Rick's Free Auto Repair Advice

What is superheat?

How is Superheat generated in car AC systems?

Superheat is usually an indication of low refrigerant charge

Superheat in a car AC system isn’t a good thing. Car AC systems work by metering high pressure liquid refrigerant into an evaporator.  The current refrigerant R-134a has a boiling point of -15.4°F. It enters the evaporator as a liquid. As air passes across the evaporator fins, the air heats the refrigerant to its boiling point, where it changes from liquid to gas.

If the system is properly charged, the refrigerant will only absorb enough heat to cause it to change from a liquid to gas. By the time the refrigerant changes to a gas, it should be exiting the evaporator and carrying a small amount of liquid refrigerant with it. Carrying some liquid refrigerant along with the gas is critical to the safe operation of your car’s AC system because the liquid also carries the lubrication oil that’s needed to prevent compressor failure.

The refrigerant gas, liquid and oil enter the accumulator where the liquid continues to pick up heat and change to a gas. A small amount of oil gets carried into the compressor to keep it lubricated. NOTE: This is why you should NEVER jumper across a low pressure switch to engage the compressor clutch when trying to add refrigerant to the system. The compressor clutch will engage, but the refrigerant gas leaving the evaporator wont’ carry any oil. The compressor can self-destruct in this mode.

Superheat occurs when the system is low on charge

When the system is low on refrigerant, it changes to a gas and stays in the evaporator where it picks up even more heat from the cabin air. Remember, the system is designed so the refrigerant removes only enough heat from the cabin air to change from a liquid to a gas. If the gas continues to absorb heat beyond that point, because the system is low on charge, the temperature of the evaporator will drop so low that humidity in the airflow will freeze on contact with the evaporator fins. The ice will continue to build up to the point where airflow is blocked almost completely. At that point, the refrigerant pressure will drop to the point where the low pressure cutoff switch turns off the compressor clutch. Or, an evaporator temperature sensor will shut down the system.

Superheat symptoms, low AC charge symptoms

AC blows cold. Then blows warm

The system starts out blowing cold air. But as ice builds up, it blocks the flow of air across the evaporator. Low side pressure drops and the low pressure switch shuts off the AC compressor clutch. At that point, no more cooling is taking place. Cabin air blowing across the evaporator melts the ice. You notice warmer very humid air as the ice melts. Once the ice is fully melted, you’ll feel only warm air until the low pressure side builds pressure and the low pressure switch re-engages the compressor clutch.

AC compressor clutch cycles on and off

The AC compressor clutch turns on and builds pressure until superheat occurs. Then the low pressure switch shuts off the compressor clutch. System pressures rise. The low pressure switch engages the clutch and the cycle repeats.

You notice a large puddle of water under your car after stopping

All AC systems drain condensation onto the pavement. But when evaporator icing is occurring, you’ll see very large puddles of water under your vehicle after you park.

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Posted on by Rick Muscoplat

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