Rick's Free Auto Repair Advice

Blower motor controller keep fan running after car is turned off

Why does the blower motor keep running after I turn off the car?

If your car uses a blower motor controller, it can keep running the blower motor after you shut down the engine. Those used to be installed only on vehicles with automatic climate control with variable speed fans, but now they’re installed on many late model vehicles.

How a blower motor controller differs from a blower motor resistor?

In a vehicle equipped with a blower motor resistor, blower motor resistorvoltage is reduced by the value of the each resistor based on the position of the fan switch. The lower voltages result in lower fans speeds. The downside of blower motor resistors is that, in order to reduce voltage, a resistor converts the energy into heat. These days cars can’t afford to waste that energy—there are simply too many electrical accessories to run.


Using pulse width modulation to control blower fan speeds

So car makers switched to pulse width modulation (PWM) to control the blower fan speeds. It’s really a simple concept: the blower motor controller pulses either the power or the ground circuit on and off depending on which fan setting you choose. blower motor controllerFor example, if you set the fan to the lowest speed, the controller may provide power for 3/10 of a second every second, while providing power or ground for 7/10 of a second every second for fan speed #3.

How does a blower motor controller know what PWM to use?

The blower motor controller is a solid state device with a power switching transistor that takes its marching orders from the HVAC control head. In some cars the signal from the HVAC unit to the controller is digital, while in others it’s pulsed or a varied voltage.

How does a blower motor controller fail?

They can fail in many different modes. They can fail blower motor controllerso that the blower doesn’t work at all, runs in one speed only, or, runs all the time, even if the car is shut off. To fix the problem, replace the blower motor controller.

How do you know whether the problem is the controller or the HVAC control head? Well, if you own a bi-directional scan tool, you can test the control head. However, since the blower controller fails far more often than the control head and is much cheaper, I advise you to start by replacing that first.

©, 2016 Rick Muscoplat

Posted on by Rick Muscoplat

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