so on.
When the A/C system in a car doesn't work, mechanics first check if there is enough refrigerant pressure in the A/C system. If there is low pressure or no pressure at all, means there is a leak in the A/C system that must be repaired first.

The system is designed so that the A/C compressor will not be powered if the refrigerant pressure is low. If there is enough pressure, but the A/C compressor won't cycle on and off, the 12 Volt power at the compressor must be checked.
A/C Compressor Clutch Coil A/C Compressor Clutch Coil.

If there is 12 Volt power at the compressor electromagnetic clutch, but the compressor won't turn on, the clutch has failed; it is a very common problem.

The part that fails in most cases is the electric coil that provides the magnetic field for the compressor clutch to engage (in the photo).

Replacing only the electromagnetic clutch coil is cheaper in parts but requires more work. If the clutch coil cannot be replaced separately, the compressor is replaced as a unit.

Often, the electromagnetic clutch may show signs of overheating, as in this photo.
Overheated A/C compressor clutch Overheated A/C compressor clutch
Sometimes the clutch bearing can become noisy. In some cars, the clutch bearing can be replaced separately. If not, the compressor is replaced as a unit.

The A/C compressor must also be replaced if the refrigerant is leaking from the compressor itself and the leak cannot be repaired.

If the compressor is equipped with the AC Control Solenoid, it will also need to be tested and replaced if defective.

In some cars, the compressor fails internally. A failed compressor may produce excessive noise when running or show signs of a mechanical failure. If some internal parts of the compressor have broken apart, the small shreds can get into the A/C system and it will require flushing off and replacing of some of the parts of the A/C system.

Sometimes the shaft that holds the compressor clutch may break off. In this case, the whole unit will have to be replaced.

A/C compressor replacement

New A/C compressor New A/C compressor
There are several types of parts available: you can order a new OEM compressor, new aftermarket compressor, a remanufactured compressor or a used unit.

The choice depends on the price, warranty and availability. If the supplier stands behind the product and provides a reasonable warranty, any type is worth considering.

Before the A/C compressor is replaced, the refrigerant must be evacuated from the A/C system.
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When a new A/C compressor is installed, it might need to be filled with special PAG oil. The exact compressor oil capacity is specified in the service manual.

The labor to replace the A/C compressor varies from 1.5 to 4 hours depending on the difficulty. After the A/C compressor is replaced, mechanics re-charge the A/C system with the specified amount of refrigerant and check its operation and the A/C system for leaks.

How the A/C system works in a car:

Car Air Conditioning System Car Air Conditioning System
The Air Conditioning or A/C system in a car is filled with a refrigerant (typically R134a) that can change its state between liquid and gas.

The system is sealed and is under pressure. Without going too deep into details, the A/C compressor circulates the refrigerant between two heat exchangers: the evaporator built inside the car under the dash and the condenser that is placed in front of the radiator at the front portion of the vehicle.

When the refrigerant changes its state from a liquid to a gas inside the evaporator, it absorbs the heat from the cabin. When the refrigerant condenses in the condenser it releases the heat to the outside air, see the diagram.