Primary Heating System

The control assembly in the instrument panel contains a blower switch, a mode knob, and a temperature knob. 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, the air delivered by the heater 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.

Mode Knob

The mode knob uses vacuum to control doors in the heater module, which control airflow through the heater module. Mode knob positions are "HTR," "VENT" and "DEFROST." Intermediate positions between "HTR" and "VENT" or "HTR" and "DEFROST" are identified by the word "BLEND." When the mode knob is in the "BLEND" area between the specific mode positions, the controls are set to provide a combination of the features of the two nearest modes.

Blower Speed Control

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.

Heater Bypass Valve

The heater bypass valve controls coolant flow through the heater core. A spring-loaded valve inside the housing allows coolant to bypass the heater core. It is normally closed at low coolant flow rates so all coolant passes through the heater core.

At increased engine speeds, this valve begins to open and controls maximum flow through the heater core to 5-6 gal./min. by bypassing the excess. This amount is adequate for all heat requirements and prevents long term erosion/corrosion of the heater core due to excessive flow rates.