OK, so everybody talks about resoldering the climate cotrol units. Yet all the specific instructions I find are on the w124.

I've got the unit from our W126 in front of me, and I am puzzled as how to get the IC board out. Something seems to hold it in place.

Of course, you could risk the entire unit and resolder from behind without removing the board. But I have a problem soldering diodes and other components I can't see (transistors? ICs?) without using a heat sink! I need to remove the board.

Honestly, had anyone actually done this on a w126? I am beginning to wonder about the accuracy and veracity of following this route.

Any advice appreciated.

Thanks!

__________________
Henry Bofinger
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)

Where are the W124 instructions you found? I've only found instructions for the up to 85 PBUs which come apart easily. The bad solders in those units usually are where temp selector and fan speed buttons join the main board. 86-up PBUs are more... uh... integrated. I have the PBU from my SDL in front of me and I can't figure out how the board comes off either.

Sixto
95 S420
87 300SDL

I'll get back to you as soon as I find them again. I found them here on this board.

All I can tell you is that the w126 is very different from previous iterations. The connectors have been moved to to an angle that seems to be less stressed than on the older models. By the same token, it seems to me the buttons are not removable, i.e. soldered in place, and therefore the board cannot be removed.

__________________
Henry Bofinger
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)

I removed the boards enough to see both sides. Here's what I did:

- remove 4 screws and the bottom cover

- remove 5 mode buttons by prying the retention clips on either side of each button. I've seen replacement buttons so I know how the clips work. Pry one side off the white ramp, then use a flat screwdriver as a crow bar to pull towards the main board the white plastic on the other side while pushing against the black button. The button will be tight in the case as you pull it out while the tabs are deformed over the ramps. Lots of words, makes sense when it's in front of you.

- pull off the pin holding the temp control thumbwheel in place. It has a hex top but it's a friction fit pin.

- pry the two hooks holding the lower end of the thumbwheel frame to the main case.

- pull out the thumbwheel keeping track of the ball bearing (for friction?)

- it takes some wiggling to get the main board out because the part of the main board where the thumbweel rheostat attaches is wedged under some useless part of the case and because the connector pins are a tight fit in their holes in the case. Steady pressure does it.

- the main board will only come out so far because of 3 ribbon cables (fan speed, illumination and daughter board)

- I couldn't find an elegant way to remove the daughter board so I turned 2 small flathead screwdrivers between the case and the back edge of the daugter board to free the tab on the back edge of the daughter board from the slot in the case.

- the daughter board is also held by tabs in the illumination prism at the front of the case. More wiggling and steady pressure releases it.

I haven't figured out how to remove the fan speed sub-board nor the illumination board. There should be enough access to inspect and resolder all the components at this point given the slack in the ribbon cables.

Now, if only I knew what to look for in terms of burned components and bad solder joints

And my apologies if I use PCB terms in the wrong context.

Sixto
95 S420
87 300SDL

This is a picture of a mode switch showing the ramps that tabs on the button hook on...

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This is a picture of a mode button showing the tabs or slots that hook on the ramps on the switch...

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Here is the main board...

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Here is the daughter board...

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Sixto,

Thanks for that work.

The thumbwheel in my case is held in place not by a hex pin, but by something slotted like a regular flathead screw. Unfortunately my digital camera is at a friend's until tomorrow, so I can't provide pictures. Also, my unit is back in place, and from what I can tell, functioning 100%. I did have a corroded solder joint though, which I am glad I repaired.

Also, I know my board is more complex from the other side - there is a diode I saw that is not in your pictures (that's where the corroded solder joint was). However, that pushbutton removal technique and the pictures are invaluable - now I know how to get there. It makes 100% sense from what I saw - I just did not dare the removal. If I do need to take it apart, now I know how.

After resoldering the cruise control unit yesterday, I took her for a test drive - both A/C and climate control seemed to work flawlessly, though only time will tell on the cruise control.

Here are some tips as to what to look for in the electronics:

* burned components - you'll probably not find any. But the usual suspects are diodes and transistors (semiconductors are sensitive to heat). They tend to be common and easy to replace. So if you doubt anything, those are the parts to replace. Unlike resistors, positioning ("polarity", crudely put, because that's not really the term used when placing "anodes" and "cathodes" and "bases" on these little devils) is very, very important, otherwise things will simply not work. But they are otherwise easily replaced, using heatsinks. You might want to invest in a good multitester that also determines flow and Hfe on diodes and transistors respectively ($65 at Radio Shack, but who knows, maybe I am preaching to the choir and you may know more about this than I do).

* solder joints - discoloration. I found a green joint - that's not good. The trick is to redo the joint, and if you hold the iron too long at it, you'll want a heat sink on the other side. That's why I wanted to figure out how to remove the board.

From what I read, usually all you have to do is take care of the bad solder joints. So start with that, without causing heat damage.

Thanks for all the help!

__________________
Henry Bofinger
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)

The thing that looks most suspicious to me is the brown vertically mounted cylinder (capacitor?) northeast of the gold horizontally mounted cylinder (capacitor?). It's marked 33116 V2 EKU with an RDE or ROE logo. It looks like something might be seeping from the base of brown cylinder. It could also be discoloration on the board but I've read that capacitors are typically suspect in these applications. The other concern is a number of solder joints have a brown tint. Is this an extra dose of protective coating or do solder joints discolor with heat? Otherwise nothing looks burned or overheated in any way I can tell.

What are the telltale signs of burned devices? Any advice on how to identify bad solder joints?

Thanks,
Sixto
95 S420
87 300SDL

sixto -

I might be able to help you on this one. Can you email me a high-res image of the board (both sides) via PM? I've done some transistorized circuit board design from scratch, and have built them, both as a hobby.

If you are talking about one of the smaller cylindrical items between the buttons, you are looking at resistors (the ones eith the colored bands) and reddish items that seem to be diodes, not capacitors.

Often you will find a protective coating on the either one or both sides of a board, covering all components. It is brownish in color, and can form drops. The components you are looking at do not contain any liquids.

The wire jumpers you see (the wires spanning different holes, with no purpose but to form a connection) can have bad solder joints, and are much harder to get right than the components, because the solder has a harder time sticking to them. You might want to start messing with those. The good news is that though they are a pain, they won't damage by heat. I have found one bad connection of this type on my cruise control unit.

Again, send me a high res of both sides, and I see what I can figure out.

HB

__________________
Henry Bofinger
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)