BMW Blower fan not working? How to Diagnose Final Stage Relay (Blower Motor Resistor) Parasitic Draw

BMW Blower fan not working? How to Diagnose Final Stage Relay (Blower Motor Resistor) Parasitic Draw

Are you having an issue where your BMW blower fan is not working? We recently received a question from a fan having an issue with his e46 where some particularly peculiar things were happening with his BMW blower fan not working. So we thought we would take a second to go over what we would do in a diagnostic situation with this car.

BMW Blower Motor Not Working?

I have an unusual problem with a ’99 328i BMW that might be solved by your BMW ace and founder: OJ.

I have been experiencing unexpected battery drain; after sitting overnight, the battery sometimes has insufficient charge to start the car. A quick recharge will restore full function but does not solve the underlying issue, as it has happened again, though not always. The battery and charging system have been tested/checked and are OK. So: a mystery.

BUT THEN: I have also noticed that the heater fan sometimes has been running sporadically all by itself; the car is off, the key removed, but I hear the heater fan start to run slowly, then speed up, then slow down, then stop for 15-20 seconds, then start again, and so on. This does not happen always, but frequently enough to make it the #1 suspect for my drained battery problem. The BMW blower fan not working correctly makes me suspect the climate control module; what else could it possibly be?

Have you guys ever seen this problem? Is there a way to check the climate control? Is there a way to repair or replace it (I presume it is long since out of production)?

Your insight and guidance is earnestly and respectfully requested!”


Thanks for reaching out about your BMW blower fan not working. We are happy to help you out! Your issue is almost without a doubt a parasitic draw of the battery. On the older e46 chassis, when the car is turned off via the key not everything is disconnected right away. A great example of why the need to keep the battery live is convenience items like interior lights that need to stay illuminated while exiting the vehicle. Once the car has been turned off and the doors have signaled to the general module that they are shut, the computer waits approximately 16 min before putting the vehicle into “sleep mode”. That’s still not the end of power consumption, various computer control units even when off need a small amount of energy to keep their learned memory as well as for systems like the antitheft. The amount of energy needed to keep these functions active is the expected “parasitic draw” of the system. I have never seen the draw for a properly working e46 exceed 45 milliamps, with a under 30 being the target for a good system. If the draw exceeds 50mA then you may have an issue that needs a parasitic draw test to be performed. For further reading, we did a blog a while back where we went over what a typically draw test looks like on an older vehicle.

A properly done draw test should identify the circuit that contains the culprit of the draw. Once the circuit is identified then you should refer to a wiring diagram to narrow down the consumers on that circuit.

Seeing as the blower fan is not working, from our experience, it sounds like you have a bad final stage relay. Here is how you would go about replacing that. Its pretty simple!

If you are experiencing this issue, or a similar electrical issue on your vehicle. Don’t hesitate to give us a call! We have an 11,000 SQFT state of the art facility conveniently located in Naperville IL ready to diagnose any complex vehicle issues.

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