* Low Pressure Switch: This switch is crucial. If the refrigerant pressure in the low-pressure side of the system (evaporator) drops too low (meaning refrigerant is being used up), the low-pressure switch prevents the compressor from running. This protects the compressor from damage due to lack of lubrication. Once the pressure is sufficient, the switch closes, allowing the compressor to engage.
* High Pressure Switch: Conversely, a high-pressure switch monitors the pressure on the high-pressure side of the system (condenser). If the pressure gets too high (indicating a blockage or overheating), this switch will shut the compressor off to prevent damage.
* A/C Cycling Switch (or Pressure Switch): Some systems use a single cycling switch that monitors both high and low pressure.
* Temperature Sensor: The interior temperature sensor informs the climate control system about the desired cabin temperature. If the temperature is above the setpoint, the system will turn on the compressor and other components to cool the air.
* A/C Clutch Relay: The compressor itself is engaged via an electromagnetic clutch. The relay receives the signal from the control system (based on the pressure switches and temperature sensor input) and energizes the clutch, allowing the compressor to spin and circulate refrigerant.
* Thermostat (within the blend door assembly): The thermostat determines when the refrigerant needs to be circulated.
In short: The compressor won't run unless:
1. The refrigerant level is sufficient (low pressure is within the acceptable range).
2. The system isn't over-pressurized (high pressure is within the acceptable range).
3. The cabin temperature is hotter than the set point.
4. The A/C system is turned "ON" by the driver.
If your 1996 Probe GT's A/C compressor isn't coming on when it should, the problem likely lies in one of these areas. Troubleshooting would involve checking the refrigerant level (a qualified mechanic should do this), examining the high and low-pressure switches, checking the clutch relay, and verifying the function of other components in the system. A faulty compressor itself is also a possibility. Don't attempt to diagnose or repair the A/C system yourself unless you are experienced with automotive A/C systems; refrigerant is hazardous and improper handling can lead to injury.
