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Old 11-28-2003, 08:06 PM
Q Q is offline
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W126 Cruise Control Repair

I've managed to repair my third circuit board by resoldering. So far, I have resoldered my Idle Speed Control, Fuel Pump Relay, and now my Cruise Control Module. This one was by far the hardest. Not because it was so hard to solder, but because there was an anti-corrosion coating that got in the way.

My symptoms were surging or non-functioning cruise control. Sometimes, it would work fine and then it would flake out again. I could put my foot on the gas pedal lightly and feel it moving in extremem opposite directions (not well regulated).

To repair, I pulled the bottom dash panel by removing the two covers on either side of the steering wheel down where the change well is. I also had to remove a screw and a friction fastener (one was missing) from the underside. The panel then slides forward and out quite easily.

From underneath, you can see two metal boxes with electrical connectors that sit side by side. The larger one is the cruise module. It is unnecessary to remove the boxes, as you can simply uncrimp the soft metal tabs that hold the circuit board in. The board slide easily out.

On the circuit board, you will find a piece of foam glued on. I peeled mine off and then dripped Goo Gone on the remaining tape strip to soften it up. It picked off easily after that.

The most important thing to do when reworking this board is to burn off the corrosion coating before you start soldering. It took me a while to figure out that this was the best way to get the solder to stick. I highly recommend getting some soldering flux for this job, as the corrosion coating played hell with my efforts. The brush on flux would have saved me a lot of grief.

Heat each joint briefly so that the solder melts and you see the corrosion coating liquify. You will notice that the solder almost pulls right off of the pad on the circuit board for some of the joints. The board design was pretty bad, as those holes are FAR too large for a lot of the pins and require too much solder to make the connection.

Once you have heated each joint, brush some flux on a small area and use a fine tip iron and some fresh solder to rewet each pin and pad. It will take a while to do the whole board, so find a comfortable place to work. Be careful of that hot iron and your surroundings.

If you do not own any soldering equipment, you can pick up everything you need at Radio Shack for under $20, I am sure. Contrast that with the $230 part replacement and you can see that it is worth the effort to give the soldering method a shot.

Good luck and message me if you need some help.

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Old 11-29-2003, 06:19 AM
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Thanks Mr. Q. Itīs your thread very interesting.

What do you think about 190 E, 1988?

Would be the same difficulty?


Mario Farias
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Old 11-29-2003, 11:26 AM
Q Q is offline
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I'm just a DIYer, and don't have any knowledge of the 190s. I wish I could help. Maybe one of the techs on the board could verify part similarity.
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Old 12-01-2003, 05:24 PM
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They are not the same part, but are very alike. The procedure should be the same, since all the CCA from that vintage are similar.
1999 Porsche 996 Carrera Convertible
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Old 02-22-2004, 06:55 PM
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Q.. thanks for this awesome post detailing how to repair the amp.

I removed the CCA from my 1988 300SEL (w126) and removed the aluminum casing to expose the board.

The anti-corrosive coating you mentioned, I assume is the coating that gives the board a shiny look? I noticed this coating doesn't go all the way to the end, but close.

You used the term "flux"... can you tell me what that is? I am planning on going to Radio Shack and getting a soldering tool.

Are you also saying that basically all thats needed is to remove the coating... and solder each of the points on the green surface? How long did yours work for? Thanks.. just trying to see if I can fix something before I need to get a $200 piece.

Thanks for your help.. you're an asset to this forum.

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Old 02-22-2004, 09:38 PM
Q Q is offline
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Yeah, the coating is that shiny sort of spilled coke looking stuff that doesn't quite come all the way up to the connector end of the board.

Flux is an acidic sort of gel. It cleans the oxidation from the metal when you heat it up so the solder will stick better.

I wouldn't try to remove the coating. I recommend first heating each joint up one at a time. You will see some of the solder make a retreat as the coating boils away. After I had gotten about half way through my board, I found that the solder would stick better after I had sizzled the coating away first.

If you solder two pins together, don't freak out. You can easily fix that problem. The best thing is solder wick, but fresh fine copper wire works fine too. If you have some speaker wire laying around, you could hold that to the excess solder as you heat it all up. The solder will literally wick up onto the fresh copper.

Just take your time. The worst thing you can do is over heat the board or components. You shouldn't spend more than a second on each pin at a time. Do not keep going back to the same pin if you have a problem with it. Move on and come back to it once it has cooled. Too much heat will lift those pads that the pins connect to, or worse, damage one of the chips.

Oh, one more thing, get a fan to blow the fumes away. They give me a headache every time.
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Old 02-22-2004, 11:51 PM
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I've said this before,

You don't need to add any solder. The board has plenty already... you need to remelt the solder and it must flow out properly on the connection/wire/board. The best flux for electronics is RMA. You can buy something like a gluestick with the flux for $2-3. Just dap a little on and heat the joint with the soldering iron. If it doesn't flowout perfectly try again with a little more flux.

Michael McGuire
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Old 02-23-2004, 12:25 AM
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Location: AL
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I'm impressed. That goo that my CC board was dipped in was so thick, I gave up and bought one from a junk yard.

I did resolder the fuel injection ECU board when it failed. It didn't have all that lacquer crap on it. It worked for about thirty minutes! Again, I gave up and bought one from a junk yard.

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