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BMW Blower Motor Inoperative, No Air Flow, Diagnosing – Motor vs. Final Stage Resistor

February 14, 2013

This answer is applicable for many BMWs in addition to the one listed below.

Q
car year: 2003
car model: 325i
I have no air blowing from ac or heater. I’m guess it’s the blower motor. Where may I be able to locate it and could it be some other problem? I noticed that from time to time the air would fluctuate on its own. Was this a sign of it going out?

A
If the blower motor is no longer working on any speed, we have two main potential faults.

This answer, while noting specific issues and locations for the E46 3-series models (99-05, 323i, 323ci, 325i, 325ci, 325xi, 328i, 328ci, 330i, 330ci, 330xi, M3), is applicable to most (if not all) BMW  models from the mid-’90s on, for an inoperative heat or AC blower motor.  The general thoughts and diagnostic theme can also be applied to earlier models and MINIs.

1) Faulty blower motor – The motor may have failed and this certainly could be supported by your notes that it would fluctuate speed at times, prior to failing.

2) Faulty final stage resistor unit – The final stage unit (blower resistor) may have failed. A common symptom of a failing final stage unit is the blower motor not operating in accordance to how the controls are set, as if the blower “had a mind of its own”.

In order to know which is at fault, you will have to either install a new final stage unit and see if the blower works, or test the blower before replacing any parts.  Of these two, it is certainly easier to replace the final stage unit (see the Bentley repair manual for procedures).  If this fixes the “no blower” issue, you’re all set.  If not, then you are looking at accessing the motor for testing and/or replacement.  With this noted, we typically prefer to diagnose before replacing parts.

1)  Access and remove the blower motor via the upper-center engine compartment firewall access panel (See the Bentley repair manual for details).

2)  Using jumper wires from the under-hood positive battery jumper terminal and a ground terminal/source, apply battery voltage to the motor’s positive and negative terminals.  NOTE:  Hold the motor housing securely and make sure that the fan cages are free to spin.  If the motor does run, it will jump when powered up and the fans will spin quite fast.  If the motor runs, consider replacing the final stage resistor unit.  If the motor does not run, replace the motor.

Bentley repair manual:

Blower motors and final stage units
(Enter your BMW model after clicking link):


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