Blower motor runs when car is off
Diagnose and fix blower motor runs when car is off condition
Almost all blower motor runs when car is off conditions occur in vehicles with automatic temperature control and variable speed blowers. The reason this happens is that these vehicles use a solid-state blower motor control instead of a standard blower motor resistor. The solid-state motor controls can fail in the always-on position. Here’s how to diagnose the problem on a Chrysler, Dodge or Plymouth vehicle with auto temp control and variable speed blower motor.
How variable-speed blower motors work
The Chrysler variable-speed blower motor works by providing full battery power to the blower motor and pulsing the ground connection on an off. The rate at which is switches ground is what determines blower motor speed. The Front Blower Motor Controller Module is the device that pulses the ground connection based on a varying voltage from the blower motor speed selection switch in the HVAC control head.
Blower motor wiring diagram
In the diagram below you can see the blower motor relay,
then a 40A fuse. Power flows from the fuse to the Front Blower Motor Controller Module. When the HVAC is turned on, the speed switch sends a voltage signal to the controller module. The voltage ranges from 8.5 to 2.5 volts. That’s the input to the controller module, telling it what blower speed the driver wants. The controller module interprets the voltage signal and then pulses the blower motor ground accordingly. The blower motor controller is located on the heater box directly behind the glove box. Remove the glove box door and you’ll see it.
Diagnose Blower motor runs when car is off
Most DIYs think that blower motor speed problems are caused by a bad switch. They’re almost always wrong. Speed switches fail occasionally, but the blower motor controller (variable speed setups) and blower motor resistors fail far more often. So don’t follow your instincts on this one. Instead, start by testing the blower motor controller or blower motor resistor. In the case of a variable-speed blower motor, start with the controller.
Remove the glove box door
Using a digital meter, connect the black lead to a ground
point. Then backprobe the dark blue/light blue wiring going into the controller. This is the speed input signal from the speed knob in the HVAC head. With the key in the RUN position and blower activated, rotate the knob and read the voltage. It should range from approximately 8.5 to 2.5-volts. If that’s the reading you get, you’ve confirmed the controller is getting the proper speed input. Now that you know the voltage varies, the only possible cause for a blower motor runs when car is off, is a faulty controller. Replace it and the problem will be solved.
©, 2020 Rick Muscoplat
Posted on by Rick Muscoplat