How the Circuit Works

How the Circuit Works

Blower Controls

The blower motor is controlled by the heater control panel which receives battery voltage at all times through fuse 25. With the ignition switch ON (II) and the engine not cranking, battery voltage is supplied to the control panel through fuse 20. The control panel is grounded at G402.

Battery voltage is applied through fuse 56 to the blower motor relay contacts at all times. With the ignition switch ON (II) and the engine not cranking, the blower motor relay in the main under-hood fuse/relay box is energized which feeds battery voltage to the blower motor. The blower power transistor controls the blower motor in all speeds except HIGH. The blower power transistor is controlled by the heater control panel. When the control panel requests HIGH blower speed, it grounds the blower motor through a resistor built-into the blower power transistor, making the blower run at high speed.

Air Delivery

The heater control panel receives battery voltage at all times through fuse 25. With the ignition switch ON (II) and the engine not cranking, voltage is supplied to the control panel through fuse 20. The control panel, which is grounded at G402, controls the blower motor, and supplies a 5 VDC reference voltage to the air mix control motor.

The air mix and mode control motors each receive inputs from the control panel. The air mix motor regulates the mixture of cold and hot air by varying the position of the heater-evaporator door. The mode control motor controls the direction and volume of outlet air. The air flow can be directed to the dashboard vents or the corner vent on the passenger's side. Both the air mix control motor and mode control motor are grounded by the control panel.

The recirculation control motor receives battery voltage through fuse 20 when the ignition switch is ON (II) and the engine is not cranking. It regulates the position of the fresh/recirc door, and is controlled by two position inputs from the control panel ("Recirc" and "Fresh").

A/C Compressor Controls

Voltage is provided at all times to the heater control panel through fuse 25, and to the A/C compressor clutch relay contacts through fuse 58. With the ignition switch ON (II) and the engine not cranking, voltage is supplied to the control panel and the relay coil through fuse 20. The control panel is grounded at G402.

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 request from input of the ECM through the A/C pressure switch. The 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 ECM, disengaging the compressor clutch.

Thermal Protector

If the thermal protector, located on the A/C compressor, senses high temperature in the compressor, its switch will open, turning the compressor off. Once the compressor cools, the switch will close and the compressor will begin running again.