C230 ACC Fan Speed Fluctuation
 

'97 C230, 70K miles. Climate control fan speed has recently become unstable, demonstrating uncommanded speed changes.

Cycling ACC off/on makes no difference. Otherwise, system cools well (for a Mercedes... ). Any ideas?

Somewhat common problem is the electronic blower control unit on the blower motor. MB part number 210-820-62-10 & list of $132. Of course the A/C dust filter (microfilter) should be checked & replaced if needed as well.

This probably doesn't apply to your situation, but just in case -- make sure that your center AC vents are not blowing cold air at the cabin temperature sensor by the rear view mirror. Happened to me when a passenger moved the vents fully up; I couldn't understand why my ACC suddenly would exhibit odd shifts in blower speed for no apparent reason...

Cabin air filter was CLOGGED. Blower motor function returned to normal following filter replacement.

The uncommanded HVAC blower speed change is back... in automatic or manual mode the blower will suddenly, and briefly, increase speed (to max?), although the blower speed indicator on the push button control unit remains unchanged. Along with the speed increase is the sound of perhaps a flap cycling, resulting in increased back pressure on the blower.

The cabin air filter and blower motor resistance unit have been renewed.

Any ideas as to what the problem might be? Push button control unit?