TC flashing & no power with 3rd-2nd downshift on throttle (S-tronic)

[PassTheMayo]

Hello guys, first post on this forum. I've been experiencing an issue with my 2016 S4 for a few weeks now. I've had the car for about 4 months but just recently I have started to really push the car sometimes. The S-tronic shifts so smooth and crisp, except every now and then.

In some circumstances (not every time), letting off the throttle, downshifting from 3rd to 2nd and pressing the throttle causes the car to go into a "limp mode" where there is absolutely no power until you let off the throttle and press it again. While this happens, the traction control light also starts flashing. I have a theory that it goes into this limp mode because the clutch hasn't finished engaging and it doesn't want to risk damage, but I am not entirely sure. This issue has happened a total of 3 times now, all from the same steps to produce it.

I've also found this thread which also has a similar issue to me, except the traction control light does not flash in their case and my transmission never seemed to re-engage until I lift off the throttle. There is also no hard fault codes stored but I do not have the VCDS software to do a full diagnostic. Does anyone know what could be going on here?

 

[ninjag]

Does it even happen when in Drive or Sports mode?

 

[Dippy]

Limp mode is usually persistent and would leave a DTC. The interaction between the TCU and the ECU as you describe is normal operation regarding clutch closure. If you have loss of power when TC is applied this is also normal. These cars can put a lot of torque down and sometimes the tyres can't cope. Are you sure this is not the problem?

How many miles done and when was the last gearbox service?

 

[PassTheMayo]

Does it even happen when in Drive or Sports mode?

Every time that it’s happened, I’ve been in sport mode while in manual shifting. I haven’t experienced this in drive.

 

[PassTheMayo]

Limp mode is usually persistent and would leave a DTC. The interaction between the TCU and the ECU as you describe is normal operation regarding clutch closure. If you have loss of power when TC is applied this is also normal. These cars can put a lot of torque down and sometimes the tyres can't cope. Are you sure this is not the problem?

How many miles done and when was the last gearbox service?

I am fairly confident that it’s not a traction issue because the engine did not try to produce any power whatsoever before it went into this weird limp mode. The car is currently sitting at 40,800 miles on the clock and the gearbox was last serviced at 35,600 for a fluid flush.

 

[ninjag]

Every time that it’s happened, I’ve been in sport mode while in manual shifting. I haven’t experienced this in drive.

I'm wondering if it would be possible that there's some weird interaction between it trying to fall back into automatic [sport] mode from the temporary manual override when you are using the paddles? Does it happen when you are in full manual with the paddles of flappiness?

You mentioned that it's not always a smooth shift, could it be possible that the wrong fluid was used with the DSG service? It would be easily done for gear oil to be used accidentally. Or maybe underfilled? Might be worth doing it again for all the cost of c.£250 and closely inspecting the fluid that comes out. If it's only happening between two specific gears, could a gearbox fluid change be worth doing and inspecting the fluid? It would be c.£180 and you need a jack to take the weight of the transmission when removing the x-brace but an easy job.

Have you got the aluminium billet fitted in the transmission mount? That solved my occasional jerky shifts, especially when going into 1st gear.

All that aside, I would get a a full diagnostic done first just.

Hello guys, first post on this forum. I've been experiencing an issue with my 2016 S4 for a few weeks now. I've had the car for about 4 months but just recently I have started to really push the car sometimes. The S-tronic shifts so smooth and crisp, except every now and then.

In some circumstances (not every time), letting off the throttle, downshifting from 3rd to 2nd and pressing the throttle causes the car to go into a "limp mode" where there is absolutely no power until you let off the throttle and press it again. While this happens, the traction control light also starts flashing. I have a theory that it goes into this limp mode because the clutch hasn't finished engaging and it doesn't want to risk damage, but I am not entirely sure. This issue has happened a total of 3 times now, all from the same steps to produce it.

I've also found this thread which also has a similar issue to me, except the traction control light does not flash in their case and my transmission never seemed to re-engage until I lift off the throttle. There is also no hard fault codes stored but I do not have the VCDS software to do a full diagnostic. Does anyone know what could be going on here?

 

[Dippy]

If this happens at relatively low revs and torque then I agree with @ninjag - a proper diagnostics scan is needed.

 

[PassTheMayo]

I'm wondering if it would be possible that there's some weird interaction between it trying to fall back into automatic [sport] mode from the temporary manual override when you are using the paddles? Does it happen when you are in full manual with the paddles of flappiness?

You mentioned that it's not always a smooth shift, could it be possible that the wrong fluid was used with the DSG service? It would be easily done for gear oil to be used accidentally. Or maybe underfilled? Might be worth doing it again for all the cost of c.£250 and closely inspecting the fluid that comes out. If it's only happening between two specific gears, could a gearbox fluid change be worth doing and inspecting the fluid? It would be c.£180 and you need a jack to take the weight of the transmission when removing the x-brace but an easy job.

Have you got the aluminium billet fitted in the transmission mount? That solved my occasional jerky shifts, especially when going into 1st gear.

All that aside, I would get a a full diagnostic done first just.

It’s possible that it’s falling out of drive when the transmission moves under load. At the time of the problem, I was in dynamic and sport mode while in full manual mode. I did actually install the 034 Motorsport billet transmission mount insert a few days ago and it has dramatically improved the feel and shift times of the transmission. I’m thinking it has to do with how much the transmission had moved before I installed this new insert. I haven’t had the issue since then but I’ll keep you guys updated. Might not be worth it to do a VCDS scan if I feel the new insert fixed the issue.

 

[PassTheMayo]

If this happens at relatively low revs and torque then I agree with @ninjag - a proper diagnostics scan is needed.

When I downshifted into 2nd, quite aggressively I will say, I was at about 4500-5000 RPM at the time of pressing the throttle. The new transmission mount insert I just got seems to have fixed the issue but I will keep you guys updated.