The Compressor — Types and How They Work

The compressor is the only moving mechanical component in the AC system. Everything else is plumbing and heat exchangers. The compressor's job is to create the pressure differential — high pressure on the condenser side, low pressure on the evaporator side — that drives refrigerant through the entire system.

The most common type through the 1990s and still found on many vehicles today. A series of pistons move in and out of cylinders as the compressor shaft turns. Each piston draws in low-pressure refrigerant gas on its intake stroke and compresses it on its compression stroke. Fixed displacement piston compressors move the same volume of refrigerant every revolution regardless of conditions.

Used on many modern vehicles, especially with variable displacement. Two spiral-shaped scrolls — one fixed and one that moves in a circular orbit — trap pockets of refrigerant between them. As the moving scroll orbits, it progressively compresses the trapped refrigerant from the outside of the spirals toward the center where it exits as high-pressure gas. Scroll compressors run smoother and quieter than piston types.

A fixed displacement compressor always pumps the same volume per revolution. To control system output, it cycles on and off using an electromagnetic clutch — a ring that engages when voltage is applied and releases when it is removed. You can hear the clutch click when the AC turns on. When the system is satisfied, the clutch releases and the compressor stops turning. Only the outer pulley keeps spinning on the belt.

A variable displacement compressor runs continuously — it never cycles off. A control valve inside the compressor varies the stroke length of the internal pistons, adjusting how much refrigerant the compressor moves on each revolution. When full cooling is needed the compressor pumps maximum volume. When less cooling is needed it reduces its displacement. No clutch cycling. Smoother operation. Better fuel economy. These compressors fail differently than fixed displacement — they can run continuously but produce inadequate cooling when the control valve fails.

A fixed displacement compressor that cycles on and off very rapidly — every few seconds — is not normal. The low-pressure switch is cutting the clutch because system pressure is too low. The high-pressure switch is cutting the clutch because pressure is too high. Normal cycling on a properly charged system is slower — the compressor runs for a minute or more before cycling off. Rapid cycling always means something is wrong with refrigerant charge or a pressure-related system fault.