87 300SDL Climate Control weirdness
 

Searched the threads and couldn't find a definitive string to help. Got *really* warm here in Detroit for the first time, mid '80's, and I had the AC on. Working fine except now the fan is stuck on "medium" speed and I can't shut off the climate control. Press high and it goes to high and press low and it stays on "medium" speed. Temp works fine, various directional levels work fine. Just won't shut off and won't even go to "low" on the fan speed. Also, rear seat duct not working but I think that one is due to the lever.

TIA

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

Picked up a used CCU, plugged it in and still can't get the fan to turn off. It is basically stuck on "auto" . Can turn on HI but hit the off button and the damn thing won't turn off. Arrgh!

Oh, and the one that was on the car appears to have been rebuilt at some point by someone who does it for a living - silver non MB looking sticker over screws on back saying warranty void if removed, etc.

Ben,

There is an electronic module in the car that is connected to the push-button CCU that controls the blower motor. It actually connects to a resistor block and it is connected to the blower motor.

My guess is that this module is the source of your problems. We can try to figure this one out if you want. Come over tomorrow afternoon and we can can sort it all out.

Will do. I'll ping ya offline. Thanks!

Thanks for the intel Arlo, really appreciate it. I pulled and examined it and couldn't see any visible cracks, etc. like the one in the W123 example. I picked up a used one off Ebay for $15 a while back just in case after reading about the aux water pump induced short issue, but now that I have mine out the part number on this Ebay one is slightly different. At any rate, going to hook up today with my fellow Detroiter Mike and pick his seasoned brain.

HUGE thanks to mplafleur for his technical expertise! Diagnosis - porcupine. Now to the hunt for a "new" porcupine (Blower Motor Regulator). New are $363! Yikes. Go to plan B.

Yup. The fan motor control was getting the correct voltages depending upon which buttons were pressed (Off = 0v, Low = 1.2v, Auto = 2.0v, High = 7v), but the fan would never spin less than a medium-high speed.

On the potentiometer thought - stumbled across this when I got home this afternoon:

http://www.peachparts.com/shopforum/off-topic-discussion/264693-looking-compound-potentiometer.html?highlight=porcupine

Mike you were absolutely correct! Met up with Roy today and got a gently used porcupine and working magnificiently. In a weird way glad it happened so I could clean up all the mouse crap! Thanks again. Really appreciate it!

No problem Im glad u got it worked out. Now Ill know if in the future I only lose the speed control not ccu functions right where to go. This site is priceless when owning an older MBZ.

Arlo

I'm glad Roy was able to find one for you and help you out. I'm going to have to keep an eye out for those at the pick and pulls.