Why the blower motor fan keeps running after the car is turned off

Why the blower motor fan keeps running with the key off

If your car has a variable-speed fan, it uses a blower motor controller instead of a blower motor resistor. It’s a solid-state device that allows the motor to run and variable speeds. However, if the blower motor fan keeps running after you turn off the car, that’s a sign of a failed blower motor controller; it fails in the ON position. Those controllers 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, voltage is

Actual Blower motor resistor

reduced by the value of each resistor based on the position of the fan switch. The lower voltages result in lower fan 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.

A blower motor controller uses 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. For 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 so that the blower fan keeps running after the car is shut off. Or, they can fail in a way that the blower doesn’t work at all, and runs in one speed only.

To fix the problem, replace the blower motor controller.

How do you know whether the problem is the controller or the HVAC control head?

The only way to know for sure is by using a bi-directional scan tool so 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