HELP! A8 TCU (TCM) related Starting Problems
 

Audi A8 3.0Tdi Quattro Sport Auto Tip, 2005 (UK Spec.) – 95,000miles

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) :confused:. 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 :rolleyes:. 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.

If it doesn't communicate with the TCU, then it either has a bad connection at the TCU, or the TCU itself it bad. Its located under the carpet on the passenger's side front on the US spec models, i believe it should be the same for the euro ones as well. Just put the passenger's seat all the way back to its farthest postion, then pull up the door sill trim holding the carpet, remove the kick panel if needed and pull up the carpet to get at the TCU box. Open up the black box and pull out the TCU. Pull off the connector and look for any corrosion on the pins, broken connector etc. If all checks out, you probably need a new TCU.

Thanks for getting back to me. I shall take a look this evening :).

My only other query is that I had tried to source the TCU location previously (hence wanting to make that it was defintely right!!). An independent supplier for Audi printed off the attached image for me from ETKA (number 1 on image)? The part number they gave me was 4E0910156F (& 092927156 for hardware). On their system, this shows up as a 'mechatronic' unit? This confused me a little as the name reference is usually the same/similar to the part you're looking for, but nothing else showed up! It aslo seems to indicate it sitting directly under the gearbox?!?!?!!?!?!

Again, any clarifications on this would be greatly appreciated.

Had a look..........
 

Had a look last night - there is a black box in the passenger side footwell (under the carpet), but this contains a Motorola control unit for the telephone. Looks like the TCU may well be as per the diagram, unless elsewhere?

If its not under the passenger's foot well it is located in the gearbox. I haven't had to deal with any TCU's from a D2 A8 so i assumed it was in the same location as every other tiptronic trans. The CVT trans also has the TCU built into the gear box. Kind of a pain in the butt now. If its not communicating and its located in the trans i would say fluid leaked into the unit.

That could be the case. And, yup, certainly is a pain in a butt location for a TCU!! The only thing is though, why is it tempremental as sometimes it will start?!!?!?!? :confused:

Anyway, I managed to get it going last night. In the past, I had thought the issue was a dodgy battery/lack of charge. Hence gave the battery a top-up (trickle charge took about 5 hours to top it up). On reconnection had the same issue as previously (PRDNS all in red). I then tried to start, but by doing a full left turn, then full right turn of the key (understand that this overrides most of the electrical comms on start??). This may not be healthy for the car, but on the fifth attempt, it fired up!

The car however went into limp mode (no umph when revving engine), gearstick stuck in P. Left it running idle for about ten minutes to warm up. Turned off.........turned on and voila! Was back to normal again :D.

I've got the car booked in with an independent Audi master technician next week. Have been told that there could be a number of nodal connections that coiuld potentially be at fault. Shall have to wait and see what the outcome is.........................!!!