The A/C refrigerant pressure switch is located in the liquid line, between the condenser and the orifice tube.

The A/C refrigerant pressure switch consists of two separate switches, the first is the A/C refrigerant pressure switch, the second is the A/C high pressure switch. The A/C refrigerant 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 A/C refrigerant pressure switch opens and signals the A/C compressor control module to shut down the compressor. The A/C high pressure switch acts to cycle the cooling fans (the main fan motor and the auxiliary fan motor) 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.

The A/C fan no. 1 and no. 2 relays are responsible for cycling the main and auxiliary fan motors between series and parallel operation. The A/C fan no. 1 and no. 2 relays share the same power feed from the ECU-IG fuse and share the same ground path through the A/C refrigerant pressure switch and the fan control switch. When the engine is running and the engine coolant temperature (ECT) is below 83°C (181°F), the fan control switch is closed energizing both the fan no. 1 and no. 2 relays. With both relays energized an open circuit condition exists for each fan motor and neither fan motor runs. When the ECT reaches 93°C (199°F), the fan control switch opens and both relays are de-energized. With the fan no. 1 relay de-energized voltage is supplied to the main fan motor. With the fan no. 2 relay de-energized ground is supplied to the auxiliary fan motor. Both fan motors will now run at full speed.

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 main and auxiliary fan motors between series and parallel operation. In series, the main and auxiliary fan motors operate at half speed; in parallel the main and auxiliary fan motors operate at full speed. Whenever air conditioning system operation is commanded, the main and auxiliary fan motors are initiated in series through the A/C MG relay, the A/C fan no. 2 relay, and the A/C fan no. 3 relay. The A/C fan no. 1 and no. 2 relays are de-energized whenever the air conditioning high-pressure switches open or the fan control switch is open. With the relays de-energized, the main and auxiliary fan motors operate in parallel at full speed.

The A/C fan no. 3 relay, when energized, completes a series circuit from the ALT fuse through the main and auxiliary fan motors. With all three relays energized, voltage is applied through the auxiliary fan motor, through the A/C fan no. 2 relay, through the A/C fan no. 3 relay, and through the main fan motor to ground. Under these conditions, the series circuit is complete and the motors both run at half-speed. If high engine coolant temperature or high refrigerant pressure causes one or more of the relays to de-energize, the main and auxiliary fan motor circuit is switched from series to parallel configuration. In the parallel circuit, the auxiliary fan motor is provided a direct ground (instead of grounding through the main fan motor) and runs at full speed and the main fan motor is provided a direct power feed (instead of through the auxiliary fan motor) and runs at full speed.

The A/C MG relay is provided a power feed from the switch side of the heater relay when the ignition switch is ON and blower speed selector switch is in any position except OFF. The A/C compressor control module supplies a ground to the coil side of the relay in response to an A/C request and appropriate system conditions. Voltage is supplied to the compressor clutch when the A/C MG relay is energized . By energizing and de-energizing this relay, the A/C compressor control module is capable of controlling air conditioning system operation. When the A/C MG relay is energized, voltage is also supplied to the fan no. 3 relay which, when energized, is connected into the main and auxiliary fan motor circuit and is responsible for initiating main and auxiliary fan motor operation in series circuit form during A/C operation.

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