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2002 BMW 330ci Convertible Air Conditioner Blower Motor Varying speed when car is at idle


Air Conditioner Blower Motor on any speed, lets say high. Running fine while driving. Come to a stop at idle or shift to N or P. A few seconds later the blower begins to vary its speed in pulsating rhythm in what would seem to be in cadence with what would be a rough or varying idle.

Anyone know where to start? Is it the relay? Any help is appreciated.

It depends on how rough it’s running. It could be perhaps a weak alternator which is not providing enough output to overcome the drop in RPMs. I would first check out what is causing the rough or varying idle, as that may be the root cause.

Does your car have a voltage display on the dash?

A weak alternator was my first thought too. The fan motor is a DC motor, its speed varied by varying the voltage. If your alternator is putting out an unstable voltage (think: fried diode) it could easily show up as a wavering blower motor speed. Normally it should be “filtered” by the batttery, but that cannot be counted on.

Have both the charging system and the battery tested. Post back with the results.