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Discussion Starter #1
300SD I recently bought had some electrical problems, previous owner had the fuse block replaced as it had burnt some terminals. Heat/AC were still non working when I got it. Tonight I pulled the control (pushbutton) assembly out as I was not getting correct voltages to my blower control. The circuit board is burnt in several places. I am so far unable to get to top of circuit board as I can't get the plastic off around the buttons.
Several questions- How do I get the piece off around the pushbuttons? It is loose, but appears to have something holding it on the buttons themselves.
Is it common for this board to burn, if so is there another cause? Blower motor itself checks good, have not checked AC clutch yet.
I am going to solder some jumpers in on bottom of board to repl;ace burnt tracks, but if this does not work, How much is this part from MB?
Thanks
 

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There are two other things I would check before worrying about the button controls. One common failure item is the resistor pack, wich looks like a bunch of coiled wires in a cube (I don't know where it is on your car) and the other is the "strip fuse" which is often in its own obscure holder (sometimes near the shoch tower unerhood) and is held in place by two screws (visible once the holder's top is flipped open).

The cicuits do fail, but their failure is not that common. As for more troubleshooding and disassembly, maybe Rivermasternc can chime in?

-Neil
 

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Discussion Starter #3
The control unit circuit board was burnt when I took it apart. I repaired the burnt tracks on the board, installed it, turned key on. With blower fuse(F11) in, nothing happened. When I put the climate control fuse in(F10), the board started smoking again. Something is shorted, causing this to burn. My problem is, I can't leave it on long enough to find it.
WIll a bad electronic temp control unit(on passenger side by glovebox,at side of car) cause this? Can a dealer test this module,also blower control module?
 

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Yes, a dealer should be able to check it out for you.

What year car is this?
 
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