Totally out of nowhere, my blower fan goes from the ANNOYINGLY TOO LOW setting, to the high setting, all on its own about 1 minute after starting the car.

(I wish Mercedes would have made the low about 50% more air flow than it gives; the low setting is just too low).

What would cause it go change speeds on its own? I tried it in both the EC (economy with no compressor), and the a/c position, and it does it on both settings. I tried the medium and high settings and those seemed to be fine, the fan speed didn't change when in those modes. I put in a new MB blower fan and regulator probably 5 years ago.

Thanks,

jeff 1991 300d, 180k

Something wrong with the switch or the blower motor resistor. Any ideas how to check those?

Is the ACC in auto mode? If so, it may be normal. Try the other 2 modes and its behavior.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Nope

That's the resistor pack for the various fan speeds. Pretty simple device. Try to unbend what's bent so the casing is not touching any of the coils. If that doesn't work get a used one from JY. If that doesn't work something wrong in the PBU.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I just deleted that previous reply because it was way wrong. What I was looking at, is the glow plug fuse.

Where is the resistor pack you're talking about, and what does it look like, etc.?

Thanks.

You didn't have your car listed before you edited your post and I assumed it was W123. I don't know much about the 91 so what I wrote probably don't apply to your car. You may consider adding year/ model to your sig.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked