Auto A/C Terms - Glossary
Low Head Pressure

The A/C system is divided into two distinct sides. The low and high side. There are typical pressure gauge readings considered to be normal for each side. When the high side pressure drops below it’s expected normal range, the system is considered to have low head pressures or a low pressure reading for the high side of the A/C system.

Typically, low head pressures are a result of either a low refrigerant charge or a restriction. In the case of low refrigerant, the system has lost refrigerant through leaks and the system should be leak tested in order to determine where the leaks are. All leaks should be repaired prior to recharging the system.

Low head pressures can also be a result of a restriction. The most common myth is that a restriction in the A/C system causes high pressures. Nothing could be further from the truth. When the refrigerant flow in an A/C system is restricted, the compressor is left without any refrigerant to pump. There is no refrigerant available to increase the pressures. Because the refrigerant is not circulating, any refrigerant in the high side of the system remains in the A/C condenser longer than it should. As it continues to cool, the pressures continue to fall. That’s because refrigerant has a pressure/temperature relationship. Whatever change you effect on one will automatically cause the same change to the other. Therefore, reducing the refrigerant’s temperature in the condenser will reduce it’s pressures, causing low head pressures on the A/C system.