The high side sensor is located in the high pressure refrigerant liquid line between the condenser and the orifice tube. The heater and A/C programmer monitors the refrigerant temperature and transfers that data to the powertrain control module (PCM). The PCM combines this data with the coolant temperature in order to determine the need for the cooling fans and prevents the A/C compressor from operating at high discharge pressures.

The low side sensor is located in the low pressure refrigerant line between the orifice tube and the evaporator. The heater and A/C programmer monitors this sensor in order to determine the low side pressure based on the pressure/temperature relationships of the R-134a.

The low pressure switch is located in the low pressure refrigerant line. When pressurized, the switch is closed and allows the compressor clutch relay to energize. This feeds the battery voltage to the compressor clutch coil. If the pressure drops below 69 kPa (10 psi), the switch opens and cuts off the voltage to the clutch. A circuit to the heater and A/C programmer monitors the state of the switch for diagnostic purposes.

The high refrigerant pressure switch is located in the compressor discharge line. The switch performs the following functions:

Pressure readings above 2465 kPa (430 psi) open the switch, cutting off the power supply to the relay, and disengage the compressor clutch. Pressures will begin to drop. When the pressure drops to 1379 kPa (200 psi), the switch closes and allows the compressor to engage. Compressor cycling, which is caused by the pressure switch, is considered normal. This can occur under the following conditions:

If cycling occurs as the result of other conditions, a blocked line or condenser is possible. Perform the appropriate service.