Strange DSG behaviour - technical help needed!

Hi all

New to the forums but been lurking for a good while and having exhausted Google and other options I thought I'd run this past the forums. Hope that someone can offer some advice! I've read a lot of DSG problems but none that are quite like this - I'll try and be as detailed as I can without going on too long. Any questions just shout!

Car

Leon TDI 2.0 DSG Sport / 2007 / 68k
Purchased January this year

History and Symptoms

The car was fine until a speed sensor went, where the car would stick in too low a gear. Before too long though, the ABS, ESP and TP lights would come on and the gear selection would sort itself out. Speed sensor was replaced under 60 day warranty and the car was fine. The warranty has since run out

Hit a bad pothole about 6 weeks ago, and the car stopped changing up from 4th even up to 75mph. Sometimes it would, but most of the time accelerating on the motorway it would just stay in 4th. I could switch to manual and up to 6th fine. Changing back to D would drop the car back into 5th.

This has got steadily worse, with the car changing down when it shouldn't (down to 2nd or 1st while approaching roundabouts!) and sometimes sticking in 2nd right up to 3500rpm before changing up.

Been driving it in manual without any problems. Gearbox feels perfect when I'm in control.

No error code has been logged, but yesterday on a long journey, I hit a pothole and the ABS, ESP and TP lights came on, suggesting another speed sensor is away. It only registers the error at or above 60mph.

Work carried out

I've had the car into a good VAG specialist for diagnosis. Due to the lack of errors logged we've gone by a process of elimination. So far we've tried:

- New Air Mass Meter
- DSG Reset and training drive
- Transmission oil and filter change.

Nothing has helped. This has brought the tech to the conclusion that I need a new mechatronics unit. Arrgghh! Worst case scenario...

I decided to see if ECU Testing could help and emailed them the symptoms. Roger there doesn't think it's the mechatronics but I'm waiting back to see if he has any further thoughts.

Obviously now we have an error code (likely a speed sensor) but I can't get the car back until next week to confirm. In the meantime...

Questions

- Could a speed sensor be failing for some time before throwing the error?
- Could the speed sensor have been confusing the gearbox ECU and causing this problem, even without an error logged?
- If it's not the mechatronics unit, what else could it be?
- Anything else we should rule out before we take the plunge and shell out £1000 for a new mech unit?

Thanks a lot in for reading this - hope someone can help!

Roki

Update

OK, some more details to add to this - the plot thickens!

As with my previous speed sensor failure, when the error triggers and the ABS, ESP and Tyre Pressure lights come on, I can pop the car into D and the automatic works perfectly! This makes me think that this is not a mechatronics issue, although my mechanic still disagrees.

The car has been in the garage for a couple of days as it hadn't logged any error codes and it's taken them a while (and maybe a fair bit of test driving) to find the error. The error is relating to a bad rear wheel bearing, although I don't know the specific code.

So, tomorrow it's going to get this work done. I'm not sure if they're going to replace the speed sensor at the same time, but I might push for them to do this anyway as it's a cheap enough part and they'll be in about that area anyway.

Is anyone else following my logic here or am I barking up the wrong tree?

Fingers crossed.

I've just read in a couple of Golf R32 road tests that "The DSG also uses the information from the ABS sensors to avoid changing gears half-way through corners". I guess it's possible the car thinks it's cornering and is refusing to change up because of that. Down-changes use different logic and work fine.

I'll post an update tomorrow.

Sorted!

Picked up the car this afternoon, and it's sorted!

Basically, the sensor ring on one of the rear wheel bearing had corroded to the point where it had cracked and broken the abs sensor. This must have been deteriorating over the last 6 weeks or longer, and despite being totally goosed it still wasn't consistently throwing the error.

Both parts replaced, and despite continued pessimism from the mechanic the car is now back to it's former self. Every day's a school day I guess! Hopefully this thread will serve to save someone else from this detective hunt!