Following this thread, my 61K mi Cayenne S started the rough shifting/thumping others have experienced. Cayenne has been used to trailer the race car. Since I was second owner and out of warranty dealer said Porsche will likely be no help. I replaced the oil myself and immediately noticed a improvement if not total fix. Fluid was very black and did not smell like new. Super EZ to replace except for apprehension on torquing down the old fill screw due to others who have cracked the case over torquing. I never got to actual torque rating but did seal the bolt with Red rtv. Fingers crossed

 

Asked the dealer to check for low boost , jerky acceleration and cruise control does not hold speed under hills/load condition. it turn out to be the TC so
Add this one to the list , TC . vent line and some hardware were replaced today Jul/25/2018 under warranty , (updated TC part # 958-341-012-DX) . will pick it up on Friday.

 

 

that is correct , not sure when this version started but will ask tomorrow , also there were none available on the western CAN/US this TC came from Ontario/Canada.
the vent line part # is 958-301-047-01 not sure if it the same as before?

 

 

For what it's worth, the -10 variant shows "Discontinued" on the Sonnen website while the -01 shows available.

 

 

I'm giving thought to having the dealer install the vent kit on my existing XFerCase.. since they seem to feel it's working fine (I can debate this - the once in a while clunk under very light acceleration when the transmission shifts from 3-4 makes me think it might need attention. I think I'll go reset the adaptations again..) Anyway, I'll be curious how much they'd want to add the splash shield to the front output and the vent kit. Wonder if it has to come out to do that..?

 

 

I think I'll get an estimate for it.. see if it seems worthwhile.

 

My bad, looks it is a new vent line # 958-301-047-11

 

Hi Guys! I'm curious: If there is no tire slippage during acceleration, what's the theory as to why failing clutch packs would cause thumping of the drive train and/or surging of the automatic transmission during acceleration?

Is the surging seen during acceleration a result of the transmission control software compensating for some variable by excessively slipping the torque converter?

What about the thumping? If there's no rear tire slip, since this is a hang-on driveline system, why would a change in the clutch pack locking rate produce a felt thump when all wheels are rotating at the same speed?

 

From my understanding of the system, it seems to me that the clutches in the transfer case don't slip as they should while it divides the power between the front and rear axles. This will prematurely wear the clutches, and put sudden loads on the chain drive that it shouldn't be handling. Seems this could be poor quality of components or a calibration issue. As far as the noises and what's felt, it seems like when components are too far worn to be compensated for, it will cause it to behave that way. I can't say for 100% certain that this causes it, but that's what seems to be the likely cause... I may be mistaken, though.

 

The only theory I have is that the transmission ratio between the two sprockets run by the chain is 1.12, which seems like a very small difference but maybe that is enough of a speed difference to cause a driveline thump if the clutch packs do not properly/smoothly engage when the acceleration event is going on which is a period of increased clutch pack torque. In my mind, that might account for the thumping even during times of equal front and rear axle speeds but I still wanted to inquire here on other theories by those that have thought about this.

And what about the surging during a transmission shift when F/R axle speeds are equal? Transmission and/or engine feedback due to unexpected changes in driveline torque during a shift, kind of like a PSM intervention acting on engine torque?

 

Just wanted to add some feedback that I found out recently.

Been speaking to a couple of independents and a local Porsche center (the techs themselves) about the drivetrain and the various issues the 958 has. My car has had the TC replaced and I am paranoid about having it go bad again (out of warranty unfortunately) and I was keen to figure out preventative measures. Clearly regular checks on the fluid and the updated vent tube etc is a great thing to do. But one thing that I noticed before here (can't find the post), but its worth raising up again though - everyone seems to be focusing in on tire wear and consistency. Everyone I spoke to seems to think that some of the drivetrain issues 'could' be related to uneven wear of the tires. Seems that its very sensitive to uneven wear and this can cause excessive wear on key components in the drivetrain as a whole (including the TC). I know the common advice from Porsche centers is to replace all 4 tires and brakes at the same time, but as the cars age and pass to second and third owners, many don't do this which could accelerate the deterioration of the components.

Is there evidence on this? Not really, but worth considering if anyone is suffering or worried about the TC (and differentials) - best to replace tires all at the same time and keep things consistent. If anyone has anything specific though - please do share. But this seems to be the advice that I am getting.