Fix blower that runs all the time – Chrysler minivan
Blower fan runs all the time and at the highest speed, 2001 Dodge Grand Caravan with the 3.8L engine.
In a vehicle with automatic temperature control (ATC) feature “front blower power module (FBPM)” is used in place of a traditional blower motor resistor block.
Here’s how the system works. The FBPM receives its power from the front blower motor relay located in the intelligent power module. When you set a fan speed, the heater control head sends a pulsed voltage signal to the FBPM. The FBPM interprets the pulsed voltage and provides full power to the blower fan. But it doesn’t provide full ground to the motor. Instead it pulses the GROUND connection on and off based on the requested speed from the control head. When an FBPM fails, it usually fails in “full-ground” mode, causing the fan to run at full blast, just like your description.
You can replace the FBPM (the part is around $100 and is not returnable), or you can test to make sure the problem isn’t in the heater control head. Connect a digital volt ohm meter (DVOM) to the dark blue/light blue wire at the FBPM (located behind the glove box). Then change the fan speed dial on the control head. A DVOM averages a pulsed voltage signal, so you won’t see the on/off pulses. But you should see a change in the voltage reading as you adjust the fan speed dial. If you see that change, the control head is working properly and you’ve got a bad FMPM. If the voltage doesn’t change, I’d suspect a bad control head. They’re expensive, so have it checked by a pro before replacing it.
© 2012 Rick Muscoplat