Gearbox ECU Problem

[RS6Rottweiler]

I am having problems with my gearbox selction and this has been traced pretty much to the ECU being the problem? The ECU was sent off for repair twice but the company claimed there was nothing wrong with it? However, It was better for a while each time it came back but soon deteriorated back to what it was. One thing I have noticed is that it is better in warmer weather and also as I work away from home a lot when I get back the battery is pretty run down. I noted that when the battery level is low the ECU is terrible but after a long run or battery recharge the ECU works great. I know that all the connections are good so no corrosion or damp issues but I wonderd if anyone knows if there is a voltage regulator in the electrical supply to the ECU and or if the ECU is voltage sensitive and this could be part off or the cause of my ECU problem?
The cost of a new ECU is £907 + vat with none available in the U.K. so not sure when one would be available from Germany, I believe Bosch make them plus it's a lot of money to find it may not actually be the ECU that's at fault? I also looked to see if anyone else manufactures the C5 gearbox ECU but so far have not come across anyone and looking at second hand units seems prices are stupid starting at approximately £450 on eBay but you don't know what your getting or how long this ECU would last, are you just buying someone else's problem, its a big risk at these prices?

Has anyone had similar issues with their gearbox ECU and if so what did you do to resolve it? Or, does anyone have any thoughts, ideas, suggestions based on what I'm finding with my ECU regarding voltage etc that would be worth checking out? Anyone know of an alternative manufacturer / supplier or a reputable second hand places to go if I need to buy a replacement ECU? Any help would be greatly appreciated?

[HPsauce]

What symptoms exactly and what codes are showing on a scan? Maybe it's actually your F125 switch?

[srichards]

I have had a few gremlins. Battery is a good place to look. I'd probably get a new one and keep it topped up when you are away.

[ahoooga]

Check for the 'year of manufacture' on the battery, if its older than 5 years just get a new ( decent) one.

[mavada]

Its gearbox TCU...
Ive send mine to Viper Tuning some years ago, great service, Isle of Man.
http://vipertuning.com

[TechNick]

What symptoms exactly and what codes are showing on a scan? Maybe it's actually your F125 switch?

^my initial thoughts too
...although my own experience is that the F125 makes itself known via fault codes quite verbosely, so would hope that it would have been spotted by now, if it were the cause

[RS6Rottweiler]

Im away at the moment so don't have the actual code but I know it is something to do with pressure and solenoids. The garage did a swap over of wires on the solenoids and the fault did not move so this links it to the ECU apparently? Symptoms are when I select drive or reverse it jumps into gear and then drive is like a constant drive requiring more revs to drive the car and there is no gear changes. I replaced the battery about a year ago so it is good. Not sure what function the F125 switch does but it's never been mentioned to me but I can ask the question?

[HPsauce]

Sounds more like a fault in the valve body then; they can be reconditioned. Presumably fluid level has been checked and is OK?

[srichards]

Mine did similar and it was the Transmission range sensor which I think is the same as the F125 switch. It changed gear ok but thumped into reverse and would do the odd thumpy change. it seemed to be acting like something was slipping. I was wondering about to taking it in and then the PRND flashy thing started so it went in immediately.

[Shoppinit]

It does sound like a bad TCU.

[HPsauce]

Mine did similar and it was the Transmission range sensor which I think is the same as the F125 switch.

Yes, same thing, different names.
They do cause very odd behaviour without throwing much of an error because they are simple electromechanical devices with no electronics inside, but are subject to wear and sit in a very exposed position.

[chunky79]

The garage did a swap over of wires on the solenoids and the fault did not move so this links it to the ECU apparently?

If the fault didn't move, it's box fault. If the ecu was faulty the fault should move.

[RS6Rottweiler]

I managed to find the original fault code - Fault Code: 18243 Pressure Control Valve 5. P1835-35-10 Short to Plus Intermittent. Originally the wiring loom was thought could be at fault due to going brittle but as stated the wires were swapped over and the fault did not move so they don't think it is the loom?

ECU was sent away and they could find no fault but was better when returned and worked for a while, sent away again and they returned it saying no fault and was no better?

I'm not sure what the F125 valve is but it has never been mentioned to me by the service centre?

New battery but I have noticed a marked improvement with the ECU when the car has been for a long run and the battery is fully charged. I work away so car sits for three weeks at a time? Not sure if there is a voltage regulator to the ECU or if one would help with this issue?

All connections checked, no corrosion visible but I have not looked at this myself as I have heard others with this problem? The ECU again does get worse when it is cold and damp>

Anymore thoughts out there?

[Shoppinit]

ECU or TCU?

[Shoppinit]

I'm assuming you mean TCU.

It sounds like a dead IO on the microprocessor in the TCU. Could be a bad solder joint on the TCU.

In the former case, you need a new TCU. In the latter, you might be able to re-flow the solder on the pcb but it's a Hail Mary solution. I fixed an S8 ECU this way. Even surprised myself when it worked.

If you can find a fellow RS6er in Fife, maybe they'd be kind enough to swap TCUs with you briefly to see if the problem goes away.