Auto A/C Terms - Glossary
Compressor Shaft Seal

The auto A/C compressor includes several different seals. Typically, body seals are o-ring seals that prevent refrigerant from leaking externally. Internally, finely machined surfaces and gaskets seal the high pressure discharge side of the compressor from the low pressure suction side of the compressor. In all cases, those seals are static. They do not move. The only moving seal on the A/C compressor is the shaft seal. That is because the shaft seal must prevent refrigerant leaks while sealing on the main shaft of the compressor and the front head or body of the compressor.

Shaft seals can be either one or two piece design. The one piece lip seal on General Motors compressors eliminates one of the sealing surfaces that often leaks. Understand that on a two piece seal, one section seals around the shaft of the compressor. The second part seals to the body or front head of the compressor. The two pieces must also seal to each other in order to prevent leaks.

The biggest problem with shaft seals is that they tend to leak refrigerant all the time. Understand that because of the motion of the shaft turning inside the seal, the shaft seal relies on lubrication to maintain a good seal. When the A/C system is not run for long periods of time, like during winter months, the compressor shaft seals run dry. When first started, the dry seal can not hold the refrigerant pressure building inside the compressor behind the seal. Hence, the constant leaks.

It is also important to understand that the shaft seal is a source of refrigerant oil leaks. Understand that shaft seals can leak oil without leaking refrigerant. The tell tale sign is the large oil stain on components around the compressor clutch or front of the compressor. Although shaft seals can be replaced, in some cases it requires that the compressor be dismantled in order to do that.

If you suspect shaft seal leaks on your A/C compressor, they should be leak tested by a qualified A/C service technician using electronic leak detection methods.