Control Module HVAC: Description and Operation

Chevrolet:

Pontiac:

With the ignition switch in the RUN position, the blower motor runs in all mode positions except OFF. The blower motor speed is controlled by a four-position electrical blower motor speed control switch at the left side of the control. The blower motor speed control switch provides a choice of a low, two medium, and high speeds.

Power is routed from the HVAC fuse in the fuse block to the mode control switch. The mode control switch breaks the circuit when it is in the OFF position, and closes the circuit in all other positions.

From the mode control switch, the circuit then goes to the blower motor speed control switch. When the blower motor speed control switch is turned to the LOW setting, voltage is conducted to the resistor. At the resistor, the voltage passes through all three resistors and is reduced to provide the low blower motor speed.

When the blower switch is in either of the two medium speed positions, it closes to send voltage through either two or one of the resistors in the resistor. Because the current follows the path of least resistance, the circuit branch leading directly to all three resistors becomes ineffective and the current bypasses either one or two resistors, depending upon which medium speed position is selected. The reduced resistance increases current flow and blower motor speed.

When the blower motor speed control switch is turned to the HIGH setting, it energizes the high blower relay and power is taken from a separate circuit that is protected by a fusible link. This separate power circuit is hot at all times and has the capacity to handle the current demands of the blower motor at high speed.

The circuit to the blower motor is completed to ground at the instrument panel through the instrument panel wiring harness.