How the Circuit Works

How the HVAC Circuit Works

Blower Controls

Voltage is applied at all times through fuse 56 to the blower motor relay contacts. With the ignition switch ON (II), the blower motor relay is energized which feeds battery voltage to the blower motor.

The heater fan switch then applies ground to the blower motor resistor for the desired blower motor speed. Each time you turn the heater fan switch to a higher speed, more segments of the blower motor resistor are bypassed. This increases the blower motor speed. When the heater fan switch is in position 4, the entire resistor is bypassed and the blower motor runs at the highest speed.

Compressor Controls

Voltage is provided at all times to the heater control panel through fuse 13 (in the passenger's under-dash fuse/relay box), and to the A/C compressor clutch relay contacts through fuse 58. With the ignition switch ON (II), voltage is supplied to the control panel and the relay coil through fuse 3 (in the driver's under-dash fuse/relay box). The control panel is grounded at G401.

When you push the A/C button ON with the ignition switch ON (II) and the heater fan switch not in the OFF position, the control panel will ground the A/C ON input of the PCM or ECM through the A/C pressure switch. The PCM or ECM then grounds the A/C compressor clutch relay coil. This energizes the coil, which closes the relay contacts and provides voltage through fuse 58 to the A/C compressor clutch. The clutch then engages and begins turning the compressor.

Evaporator Temperature Sensor

The evaporator temperature sensor is located on the evaporator housing. If the temperature at the evaporator gets too cold, the evaporator temperature sensor sends a signal to the heater control panel to turn off the A/C compressor clutch. This prevents condensation from freezing on the evaporator fins and blocking air delivery into the passenger compartment.

A/C Pressure Switch

The A/C pressure switch is located in the condenser outlet line where refrigerant is in a high temperature/high pressure liquid state. The switch will sense abnormally high or low pressure, and open the circuit. This removes the ground signal from the PCM or ECM, and the compressor will be turned off.