Relays and Sensors Description. Pontiac Sun Runner - 4WD (Canada) 1998

For 1990-2009 cars

### A/C Refrigerant Pressure Switch

The A/C refrigerant pressure switch is located in the top of the receiver/dryer. The A/C refrigerant pressure switch cycles the compressor on and off under conditions of abnormally low or abnormally high refrigerant pressure.

If the pressure drops below 193 kPa (28 psi) or rises above 3137 kPa (455 psi), the A/C refrigerant pressure switch opens and signals the A/C compressor control module to shut down the compressor.

Abnormally low refrigerant pressure may be caused by the following conditions:

• | An insufficient refrigerant supply

• | Very low refrigerant temperatures

Abnormally high refrigerant pressure may be caused by the following conditions:

• | Extremely high refrigerant temperatures

• | An overcharging of the system during vehicle service

### A/C Compressor Clutch Relay

The A/C compressor control module energizes the A/C compressor clutch relay, causing the A/C compressor clutch relay to apply voltage to the compressor clutch. The A/C compressor control module is capable of controlling the air conditioning system operation by means of energizing and de-energizing the clutch relay.

### A/C Condenser Fan Relay

The condenser fan receives voltage from and is activated by the A/C condenser fan relay. The condenser fan runs continuously as long as the A/C compressor is engaged.

### A/C Switch

The A/C switch is mounted on the heater control unit. The A/C switch signals the A/C compressor control module that A/C system operation has been requested.

### Evaporator Temperature Sensor

The evaporator temperature sensor is a thermistor. A thermistor is a device that translates temperature into an electrical resistance. Thermistors are connected into electrical circuits in order to determine the temperature in critical areas. A thermistor is used in engine control systems, such as engine coolant temperature (ECT) and intake air temperature (IAT) sensors. A thermistor is used in the A/C system in order to determine temperatures at the evaporator core. If the evaporator core temperatures drop to approximately 0°C (32°F), frost and ice will start to form on the evaporator fins. Ice formation on the evaporator fins will impede airflow through the evaporator and reduce the system's cooling ability.

The A/C system incorporates a thermistor, that is electrically connected to the A/C compressor control module, in order to prevent any ice formation on the evaporator fins. When the evaporator temperatures drop to 0°C (32°F), the A/C compressor control module reads the corresponding thermistor resistance as a system cutout signal. When the signal is received, the A/C compressor control module shuts down the compressor until the evaporator core temperatures increase to 3°C (37°F).