Gearbox/TC?

[MountainMutant]

Been out for a drive this afternoon without the family on board so time for a bit of sport mode and the paddles.

Had a few occasions where the car on hard acceleration changes to the next gear but then nothing. Revs rise as if in gear but no more power.

Is this the dreaded gearbox failure or maybe the TC?

No errors showing up.

Most of my driving is done at a far more sedate pace so not noticed it before.

Sods law to get problems when I'm thinking of getting a C7!

MM

[HPsauce]

I had very similar behaviour on my S8, which I think is basically the same gearbox.
After about 2 years of chasing down an intermittent problem, and changing loads of bits and pieces trying to figure it out, I nailed it!

It was a misadjusted brake pedal switch!

Probably the same setup on a C5 as a D2; the switch has two separate pairs of contacts inside, with a tiny, tiny difference between them. You can monitor them with VCDS, which is how I finally got to figure it out.
One switch basically operates the brake lights, the other mostly informs the ECUs that the brake pedal is pressed. And that latter signal can arrive without any brake lights appearing or actual brakes operating.

In my case a tiny adjustment (screwing the switch in the relevant direction in its mount) ensured that both switches were "off" unless I was very definitely starting to press the brake pedal. Beforehand it was on a hair trigger and sometimes the ECU thought the brakes were on without any relevant action by me or brake lights coming on.

In that mode engine power is severely restricted though the gearbox will eventually upshift on its own; typically at higher revs.
I think it may be a variation of (or related to) the hill-descent logic which does things to gears/throttle when you are braking but not losing speed.

[MountainMutant]

Where is the switch HPsauce?

Problem seemed to occur in higher gears/speeds.

[HPsauce]

I don't know exactly on a C5, but typically (as on a D2) it's mounted above the brake pedal lever with a spring-loaded pin pressing down on the lever.
So when you press down on the brake pedal the pin moves out of the switch body. It's adjusted by screwing the switch body in and out of its threaded mounting bracket.
On a D2 you have to remove all the trim below the steering column and some tough ventilation ducts behind that to get at it.

Of course that may not be your problem. But VCDS can be used to monitor the switch, so you can see how "hair trigger" it is quite easily with physically getting to it.
In my case the slightest touch of the pedal could change its state.

It's a plastic pin pressing on a metal lever so over time I guess can wear down slightly. And IIRC the screw adjustment didn't have a locknut.

And of course there are no error codes because it's doing exactly what it's designed to do.

[Ben_r1]

Relax, I think if it were the gearbox struggling to engage gear you’ll get the warning light first time.

[mavada]

https://www.flickr.com/photos/surgenor/ ... 52/detail/

[MountainMutant]

Thanks Mavada
So we think this is the problem then? Common fault?

Does the steering wheel need to come off? Don't usually work on my car but if the trim comes off ok worth a go.

MM

[HPsauce]

I think that was just posted as a guide to finding it.
IF it is the potential problem (USE VCDS to test sensitivity!) then just adjust it to be less sensitive and see what happens.
I very much doubt the steering wheel needs removing just to get some lower dash trims off.

[Daveperc]

If you are fairly small and flexible then you can do it without removing any trim.

Pretty common fault - also affects cruise control etc.

Dave

[MountainMutant]

So just to confirm before I have a look does the switch need changing or just screwing in a bit?

I'm not small and flexible so will have to remove the trim!

[bilko1]

I'm not convinced it's adjustable, you just turn it 90 degrees while pushing it in to the bayonet type fitting and it sets itself. There is no adjustment as such.

[HPsauce]

It certainly looks different to the D2 version, so maybe it isn't adjustable?
I'd still use VCDS to test it first, unless replacement is very cheap and easy.....

Read this too; looks to be very specific how you install a new one! http://forums.vwvortex.com/showthread.php?1158315