Yesterday I took my automatic climate control system out of my 84 300SD. I used a solder iron to reconnect all my connections following the posts I had found on here as my unit was not functioning properly. When I reinstalled it and turned the key to the start position, it worked good for a few seconds, then the acc would not turn off in the off position button and fan would change on its own. Next I saw and smelled some smoke, so I immediately turned it off. It was on for maybe 30 seconds. This morning I took it back out and saw a black spot on the green grid between two solders. I believe it is the connection between pin numbers 9 and 10 on the bottom edge of the plate. Does this black spot meaned I ruined it? Please advise.

My original problem was heat all the time. I checked the monovalve and found it was unplugged. So I plugged it back in and while I was at it I unplugged the aux water pump (I think that is what I unplugged, it was the connection right next to the battery). It stopped blowing hot. Then I went on to fix the CCU.

Also, I was told the compressor was not working by the person I bought it from in April. Have not checked into that at all.

Thanks!

It's possible it's fried. The aux water pump is usually toward the front on the engine located on the driver side. At least it is on the w123. IIRC the connecting points are the same since both used the OM617. IT is also possible that a locked up aux water pump fried the CCU also.

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RRGrassi


70's Southern Pacific #5608 Fairmont A-4 MOW car

13 VW JSW 2.0 TDI 193K, Tuned with DPF and EGR Delete.

91 W124 300D Turbo replaced, Pressure W/G actuator installed. 210K

90 Dodge D250 5.9 Cummins/5 speed. 400K

In the 617 engine used in the 300D and SD, the aux water pump is not fused or otherwise protected. Mercedes discovered over the years that a frozen pump (due to rust, corrosion, crud in the coolant, etc.) drew so much current (an electric motor draws max current at speed = zero) that it could damage the CCU. Therefore, later versions of the CCU had protective circuitry installed to avoid that problem.

For your car (and mine) the fix is to install a fuse (1-2 amps) in one of the wires to the aux water pump. Use as low an amperage as works without burning out in normal operation. See this link for a DIY:

http://www.peachparts.com/shopforum/showthread.php?t=157679

The aux water pump itself can be taken all apart and cleaned but it isn't so expensive that you can't just replace it with a new one: less than $100 if you shop around. You can also simply bypass it with a piece of pipe between the hoses. Its purpose is to aid hot coolant flow through the heater core when the engine is running at low speeds (city traffic, stop-and-go, etc.) and make the heater work more smoothly. In most cases, you won't even notice it's missing. Cost: cheap/free. Pipe: not sure whether copper should be avoided, perhaps others can advise.

The monovalve must work for the ACC system to operate as designed. People have jury-rigged a manual valve (open for heat, close for a/c) or retrofitted a manual system from a 240D or something similar. There are repair kits for the monovalve; you don't have to replace it unless the electrical part is damaged.

You should be able to repair the fried traces on the CCU board by carefully soldering wires to jump the fried traces. I had to do that to my '85's CCU, replacing an earlier repair that was poorly done. I installed the fuse in the aux water pump at the same time.

Jeremy

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Jeremy,
Thanks for the reply. The CCU was working Ok with the aux water pump running and the monovalve unplugged. Then I plugged in the monovalve and unplugged the aux pump. So the black spot was not caused by the aux pump. I will still plan on installing a fuse when I hook it back up, but I'll probably wait until it gets cold here to hook it back up.

The black spot must have been caused by me joining two points while resoldering.
QUESTION: So if I jump the black spot with a wire, should I take the solder off around it? Or what should I do when I jump it? It will be a little tricky because it connects the bottom to the pins on the side. I guess I could drill a small hole where the solder was and connect the wire there.....

I just looked at it again. The black spot is BETWEEN two solders. It doesn't appear to be blocking any circuits. Another question: Will I do anymore damage by plugging it in and seeing what happens? I only let it run for a few seconds, is it possible that it just shorted out the connection between the two solders but the circuits are still intact?

If the black is between the traces on the board more than likely you let the magic smoke out of one of the components on the other side of the board rather than burning a trace off of it. It's probably toast.

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1983 300D
1978 BMW R80/7
1997 Jeep TJ

I'm not quite sure what that means? Compontents? Like what?

Also, when it was smoking for a few seconds, it still was working somewhat. So back to my previous question: Will I do anymore damage to it by installing and using it?

Here are some pictures:In the 3rd one the flathead is pointing to the spot that is black. (You can click on them to make them bigger and see them better)

In thinking more about what you said, I think by components you mean the aux water pump or something else like that. But the only possible thing that was changed was plugging in the monovalve and I had ran it for a day. It smoked immediately after install after I resoldered it, so it must be correlated to the resoldering. I'm thinking that some of the solder got too close to each other.

It is the junction between the circuits connecting pins 9 and 10.

Can I damage anything more by continuing to use it?

Is it possible that it just shorted the newly joined solder connection and now it is not connected and I can keep using it?

It is entirely possible, however unlikely, that the flux was conducting power and simply burned from the heat. (Flux is a conductor). Try to clean all flux off of the board, we use flux remover in the electronics industry, not sure what chemical that is. Maybe alcohol? As the flux remover evaporates water droplets are left behind so make sure the board is completely dry before you try it again. If it works, then you are ok! If it doesn't then a risistor or other component was probably taken out and that may be repairable as well.

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1983 Mercedes Benz 300D Turbo (Daily Driver)
1979 Mercedes Benz 300CD (Needs Lotsa Work)
1963 Chevrolet C10 Stepside Shortbed (Not Stock)
1999 Chevrolet Silverado Z-71 (Fun to Drive)
2007 Nissan 350Z (Wifes Car - Lucky!)

By the picture I would say that you bridged the 2 points with a bit of solder. All it takes is a small thread.I would try the unit again, there is a good chance it just burned the bridge clean. the CCU in my 1982 only has a couple of diodes and the rest of it is the relays and switches. not a whole lot that can go wrong. the circuit board traces are the weak link. Those solder joints along the edges close enough together that it you need a small iron to do the job properly. I would also test the ccu without putting it in the case so you can aboserve the operation closer. be sure not to let it touch anything metal while it is pluggged in or it will smoke again. Only do this if you are comfortable with it. hope this helps.

Thanks for the last two posts. They both helped.

I did use flux to clean off the board, so maybe that contributed to the smoking. I will wait until tomorrow to make sure it is dry and try it again. I'll keep everyone updated. If not I'll order a used one off of ebay.

Thanks for everyone's help!