G 2500 3/4 Ton Van V8-6.5L DSL Turbo VIN F (1997)
Control Assembly: Description and Operation
Base Heater 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 "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 four blower speeds "Lo," "Med 1," "Med 2" and "Hi." The blower switch receives power from the HTR and A/C
fuse when the ignition switch is in the "ON" position.
In the "Lo" speed position, the circuit continues through the heater wiring harness to three resistors in the blower motor resistor relay.
In the "Med 1" speed position, the circuit continues through the heater wiring harness to the blower motor resistor relay, but it bypasses one of the three
resistors.
In the "Med 2" speed position, the circuit continues through the heater wiring harness to the blower motor resistor relay, but it bypasses two of the three
resistors.
In the "HIGH" speed position, the circuit continues through the heater wiring harness to the blower motor resistor relay, but it bypasses all three resistors
and closes the relay's normally open contact to provide full power to the blower motor from the HVAC maxifuse(R).
From the blower motor resistor relay, 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 the generator or A/C compressor bracket.
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.
