Climate Control Weirdness

[TostitoBandito]

Recently my climate control has begun acting strangely. It's the dual zone automatic climate control unit.

• It'll fluctuate fan speeds rapidly, seemingly for no reason. The fluctuations seem to mostly be from the extremes, so minimum to maximum speed and vice versa. Major changes.
• Changes in fan speed aren't necessarily reflected in the display indicating the fan speed. It might be blowing full speed, but the indicator will only be at 25% or so.
• While it's in one of these states and blowing at what sounds and feels like full speed, I can increase the indicated speed manually all the way to full and then back down again with no discernible effect on the actual speed.
• If I turn the climate control all the way off by decreasing the fan speed to zero it will mostly shutoff and stop blowing on me, but I'll still hear a fan going underneath someplace.
• Similar to this, sometimes a fan will sound like it's going full speed, but very little air is coming out of the vents.

It does this most frequently at startup and will sometimes settle down eventually. Sometimes not. In all cases the vents are wide open and varying the vent temp knob doesn't really change anything except the temperature slightly as it's supposed to.

I've done some searching and it looks like this might be due to a bad FSU. Does anyone else have any thoughts, or is that the first thing I should try fixing?

[deboM3]

Yeah, common problem. It's fairly cheap too, I would start with that. I'm pretty sure mine had those symptoms or similar when my fsu went bad. Good luck

[mmark.]

Imo, if the problem persists after FSU replacement, the resistor in the climate control unit is burnt out.
Lots of info on the net.

m

[Ex36]

It's not the resistor, it's the capacitor. Do a search on how to replace it--you can order a bag of them for a couple dollars on Amazon. I've had to do this on each of the M3/4/5's I've owned. Both times the climate control just started acting bizarro (symptoms like you've mentioned), and both times replacing the faulty capacitor fixed it.

[deboM3]

come to think of it, I did replace that as well.

Sorry, I may be remembering wrong, I thought it was the FSU when the blower was acting all crazy, but maybe it was the cap.

[TostitoBandito]

The main climate control unit is actually relatively new, so the capacitor is probably still ok. The whole thing died 5 or so years ago and was replaced.

I'll start with the FSU and go from there.

[LockDots]

It's not the resistor, it's the capacitor. Do a search on how to replace it--you can order a bag of them for a couple dollars on Amazon. I've had to do this on each of the M3/4/5's I've owned. Both times the climate control just started acting bizarro (symptoms like you've mentioned), and both times replacing the faulty capacitor fixed it.

I believe that was to address the intermittent display and operation of the climate controls, not incorrect fan speed.

[marvinstockman]

Any problems running the system in manual?

[TostitoBandito]

Any problems running the system in manual?

Yes. As I said, when it's doing this thing the fan will stay on full power regardless of what I manually set the speed to. If I set the speed to off, it partially turns off to the extent where it isn't blowing into the cabin but I still hear a fan going somewhere underneath. When the issue isn't occurring, it functions normally when I adjust speed and turns completely off if I turn it off. It's intermittent as to which "mood" it's in.

[earthwormjim]

Your final stage unit is on its way out, it's not the climate control or the infamous watch dog timer cap. If it was the cap, the display would be unresponsive to button inputs.

[TostitoBandito]

Cool, good to confirm my suspicions. OEM replacement part was around $50, and looks pretty straightforward to install up under the drivers side knee panel.

[earthwormjim]

Cool, good to confirm my suspicions. OEM replacement part was around $50, and looks pretty straightforward to install up under the drivers side knee panel.

Make sure you have torx screw drivers, my M3 had torx, my 318is had philips...

[Gregarious7]

I'd like to hijack this thread.

Recently went from a fan delete mod to a SPAL electric fan and had most of the cooling system replaced.

The AC fan seems to fluctuate with MY ENGINE REVS. For example, if I'm accelerating, the ac fan will increase in noise. Then if I shift or have it in neutral, I can hear the AC fan decrease in speed and noise when I engage the clutch.

It will also after a while, stop blowing cold.

Does this sound like the FSU as well? My mechanic suggested that it's the compressor.

[BeerSultan]

Why would the AC fan speed be related to the compressor? Sounds like electrical load, which would put my suspicion on the alternator first.

Having the fan variate in speed w/ RPMs vs. the loss of cold air sounds like 2 distinct problems.

[TostitoBandito]

Seems like it'd be the compressor or something related in the AC circuit.

The FSU is purely electric and not mechanically linked to the engine at all, so it wouldn't have a clue what your revs are. The FSU, from what I gather, works with the auto climate control unit to control the fan speed based on internal temp readings/settings or manual user input. It's the fan in general and not specific to when the AC is on. Typical symptoms of the FSU seem to be a lack of ability to control fan speed, and/or the fan being either full on or full off at all times.

[earthwormjim]

I'd like to hijack this thread.

Recently went from a fan delete mod to a SPAL electric fan and had most of the cooling system replaced.

The AC fan seems to fluctuate with MY ENGINE REVS. For example, if I'm accelerating, the ac fan will increase in noise. Then if I shift or have it in neutral, I can hear the AC fan decrease in speed and noise when I engage the clutch.

It will also after a while, stop blowing cold.

Does this sound like the FSU as well? My mechanic suggested that it's the compressor.

You're talking the HVAC fan right? It's probably the fan slowly dying. When you give the engine gas, your alternator voltage goes up. The HVAC fan may not be able to maintain proper speed at idle or battery level voltages, but the extra volt or two when the engine is under load, could get it up to speed.

That was not a fun job, I believe BMW starts with the HVAC fan and builds the car around it at the factory.

[Gregarious7]

Yes that sounds right.

And by HVAC fan you mean blower motor?

[earthwormjim]

Yes that sounds right.

And by HVAC fan you mean blower motor?

HVAC, blower, squirrel cage, yes.

[TostitoBandito]

Just replaced the FSU, all is well again. Thankfully I had some experience standing on my head in the driver's seat from when I did the clutch bushings and brake switch, so this didn't take too long.

The new FSU also looked a bit different than the old one, it looks like they upgraded the part. It has more and thinner "spikes" than the original part had.