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Tech Article

Audi C5 A6 Quattro 4.2 V8 Transmission Control Module Ruined? Maybe Not ...

Last Updated: 09/24/2017
Parts Group: Transmission control module
Is your transmission control module soaked with water? Normally, this means you're out hundreds of dollars for a replacement unit. But, perhaps not ... is the point of this article. If you're not here for the technical analysis but would rather buy a used unit from us, guaranteed to work and fit, then please select the link below: The transmission control computer on one of my Audi cars was literally submerged in water, yet I got it to work again. Today, I drove my car around for miles and miles, testing the fixed computer. It is indeed functioning well. I’m delighted. A big thank-you goes out to my brilliant tech advisor, who’s worth her weight in gold.

This story begins when a gentleman in Las Vegas owned a lovely silver-colored high-performance, luxury-model Audi A6 Quattro with a 4.2 V8 engine. It’s the type of car that nowadays costs $65K or more at the Audi dealer.

The evidence shows that the gentleman had taken exceptionally good care of the car. Then, there was a fire under the car. My guess is that he drove over a piece of roadway debris that melted itself to the hottest part of the exhaust and caught fire. This ignited some oil that had seeped out from the engine or powers steering over the years and had blown backwards onto that area. The fire burned under the gearshift lever, melting the components there. Someone helpful emptied a fire extinguisher onto the flames, which is how the inside of the car came to be coated with a fine, yellow dust. It’s especially thick on the passenger side floor mat, so my guess is that whoever was wielding the fire extinguisher was standing by the open driver side front door, aiming at the center console and overshooting onto the passenger front floor mat, which was caked thickly with the powder. The powder even got inside the CD player, which now no longer works.

Next, the Las Vegas Fire Department presumably showed up. A friend of mine used to work in a Fire Department. I didn’t get the impression that these folks do much hand wringing as to the effects of putting a fire out; they simply proceeded with enthusiasm. This would explain why the two-inch thick carpet-and-underpad was soaked with water, and why there was an inch of water at the bottom of the floor pan of the Audi.

This is probably a good time to mention that the automatic transmission is vastly complex, and is electronically controlled by a computer module dedicated to the task. The module is safely out of the way of people bumping it, under a thick carpet, under the passenger side foot well, which is a good place for it unless someone happens to empty gallons and gallons of water, right there. And indeed, this is what happened.

Post-disaster, I bought the car for $300 with a clean title. It looked irreparable but now I’m not so sure any more. I’m hopeful. I’ve watched the movie “Seabiscuit” several times and I like the premise that one doesn’t give up on an animal or a car or (especially) a person just because there are some challenges that at first glance appear to be overly daunting.

When I was 20 I considered buying a car that was in such bad shape that my guy friend, who had a very fast and pretty car of the same model, ridiculed the beat-up car. Indeed, it was a sad case. It had structural bodywork problems, and the dashboard was structurally trashed, too. Even so, I bought the car and methodically fixed every problem, fundamentally and well. I ended up with a very pretty car that, ironically, was faster than my guy friend’s car, in street races.

Similarly, some of the most delightful people in my life are individuals who at the time were convinced that they were unlovable or deeply flawed and yet, as we spent time together, they ended up evaluating themselves much more happily. In turn, they also enriched my life. Every time, I was glad that I hadn’t just gone with the superficial assessment.

So it is with this Audi, too. The most difficult-to-salvage part is very expensive: the transmission control computer. I can buy an entire transmission for the same price as I’d have to pay for a replacement transmission control computer. It was with great dismay that I saw the transmission control computer emerge from under the soaked carpet of this car. It had been sitting a the bottom of a minor lake, inside the car. Water literally dripped out of the computer. Even worse, someone had previously removed the watertight protective case from the computer, which is really bad luck. It’s sort of like disabling the air bags right before having an accident.

Fortunately, I am friends with brilliant people. One of these graces me with her technical advice, and she advised that I buy some chemically pure acetone (as in, hardware store, not cosmetics aisle). Next, I was to buy a new toothbrush and some chemically pure (as in distilled) water, and then make a mix of 65% to 75% acetone, with the rest being distilled water. Far away from spark or fire, with the computer main board in a bath of this blend of fluids, I’d then gently brush the electronic components with the new, clean toothbrush -- that will never be used on human teeth again in the future. I’d do the bath and brushing for 30 seconds or so. And yes, I’d wear nitrile gloves.

Afterwards the part gets removed from the bath and then it needs to be dried thoroughly, using Nevada desert air (which is what I have, where I am located) or using a desiccating box that one can construct with iodized salt and/or rice, and an electric fan to circulate the air. Yay for Nevada! I took the simple approach. A day later (probably sooner, but I wanted to be safe) the main circuit board was bone dry.

I assembled the computer and installed it in my black Audi A6 Quattro. It was a tense moment. The computer has so much control over the transmission that it’s on principle possible for the computer to give bad commands that could cause my $2,500 transmission to self-destruct. But, I was brave, and I proceeded. Result: the transmission worked perfectly. Park, reverse, neutral, drive … downshift, upshift, manual control, sport mode … everything. Wow!
I asked my brilliant tech advisor why this worked. She explained: "Water usually isn't pure. Pure water actually has no conductive ions, so by pouring pure water on electronics nothing happens. However minerals mostly salt has the ionic structure to carry electrons. So when contaminated water touches circuitry, it creates a short [circuit], the electricity passing through will generate an electron field which heats the salt or ions, causing it to burn to the board, the water will evaporate but the salt leaves an electrolyte trail that keeps it shorted, the acetone washes away the ions and electrolytes and cleans the short [circuit]s away, the water is to avoid wasting acetone and avoid etching the chips. Acetone only works if the host chip isn't damaged and it's just a minor short [circuit]."

I’m impressed! But wait, there’s more. Last year, I’d bought another such Audi from a gentleman from a rainy part of Oregon, who explained that the original transmission control computer had gone bad, so he’d replaced it. The bad computer had been tossed behind the passenger seat, on the floor somewhere. I took the gentleman at his word that the transmission control computer was bad, but I couldn’t bring myself to throw it away.
I wondered if, being from a rainy climate, the transmission control computer wasn’t perhaps also water-damaged? I decided to try the acetone treatment on it too. It worked!! I tested that in my car too, and the transmission behaves perfectly, in every way, mile after mile. I’m delighted.
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