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Tech Article

Audi C5 A6 Quattro 4.2 V8 Transmission Control Module 4B0927156BS: Buying or Renting: A Better Business Model?

Last Updated: 09/24/2017
Parts Group: Transmission control module
If you’d prefer to skip the technical analysis and simply buy the part, in good used condition, please use this link.

This article is about the value of having multiple identical parts to swap out as an aid in troubleshooting. It describes some testing that I did a few days ago, while driving my 2000 Audi A6 Quattro 4.2 V8 (the model with the ZF 5HP-24A transmission) through a semi-deserted neighborhood hereabouts, using (in succession) three separate transmission control modules.

I already knew that, with any of these transmission control computers installed, the car drives well. It shifts into every gear but I was looking for subtleties beyond the basic functionality.

Downshifts were buttery smooth, but I found that if I cruise at just under 30 miles an hour, and then I gently accelerate just beyond that, there’s an upshift and in that car, I wasn't super happy about the smoothness of that upshift, sometimes. Occasionally it was very smooth, other times just sort-of-smooth, other times not smooth.

I wasn’t sure if the cause was in the transmission or in the transmission control computer. Fortunately, I had the benefit of being able to take out the transmission control module and installing another one, within just a few minutes.

I found it very useful to be able to do this swap so quickly and to be able to change just this one variable, to isolate the issue.

As it turned out, the car behaved identically regardless of which of the three transmission control computers was installed, so the principle of Occam`s Razor would suggest that all three of the transmission control computers are probably fine.

There was another benefit too, of having multiple identical parts: while removing the first transmission control computer, I noticed that it was quite warm to the touch. That led me to wonder whether that temperature was normal vs. too warm. It was very useful to be able to install another unit and go driving for the same amount of time, and then feel and compare its temperature to the first unit.

Buying a used part has the benefit of saving money, but with a new part, there is often additional value in being quite certain that the new part is good. With most used parts, if it’s bad you can send it back and get a refund, but it would be nice to be able to trust the used part enough to utilize it for troubleshooting too.

That’s why I’m taking the approach that, before I mail you a 4B0927156BS transmission control computer, I make a video of that actual, individual part working in an actual driving, well-functioning car, so that you can have a high level of confidence that it’ll work when you receive it. Even so, I understand that shown-to-be-good isn't quite as certain as known-to-be-good.

In case you mainly need a unit for troubleshooting, I also rent these units. The process works as it does for buying but then you have 30 days in which to use the shown-to-be-good transmission control module to help troubleshoot your car and perhaps rule out that the transmission control module is the problem. If this saves you an unnecessary transmission swap or rebuild, great.

If you send the part back, and within 30 days of us sending it out, we receive it back, then we’ll re-install it in our test car. If it still works and it’s still sealed and undamaged, you get a $280 refund. In effect, you’ll then have paid $120 to rent a shown-to-be-good transmission control module.
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