My Mercedes Benz 1982 - 300SD has the following problem: Every time that I need to use the CCU push buttons it does not work. Sometimes after push the Defrost button on and off couple times I can get the fan and AC running but many times even doing it the fan and AC compressor does not work. When I get the fan and AC running using the Defrost button, I wait for almost three minutes running and then I switch to another button to keep it running. I checked all fuses and relays related with AC and CCU system and all are OK.
Not matter if it is winter or summer the symptom is the same and if it the system work at the first time it can also quit work at any time. I read many posts and threads about this issue and I got good tips from others but now I am wondering from where the problem is coming from and how to start to fix this problem since summer is started and a good AC is needed.
You help is extremely appreciated.
Thanks.
Nyko 46.

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Nyck - TX - USA

I have a similar problem althought not as severe. Many of the posts say it is the brushes on the motor. This is an easy fix if you do not have a Siemems blower unit like mine, where the brushes are not accessible. If you can do it, replace the brushes to eliminate this factor. I belive there is a good DIY artilcle on this.

I talked to a friend who said he sent his CCU off to be rebuilt and this fixed the problems. I had mine out, took it apart and cleaned it thoroughly. It did not change anything but it does look a lot better. I replaced the tiny light bulbs and now it looks nice and bright at night. I was going to resolder some of the traces but I could not see any cracks, so left it alone.

I need to look back at some posts for a reference, but evidently there is a guy in Georgia who rebuilds these control units.

Good luck and let us know how it turns out.

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85 300D 75K Anthracite Grey 0-60 in 13 seconds **For Sale**
84 300D 333K Black (The Velveteen Rabbit) 0-60 in 14 seconds
00 Toyota Sienna 208K (Sold)
15 Subaru Outback 43K
11 Subaru Outback 67K
98 Ford Taurus 100K (Gertie - Was Grandma's - drove it to church and shopping - really) Daughter's car now.
30 Model A Ford 2 Door Sedan (Sold) 0-60 in . . . Never reached 60

I believe that JimmyL has posted pictures of how to change the brushes on the siemems blower motor

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

Your symptoms sound like a dying CCU (push button unit).

Defrost is the default mode. Many people with bad units can only get anything to work on defrost. Then when you think you switch to other modes, in reality it stays in defrost.

It sounds like you are in store for another unit. You can take your chances on ebay (I did and it has been working fine for almost a year) or you can order a high quality rebuild with a warranty for around $200 and some. I forget the link, but I'm sure others will post it.

I guess you could try soldering it yourself, but the downfalls are you can make the problem worse and void being able to send it away for a core charge for the rebuild.

I bought a rebuilt one from programma and had good luck, and I would recommend doing the same instead of wasting so much time trying to solder and stuff.

I agree, classic CCU symptoms. However, a bad blower motor of a bad auxiliary water pump can smoke the CCU, so don't just toss in a new CCU without checking them out. I got a rebuilt unit from GDL, it's been perfect for several years.

I couldn't find this tutorial.
Can you please tell me where to find it?
Thanks

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Nyck - TX - USA