Relays, Switches and Sensors

A/C Dual Pressure Switch
The A/C dual pressure switch is located along the right front inner fender, in the liquid pipe, between the receiver/dryer and the evaporator.

The A/C dual pressure switch consists of two separate switches, the first is the dual pressure switch, the second the A/C high pressure switch. The dual pressure switch acts to cycle the compressor on and off under conditions of abnormally low or abnormally high refrigerant pressure.

If the pressure drops below 196 kPa (28 psi) or rises above 3140 kPa (455 psi), the dual pressure switch opens and signals the A/C amplifier to shut down the compressor. The A/C high pressure switch acts to cycle the cooling fans (A/C condenser fan and radiator fan) from low to high speed operation. The change in refrigerant pressure affects the temperature of the refrigerant because the refrigerant pressure and temperature are proportional (temperature increases when pressure increases).

With the A/C on and refrigerant pressure below 1226 kPa (178 psi), indicating low refrigerant temperature, the A/C high pressure switch is closed, thus allowing both cooling fans to operate at low speed. If the refrigerant pressure exceeds 1520 kPa (220 psi), indicating high refrigerant temperature, the A/C high pressure switch opens, thus allowing both cooling fans to operate at high speed. When the refrigerant pressure and temperature return to normal, the fan speed is reduced to low. The change in fan speed improves the condenser's ability to lower refrigerant temperatures and pressures.

Reasons for abnormally low refrigerant pressure are either an insufficient refrigerant supply or very low refrigerant temperatures. Reasons for abnormally high refrigerant pressure include extremely high refrigerant temperatures or an overcharging of the system during service.

A/C Fan No. 2 Relay
In conjunction with the A/C MG relay and the A/C fan No. 3 relay, A/C fan No. 2 relay is responsible for cycling the condenser fan motor between six and twelve-volt operation. At six volts,

the condenser fan motor operates at half speed; at twelve volts the condenser fan motor operates at full speed. Whenever air conditioning system operation is commanded, condenser fan and radiator fan motor half-speed is initiated through the A/C MG relay and A/C fan No. 2 relay. A/C fan No. 2 relay is de-energized whenever the air conditioning high-pressure switch opens. With the relay de-energized, the condenser fan motor operates at full (12-volt) speed.

A/C Fan No. 3 Relay
A/C fan No. 3 relay completes a series circuit from the condenser fan fusible link through the condenser and radiator fan motors. With all three relays energized, voltage is applied through the A/C fan No. 2 relay, A/C fan No. 3 relay, and the radiator fan motor to ground. Under these conditions, the series circuit is complete and the motors both run at half-speed (six volts a piece in a 12-volt circuit). If high engine coolant temperature or high refrigerant pressure causes one or more of the relays to de-energize, the condenser/radiator fan motor circuit is switched from series to a parallel configuration. In parallel circuit, the condenser fan motor is provided a direct ground (instead of grounding through the radiator fan motor) and runs at full speed.

A/C MG Relay
The A/C MG relay, when energized by the A/C amplifier, applies voltage to the compressor clutch. By energizing and de-energizing this relay, the A/C amplifier is capable of controlling air conditioning system operation. This relay is also connected into the condenser fan motor circuit and is responsible for Initiating condenser fan motor operation.

A/C Switch
The A/C switch, mounted on the heater control unit, signals the A/C amplifier that the A/C system has been turned on.

Evaporator Temperature Sensor
The evaporator temperature sensor is a thermistor which is a device that translates temperature into an electrical resistance. Therefore, thermistors are connected into electrical circuits to determine temperatures in critical areas. Thermistors are used in engine control systems as Engine Coolant Temperature (ECT) and Intake Air Temperature (IAT) sensors. They are also widely used in air conditioning systems to determine temperatures at the evaporator core. If evaporator core temperatures drop to about 0°C (32°F), frost and ice will start to form on the evaporator fins. Ice formation on the evaporator fins seriously impedes the airflow through the evaporator and reduces the ability of the system to cool the vehicle's interior.

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