A8 TCU (TCM) related Starting Problems

[Satty B]

Hi there, my beloved A8 has been giving me quite a headache over the past few weeks. Intermittently (now more often than not), it simply won’t start (PRDNS all show up in red) :zx11:. Have checked with VAG-COM, which consistently points to the ‘no signal/communication’ with the Transmission Control Module on a number of fronts (ie start authorisation/engine etc). If and when it does get going, it runs like a dream and all fault codes disappear . :o

I have attached the VAG-COM log for anybody who would like to/coulreview.

In the past I had thought that it was a power issue – on topping up the battery and reconnecting, the car would fire up first time, but wasn’t guarantee on how long it would last (never broke down once started, always the following day/few days later). I’ve had this checked out, and have been told the battery/alternator are fine. This may have just been a coincidence, or somehow I may have been resetting the problem? Oil and filter for the gearbox has also recently been changed (well after the problem started). Have also had the odd day where it won’t fire up in the morning, but later on in the day, it has started!!

My query is this, could this be just faulty connections with the TCU (TCM)? More importantly, where exactly on the car is the TCU located so I could check?!?! The worst case obviously is that the module may be faulty and need replacing, is this a common issue? A brand new one is in the region of £1400……..

Any help/thoughts/pointers on this would be greatly appreciated.:beerchug:

[john14335]

Sadly you're not alone, my 3.0TDI D3 has the same fault - the intermittent nature of it is a pain in the posterior. I know it's been a while since your post so maybe you've already found the solution but my car's booked in on weds to (hopefully) sort the problem. I'll update the post when i know the result

[Satty B]

Hi there. I took my car to a very reputable Audi specialist who initially though the starter relays may partly be at fault. This fix seemed to the do the job for 3 months until the problem reoccured. Turned out in the end it was the TCU that was damaged and had to be replaced (special order part). Not a cheap fix (about £2k), but since then (touch wood) I've had no more problems with the car. Hopefully yours is a different issue!!

[john14335]

Hopefully it isn't the TCU - the car currently is showing the airbag light due to a faulty seatbelt pretensioner which my dealer assures me is causing the airbag system to send incorrect signals to the rest of the car's control modules. The pretensioner will be replaced on Weds so hopefully all will be well!! If not, looks like Christmas will be cancelled :beerchug: