Heating systems use a heater fan attached to a permanent-magnet, variable-speed blower motor to force warm air into the passenger compartment. The higher the voltage applied to the motor, the faster it runs.

A switch mounted on the instrument panel controls the blower operation. In most heating systems, the switch controls blower speed by directing the motor ground circuit current through or around the coils of a resistor block mounted near the motor.

When the switch is off, the ground circuit is open and the blower motor does not run. (Some systems used in the 1970s, however, were wired so that the blower motor operated on low speed whenever the ignition was on). When the switch is turned to its low position, voltage is applied across all of the resistor coils and the motor runs at a low speed.

Moving the switch to the next position bypasses one of the resistor coils. This allows more current to the blower motor, increasing its speed. When the switch is set to the highest position, all of the resistors are bypassed and full current flows to the motor, which then operates at full speed.

In some GM systems, a relay is used between the high switch position and the blower motor. Ford incorporates a thermal limiter in its resistor block.

Current flows through the limiter at all blower speeds. If current passing through the limiter heats it to 212 F (100 C), the limiter opens and turns off the blower motor.

When this happens, the entire resistor block must be replaced.

In summary:

1. The fan control switch routes current through paths of varying resistance to control motor speed.

2. An electric motor drives the heater fan.

3. Blower motor resistors are installed on a “block” near the motor. Some resistor blocks have a thermal limiter.

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