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Tech Article

Audi C5 A6 Drowning a Transmission Control Module

Last Updated: 04/14/2017
Parts Group: Transmission control module with protective case
One reason for the ZF 5HP-24A transmission failing in an Audi A6 Quattro with the 4.2 V8 is that the Transmission Control Module (under the passenger seat footwell carpet) gets an unwanted immersion in a puddle of water that forms underneath the carpet. I’ve heard about it but haven’t seen it -- until today.

Granted, it wasn’t a 4.2 but a 2.7 Turbo, but the cars are very similar. As I recall, the Transmission Control Module for the 2.7 car isn’t under the passenger seat footwell carpet, but I checked just to make sure.

As I cut away the carpet, a soggy mess greeted me below the carpet. The stench had the smell of old, stale water ... whatever was there had been there for a long time. There was certainly enough water to have seeped into the enclosure of the Transmission Control Module. Unless the enclosure was waterproof, this could hardly have failed to cause problems. From the analysis I’ve performed on these enclosures, they don’t seem likely to keep water out to that extent, even though they are sealed to some extent.

Conclusion: as part of diagnosing a transmission failure, I plan to take a peek under the passenger seat footwell carpet. Unless it’s bone dry, I’d suspect the Transmission Control Module. Not that replacing that might cure every problem; this transmission is a very obedient follower of the electrical commands it gets from that the Transmission Control Module.

If these commands were to cause the wrong solenoids to energize at the wrong times, then as I recall the schematics there are at least two scenarios in which the transmission could essentially use one clutch to drive the other that’s locked up. So a bad transmission control module might have caused transmission damage so both might need to be replaced.

Perhaps ZF and Audi thought of that and built in a safeguard -- but until I know they did, I’m wary. I’ve heard of at least two stories where someone swapped out a failed transmission for a supposedly good one, and yet the problem didn’t get resolved. One of those stories, I heard in person, and it was a sad tale.

It seems to me that swapping out the transmission without taking a good look at the Transmission Control Module might be compared to treating a burn victim who’s still sitting in the fire.
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