Sounds like the typical solder joint failure that brought me to this wonderful site in the first place. If you search a while on ccu solder joint repair or re-solder or something like that you will find allot of info on how to repair yourself through various links and old threads. Though my 87 300D ccu was different than the pictures shown for w123 ccu, the repair is very similar and I think possibly easier due to only one board to mess with. I am unfamiliar with sd/sdl models and if they are same/more similar to w123 in 87. Though I am not a professional at soldering, with a good magnifier lots of light and patience my ccu still works flawless two years later. Oh and the not shutting off is telltale solder joint symptom. There was cracked joints all over from thumbwheel temperature control, speed buttons, aux pump area (little hot spot also) and pushbuttons which I believe cause the no shut off problem. If you are unsure of your soldering capabilities you may want to experiment with a junkyard ccu first. I had nothing to lose as mine didn’t control anything anymore and wouldn’t shut off. In Las Vegas a/c is a requirement. Hope this helps

Arlo

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W211 E550
HVAC/R Certified Technician