I found a Temperature Controller for a W124 1991 300E that at a Junkyard that I hoped to use on my 300D 1987. I was hoping to get around spending $300 for a new unit.
The physical configuration and electrical connector points are Identical; however, the part numbers do not match. Are these units interchangeable? I can see no reason why their electrical layout/configuration/pinout would be different since their basic functionality should be the same. I think temperature and button selection would be the input and damper postioning and blower fan output would be the outputs?
Could I swap these units?
Also what causes them to fail? Is it simply a matter of cracked soldering connections that can be fixed by resoldering?

i think sometime between 1987 and 1991 they switched from Celcius to Farenheit on the temp dial and also used some different materials. that might be the difference in the part numbers. ive read a bunch about the 124 a/c system in my service manual, and i dont ever remember it distinguishing a difference between the 87 and 91, but i could be wrong.

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1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350

I can see no reason why they should be different, but I know that they are. You cannot use the 1991 300E PBC in your 1987 300D. At least not without also changing other components and/or wiring.

The one difference of which I am aware has to do with the signal from the PBC to the Klima relay indicating activation of the a/c compressor is desired. On the diesel PBC this signal is a grounding of the line from the PBC to pin 7 of the Klima. (I don't recall from which connector/pin on the PBC this signal originates. Believe it or not, I don't have the wiring diagram for the 124 memorized. At least not all of it...) On the gasser this signal is +12v, and it may not go to the Klima - the gas car may use the MAS to control compressor activation.

You'd have to spend mucho time staring at the wiring diagram to gain any confidence this swap would work. Even then there would likely be electrical hacking required.

There are businesses out there which refurbish & resell these things.

The common failure on the 124 is the pins for the potentiometers (there are two) operated by the temp. wheel come loose. It's easy enough to disassemble the PBC and resolder these.

What is your climate control doing wrong? Try unplugging the aux. water pump. Wierd stuff happens when they short out, and they all short out eventually. It's located underneath the window washer fluid reservoir.

- JimY