My fan is stuck on high. I installed a switch at the fan motor temporarily. I put a brand new control unit and same thing. Any ideas where to look first? Need a copy of the manuals on CD I suppose. It's an '82 300D Turbo.

You are right, you need a wiring diagram. I don't have one to look at as I am at home but I would look into the defrost positions control of the blower controller. The defrost is always at high speed.

I would also look at the blower resistor schematic and see if they were still using the overheat bypass switch like earlier cars. This was a switch which shunted the resistor coils if they were getting to hot. The blower would go full speed and the coils would cool. Don't remember ever seeing that fault on a 123 body.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician