I’m inspecting various Audi C5 A6 Quattro cars to learn more about the differences in their transmission control modules. I have seen many forum post opinions as to what belongs where, but I prefer to know things for certain from direct observation. That’s why I like to look at actual, physical cars and see in person what exactly is installed, and where — and how original that part seems likely to be.

In both of the two 4.2 liter cars that I inspected, the transmission control module part number starts with 4B0927156 and then has a two-letter code behind it, either BS or CT. The BS letter codes was observed in a 2000 model cars as per the VIN, and the CT letter car was observed in a 2001 car as per the VIN.

The module with the BS-code was in our project car. It was clear to me that someone had already replaced it because it had an eBay seller name written on the aluminum case with pink paint-stick paint, in several places.The rubber seal on the black plastic case that contains the module was missing. Attempts to contact the eBay seller so as to learn more … have so far yielded no results.

There is the possibility that the module with the BS letter code, i.e., the module that was in our project car, does not have the same letter code as had been in there originally. For all we know the original module had a different letter code, and the then-owner had decided to buy the module with the BS letter code on it instead, hoping in vain that it would work just as well. According to some forum posts, modules with different letter codes are NOT interchangeable.

On the car with the module with the CT letter code, the module might well have been the original. The black plastic case that contains the module normally has a rubber seal all around. The seal was there and seemed to be pristine. The location of the part seemed undisturbed.

On both of the 4.2 liter cars, the black plastic case was under the front passenger footwell directly below where the feet would be, in an indentation that seemed to be in the floorpan bodywork. The case is small with snaps all around, with an impressive-looking rubber seal all around. The case appears to have two vent holes at the bottom. My concern would be if those holes allowed water to leak inside the case and damage the module. According to some forum posts, such leaks has been known to happen. However, these are not vent holes; they are part of the casting for the two Phillips screws  that attach the TCM to the protective black plastic case.

On two of the 2.7 liter cars that I inspected, the black plastic case instead had a large hinge on one side, and the entire case was larger than on the 4.2 liter car. Here is a picture. The larger case, the one at the top, is from the 2.7 liter car:

The top half of the case for the 2.7 liter car can detach at the hinge from the lower half.

The part numbers for the case for each car can be seen only when the transmission control module has been removed from the case; it is on the inside of the case. For the 2.7 liter car the part number is 8D0-927-355-A. For the 4.2 liter car the part number is 8D0-927-355-B.  The last letter is allegedly a revision code, so the simple, smaller case is allegedly a revision of the more complex, larger case.

On a 3.0 liter car that I inspected, I could find no black plastic case under the carpet of the front passenger foot-well area.

There’s quite a diversity as to these parts.