The compressor is the heart of the air conditioning system. The compressor pressurizes the refrigerant so that the refrigerant can dump heat as it passes through the condenser. Cooling the hot pressurized gas causes it to turn back into a liquid. This allows the refrigerant to reabsorb heat and produce a chilling effect as it flows through the evaporator. Historically, A/C cooling was controlled by cycling the A/C compressor on and off. A temperature sensor or switch monitored the temperature of the air in the evaporator. If less cooling was needed, the relay for the A/C clutch was deenergized to disengage the drive pulley for the compressor.
If more cooling was needed, the relay for the clutch was reenergized to reengage the clutch. By cycling the A/C compressor clutch on and off, the average cooling output of the A/C system could be held fairly steady. The disadvantage with this approach is clutch wear and a noticeable “thump” when the clutch engages and changes the load on the engine. A variable displacement A/C compressor works differently. Instead of cycling on and off, a variable displacement compressor uses a wobble plate or swash plate to change the displacement of the pistons inside the compressor to vary cooling. Changing the position of the plate shortens or lengthens the stroke of the pistons. Increasing stroke (displacement) increases refrigerant flow and cooling, while decreasing stroke decreases refrigerant flow to reduce cooling. The flow can vary from as little as one percent up to 100 percent.
With a variable displacement compressor, no clutch is needed. The A/C compressor runs all the time. Getting rid of the clutch eliminates a common source of trouble as well as the thump that occurs when the clutch engages. The result is smoother engine operation, less noise and better control over cooling output. The A/C control module or PCM changes compressor displacement using inputs from an evaporator temperature sensor, refrigerant pressure sensor and one or more air temperature and sun load sensors.
Problems in any of these sensors or the control module or PCM can affect the normal operation of the compressor. Compressor failures can occur as a result of high-mileage wear, or more commonly, loss of refrigerant or lubrication. Most late-model compressors have little or no sump to store oil because the oil circulates with the refrigerant. If the A/C system develops a leak, oil escapes with the refrigerant, starving the compressor for much-needed lubrication. Using the wrong type of lubricant also can damage a compressor. Different compressors require different viscosities of PAG oil (50, 100 or 150).
Follow the OEM recommendations for the type of PAG oil that should be used. Some replacement compressors come with the correct oil already inside while others contain a shipping oil that must be drained before the compressor is installed. Replacing an A/C compressor requires disconnecting the pipes or hoses to the compressor. Before this is done, the system must be connected to a recovery machine to remove any refrigerant that is still inside. It is illegal to vent refrigerant into the atmosphere. If the A/C system is dirty, it must be flushed with an approved flushing chemical to remove the contaminants.
On many applications, the condenser also must be replaced because it cannot be completely flushed. A new accumulator or receiver/drier and orifice tube is also recommended. Once the new compressor has been installed and the lines reconnected, a vacuum pump must be used to purge air and moisture from the system. If this is not done, air will prevent the system from accepting a full charge of refrigerant resulting in noisy operation and poor cooling performance. Moisture that has not been removed can react with the refrigerant and oil to form acids and sludge that can damage the new compressor.