300 SDL Electrical Woes - PeachParts Mercedes-Benz Forum

oilslick · #1 07-02-2009, 08:54 PM

Hello,

I haven't posted on here in ages. I have spent quite a bit of time researching the archives to find a solution to the problems my '87 SDL is having. But I can't find the definitive answer I am looking for. I throw myself at the mercy of the collective wisdom of this forum.

My A/C works but every now and then the compressor kicks off. I lift the hood and remove fuse #5 and then reinstall it and the compressor kicks back on. It may work for 2 minutes or two months but everytime I do the same thing and get through.

During the winter we limped through with a heat situation. Sometimes we wouldn't have heat. You could shut the engine off and restart it (leaving the heat/temp selector/fan on) and then we would have heat. I assume a bad monovalve but maybe not.

Just recently the center vents stopped working so I replaced ALL the vacuum "pods" under the dash with new pods (not rebuilt). The center vents still won't work. The defrost blows continuously. I have vacuum going to the vacuum switchover valves but nothing is functioning like it should.

All of this must be connected somehow. I have read how the monovalve could go bad and fry the pushbutton unit but automotive electronics are NOT an area that I can handle (unless I am walked through it step by step) so I am looking for advice here please.

If you were chasing these problems where would you start and what would you ASSUME the problem would be?

Kevin

powerpig · #2 07-02-2009, 09:09 PM

I don't know if this helps, but I had a lot of problems with the heat and air with my 87 300SDL. It was always flaking out. I recently replaced the ECU unit. (the thing with all the buttons on it) Now it works great. Air and heat when it's supposed to be happening. You can order a rebuilt one from Peachparts here on the forum.

oilslick · #3 07-03-2009, 12:14 AM

Nobody else? Bump!

compress ignite · #4 07-03-2009, 01:35 AM

I'd pick up a "New to Me" CCU @ a JY and see if it fixes the problem.
(You might have to try a couple)
'Could your A/C compressor clutch be trying to go south on you? (Fuse Action)

oilslick · #5 07-03-2009, 01:41 AM

I have two or three ccu's I have pulled from some junkers and each one does the same thing, no change. The ccu's work in other cars just fine. I am leaning toward the change-over valves located behind the ccu. I have vacuum going all the way to them but no vacuum going to the vacuum pods.

I have wondered about the AC compressor clutch. I plan to take her to the repair shop tomorrow. I have owned this car for almost three years with no problems whatsoever (except replacing alternator). This will be the first "real" money I will have spent on it. Fives years ago I would have dived right into this thing but I ain't as interested in these old cars like I used to be. It's just a way to travel for now. She's a really nice car, not a rag, so I want to get another 100,000 out of her at least but I want everything to work right while I do. Wouldn't be surprised if I trade it for an older w123 chasis.

Thanks!

Kevin

wgilmore · #6 07-03-2009, 03:22 AM

Quote:

Originally Posted by oilslick

Nobody else? Bump!

That's the problem with these old A/C systems - too many electrical issues.
You really will need to get into the electrical to solve this or send to a shop.
If you want to check -
compressor speed sensor function and signal
Tach signal to KLIMA
KLIMA relay
Ground loss - -
just a start from my point of view.
Note that fuse 5 feeds much of the junk that the CCU talks to and the CCU and Monovalve.

Regarding the vacuum issue, check the vacuum control switches.
Do you have the factory service manual? You really need to get one if you don't want to shell out loads to the dealer or indy.

My AC (cooling) does a similar thing - compressor won't cut -in - occasionally , I'm just not going to get into right now. So , I shutdown the car, give it a few minutes - everything is fine for weeks - months.

oilslick · #7 07-03-2009, 08:22 AM

I get lost in the terminology. I could troubleshoot the problems but unfortunately for me every time I search the archives the explanations are given using the electrical terminology (of course) and I don't have a clue as to what they mean. LOL! I understand "power", "no power". That's it! LOL!

Kevin

mplafleur · #8 07-03-2009, 10:36 AM

Quote:

Originally Posted by oilslick

I have two or three ccu's I have pulled from some junkers and each one does the same thing, no change. The ccu's work in other cars just fine. I am leaning toward the change-over valves located behind the ccu. I have vacuum going all the way to them but no vacuum going to the vacuum pods..

Thanks!

Kevin

Those pods are for the most part not driven by the CCU, but by the temperature control module that is located above the passengers right foot in my SD. I'm not sure where they put it in the SDL.

I replaced mine once and have had good results.

AlanTbird · #9 07-03-2009, 12:44 PM

I've had all those issues.....replaced the dash unit twice and then the KLIMA.....after the replacement Klima was intermitent...I gave it a good whack cause it wouldnt seat right in the pins and its been fine since.....My heat issue, I have decided is the T-stat which I plan to replace when I am doing the injectors and glow plugs bfr next winter.