I have been working on this climate control system since I got this car - just got done figuring out a blower motor problem after the PO had replaced the brushes, but reinstalled the motor backwards.

Just replaced the Climate Control Unit and I have no low speed fan...Blower speed changes when on Auto and Hi speed works...

Any thoughts?

Thanks, ryan

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RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

Im having the same thing happen right now. Generally, I think the first step is to re-melt the solder connections on the climate control units printed circuit board. Interested to see what other folks say.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

The 123 based ACC uses a stepped resistor and relay arrangement to reduce the blower speed. I would check the resistor pack and its wiring.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Yes, but my understanding of the system is that the Hi and Lo fan buttons override the resistor involvement and that the resistor is in play only when the Auto fan setting is pressed.

But, I may be completely mistaken...

__________________
RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE

The resistor is in series with the motor all the time. There are relays in the PBU that bypass portions of the resistor to create the various speeds. As the segments of the resistor get bypassed, the voltage applied to the motor, and the motor speed, increases.

At high speed the entire resistor is bypassed.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech