Control Assembly, HVAC

The control assembly in the instrument panel contains a temperature knob, a mode knob, and a blower switch. The controls are back-lit when the headlamps are on. Electrical voltage is provided through the instrument panel lighting circuit. Brightness of the dial illumination is controlled by the instrument panel dimmer switch.

Temperature Knob

When the temperature knob is in the BLUE (Cold) end of its travel, air delivered by the system is unheated. When the temperature knob is in the RED (Hot) end of its travel, all air passing through the heater module is heated before it is discharged. Intermediate positions of the temperature knob result in a mixture of heated and unheated air to provide more moderate air temperatures.

As the temperature knob is moved away from the BLUE (Cold) end of its travel, the temperature valve opens to allow an increasing amount of air to pass through the heater core. The farther the temperature knob is moved toward the RED (Hot) end of its travel, the greater the airflow through the heater core. Heating a greater portion of the total airflow in this manner results in a warmer discharge of air from the outlets.

When the temperature control is in the full RED (Hot) end of its travel, the temperature valve blocks off the passage that allows air to bypass the heater core and causes all of the airflow to go through the heater core for maximum heating. This method of temperature control provides a very rapid response to any change in temperature selection throughout the entire range when the engine is warmed up.

The temperature door is electronically controlled with an electric motor in place of a cable system.

Mode Knob

The mode knob operates a rotary vacuum switch that routes engine vacuum to specific hoses in the vacuum harness. These hoses control vacuum actuators on the HVAC module. Each actuator operates a valve (a door-like hinged deflector) that routes airflow to the various outlets of the heater system.

The mode knob positions are MAX A/C, NORM A/C, BI-LEV A/C, VENT, HTR, BLEND, and DEFROST.

Blower Switch

The blower switch provides a choice of three blower speeds. The blower switch receives power from the fuse block when the ignition is ON. In the low-speed position. the circuit continues through the heater wiring harness to two resistors in the resistor assembly near the blower motor.

When the blower switch is in a medium-speed position, the circuit continues through the heater wiring harness to the resistor assembly, but it bypasses one of the two resistors.

When the blower switch is in the HIGH position, the circuit continues through the heater wiring harness to the resistor assembly, but it bypasses both resistors to provide full power to the blower motor.
From the resistor assembly, the circuit goes to the blower motor terminal.

The blower motor circuit is completed to ground by a wire in the heater wiring harness that goes from the blower motor terminal to a terminal at the cowl panel sheet metal near the blower assembly in the engine compartment.