ok, so i have a 1984 300sd and my heater control has never worked right, "i have hot air coming out the defrost and vents only" and i"ve read on here that the heater controls on these cars are known for problems "i believe the solder joints between the relays and the bread board?"
so i'm at the u pull yard and spot a 198? 300d with what looks like the same heater control, snach it out, thay charge me 13.99 plus tax for it.

So now i'm at home looking at it and it looks brand new compaired to mine, i pull it apart and can see were it's just started to discolor the bread board.

So for question #1 are the auto heater controls the same for a 300d to a 300sd? #2 what exactly has to be resoldered on these?
and #3 what do you have to do to stop it from doing it again?

any help would be great.
thanks,
jason

Compare the size of the buttons, and the number of pins on the plug, mounting points also.
The failed part on the board is referred to as a cold solder. Over time they rattle loose, you'll be able to see a round hair line crack surrounding a pin, kinda like a volcano. You need to closely inspect each solder joint.

In the first two pics is a unit that came from the same chassis, but doesn't fit the one in my car. Different mounting points too, but you won't know that untill you get the old one out, guess how I know.
Note the different fan switch.

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83 SD

84 CD

The controls for the W-123 and the W126 are not identical and there will be certain features that won't work correctly if you exchange them.

Furthermore, if the system is blowing defrost air all the time, the problem is not with the CCU. The issue is with the vacuum pods that open and close the vacuum doors. One, or more than one pod has a leak and it must be found, repaired, or replaced for the system to work correctly.

There is a wealth of info in the archives on the vacuum pods if you take the time to do a search for it.

both control units look the same as far as the buttons go.
as far as symptoms i have no temp control "hot air only" it only comes out the defrost and ac vents, no matter what button is pushed and i had the 126's control unit out and it smelled burnt, i never took it apart though.
thanks again.
jason

Okay, before you install the "new" one, you may want to remove the plastic cover and inspect for cold solders, or at least plug it in to make sure it works before you install it.
Good luck.

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83 SD

84 CD

First things first.....Hot air only.... Look at your monovalve. You may have a ruptured rubber membrane in it. The monovalve is located on the firewall, passenger side. Search monovalve failure on this board and there are many, many excellent posts and pictures showing you what it looks like and how it goes bad. A bad monvalve will keep giving you heat only. Also you state that the CCU in your car smells burnt. The next thing to check is the aux water pump which sits next to the monovalve. When this pump seizes, it will stop pumping and then will eventually burn out the CCU. Search for aux. water pump also. This board has an enormous wealth of experience and has many well written and well documented procedures to follow. On your car if the aux water pump is bad and you put in a different CCU, you will burn out the new one too. A lot of people have put an in-line fuse in the power wire going to the aux water pump, so it will blow the fuse first instead of burning up the CCU.

Good Luck....Use the Search Function...It works very well.

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John
1986 300sdl
1985 380se

Vacuum pods are controlled electrically by the CCU. Odds might be substantially in favor of the problem being vacuum related, but the CCU cannot be ruled out as the culprit.

The solder joints that have a problem are along the edge of the main board where it connects to the side boards. When you open the Pushbutton Controller, you will find the circuit boards are arranged in an "H" pattern. What happens, common in the public arena, is the main board flexes from "just push a little harder" syndrome.

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Sam

84 300SD 350K+ miles ( Blue Belle )

i have been watching the posts here on both the monovalve and the aux. coolant pump, i have the coolant pump unplugged now. i just want to be sure that the aux. coolant pump is the only thing that likes to smoke the ccu.

i saw some pictures of here that had burned up two or three of the relays on the main board. i would think the aux. pump is only one relay.
this is a picture of the one i got out of the w123.

Is it the joints on the edge's were the small boards are attached to the large one or the connections for the relays?

i have a friend that is pretty good at fixing circuit boards i could have him look at this one before i use it.

I plan on shotgunning a mono valve rebuild, aux. coolant pump with fuse and the ccu. to see if wants to start working right. I do hear the blend doors moving when i shut the system off.

Does the ccu. use vacuum from the door lock system or the engine vacuum system?

My door locks work good, the shutoff system is a little odd at times, mostly extreme cold though.

I thank all of you again for the insight, I've been a mechanic for 20 years but this is my first MB diesel, I see why you love them.

jason

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The solder joints that are suspect are where the small circuit boards attach to the large one. They develop hairline cracks that aren't visible unless highly magnified.

The climate control uses engine vacuum. The best/essential device for troubleshooting the vacuum operated pods is a Mightyvac.

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Sam

84 300SD 350K+ miles ( Blue Belle )

When you get the cover off, look for areas such as in the circle.

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83 SD

84 CD