kino1161

While driving in the GTS this weekend and while accelerating from a traffic light, the truck, all of a sudden, had a hard "thump" as if I had shifted from auto to manual or something. For comparison, it felt more like someone had it the truck from behind. The truck continued on driving as normal after that for the remainder of the weekend. No CEL was activated or anything. I changed my coils and plugs about 8 - 10K miles ago. What could have caused this?

08 GTS
91K miles

Andy E.

Assuming that you have Tiptronic, you may have inadvertently shifted into 1st while stopped; using the poorly placed steering wheel Tip buttons. Only once you release the brakes will the transmission downshift; making for a thump as you described.

The placement of these buttons leaves a lot to be desired as I've done this too many times in the past. It may be attributed to my thick fingers....

oldskewel

I've noticed a very rare thing like the OP mentions, in my 2004 S.

So if I want to rule out your suggestion here, I'd like to recreate that one on purpose to see if it's like the one I'm concerned about.

To do that, is it as simple as: stop (keep foot on brakes), tiptronic downshift command to 1st gear (from the normal 2nd), release brake and start driving away?

I've not tried that yet, but the clunk I'm hearing (rare, like once every couple of months) does not seem like it could be created with inputs of any type unless there is a malfunction somewhere.

kino1161

Andy, thank you, for your response. I have a question though. If I inadvertently shifted to 1st, why would the downshift be so violent (for a lack of a better word). :-) It really felt was like I was hit from the rear.

Andy E.

How hard/fast did you accelerate from a dead stop? Do you heel-toe or use two feet when driving? The system will not downshift into 1st at a complete stop with the brakes depressed; regardless of what your instrument cluster tells you (with respect to the selected gear that you've actually engaged).

After releasing the brake pedal, the system will then immediately downshift; provided of course that the downshift was initiated within a few seconds prior. In the automatic mode, the system will allow for a temporary downshift or upshift via the steering wheel buttons, after which it will resort back to full auto.

Once again, I am using my personal experience driving my '09 GTS. My experience was a thump, but not to the point where I felt that I was rear-ended.

kino1161

Oh. Ok. It was hard thump (jerk). Thanks, for your help. I am curious to see if anyone else has experienced this and what rectified it but the truck continued to run as usual thereafter.

Petza914

I think of 2 potential things. One is that unless the vehicle is in Sport mode, it always starts in 2nd gear, which drives me nuts. First thing I do when I get in the Cayenne is hit the Sport button, then the Normal suspension button. This drops the car to Sport height, enables the 1st gear start, then makes the suspension the normal stiffness. If you weren't in Sport mode and it started in 2nd, but then decided that because of load or your throttle inputs, it needed to be in 1st, it might have done a downshift, and because you were in the gas, it felt abrupt.

If you were stopped on an incline at the time, the vehicle might have entered Auto-hold mode to prevent it from rolling backwards when you transition your foot from the brake to the gas. I've noticed there is a little resistance right before the disengagement of the feature and that could be what you felt. If you combine Auto-hold and an abrupt 2nd to 1st shift upon departing, it could be a decent jolt as you described.

kino1161

Understand. Thank you, for your input, Petza. I hope it doesn't happen again. :-)

Petza914

If it does, make a mental note of things like if you were in Automatic shifting mode or manual, were you on an incline, how much throttle was applied to pull away, etc

kino1161

Will do and thanks for your help.