Why a heater fan motor keeps running after the car is shut off
The controller is what keeps the heater fan motor keeps running after the car is shut off
If you have a vehicle with an automatic climate control system and a variable speed heater fan motor and the heater fan motor keeps running after the car is shut off, chances are the heater fan motor control processor is toast or the HVAC control module is damaged.
Why the heater fan motor keeps running after the car is shut down
On many GM and Ford products with auto climate control and a variable speed fan, the default failure mode is the fan ON position. That has never made sense to me because it just runs down your battery even with the key out of the ignition. You would think carmakers would only provide power to the controller in the full ON key position. But they don’t.
On other brands, a failure in the blower speed controller just prevents the fan from working at all.
To turn off a running heater fan motor, you can yank the fuse or disconnect the heater fan motor controller.
How the heater fan motor controller works
Electronic variable speed heater fan motor control
Variable fan speed is often used in vehicles with automatic climate control and providing variable fan speeds is accomplished with a solid-state fan speed controller module.
There are a lot of variations in how each carmaker designs variable fan speed, but it boils down to this; the controller receives a fan speed setting from the speed switch. Then it pulses the ground connection to the heater fan motor. The longer the ground connection is off, the slower the fan speed. So low speed might be something like 2/10ths of a second on and 8/10ths of a second off. At high speed, the ground connection is always on.
The blower fan controller isn’t cheap
Just as an FYI, the retail price of a GM heater fan motor control processor is $280.34. There are a few places online that rebuild these puppies. Search for “HVAC rebuilder.” I know of some reputable PCM rebuilders, but don’t have any experience with HVAC rebuilders. So you’re taking your chances.
On newer GM vehicles the controller is located near the heater fan motor. On older vehicles, the controller is sometimes mounted in the engine compartment near the firewall.
© 2012 Rick Muscoplat
Posted on by Rick Muscoplat