khowell

I have a 2005 CTT with appx 65k miles.

Since I acquired the car (about 62k), I've noticed a little "looseness" in the drivetrain, particularly noticeable at low speeds.

I have a hard time describing it, because there isn't any clunk, thump, vibration or other noise. On deceleration from low speeds (e.g. around town 30ish and below) I feel a distinct front and back slight lurch as though there were some lash at some point in the drivetrain.

Except for the lack of a thump or something similar, it sort of feels like a loose transmission mount did in my old Jaguar XJ12.

While I was replacing the coolant pipes and enjoying the under-car access to the infamous T fittings hoses, I checked around for obvious driveshaft bearing issues and transmission mount (although I probably really didn't know what I was looking for on the transmission mount).

Is this a symptom caused by the valve body, or is there some other mechanical issue I should be exploring?

Again, no obvious thumps, bumps or other similar sounds. The transmission shifts fine (a little stiff shifting on cold mornings until things warm up), operates fine and appears to have no operational issues.

At "speed" (anything over about 45) I don't notice this issue while accelerating or decelerating.

Any ideas on what to check while I'm enjoying some more quality time crawling around under there?

--Ken

AGARubberDuck

I'll be watching the replies here as my 04 tt behaves the same way. Rush hour stop&go traffic is especially annoying when I'm constantly on and off the gas pedal (usually no need for a brake since these things seem to jake-brake.)
Being there is no thunk, and it goes away entirely in 3rd and above, I've just gotten used to it as a quirk of a highly-overpowered power plant on an suv drivetrain. The Pdealer didn't say anything about it during the PPI or subsequent visits, but I didn't point it out either. I'd love to pinpoint a 'problem' and fix it though.

mcbit

Every Cayenne that I have driven, and that is quite a lot now, has had that same backlash in the drive-train.

Brainz

I have noticed this on my 2006 CTT as well. I've identified that vacuum leaks on the intake manifold make it considerably worse, which makes some sense. My theory is that the transmission control module seems to want to keep the torque converter locked up as much as possible for fuel economy, and that low speed, part throttle driving is near below 2000 rpm where the "lash" occurs.

The lash problem was much much worse when I had an intake leak as I suspect the lower manifold vacuum level tricked the TCM into thinking there was a more open throttle and hence kept the trans locked up in a way that exacerbated the lash. When I fixed the vacuum leak, the lash was almost (but not 100%) gone. I've also somewhat confirmed the theory by driving in a lower gear to keep the RPMs up -- the "driveline lash" doesn't seem to exist above 2000 rpm.

Note that the vacuum leaks also seemed to exacerbate an annoying 1/2 and 2/3 shift flare under low speed and medium acceleration, and a slipping transmission sensation under certain medium and high acceleration conditions. In my experience, the valve body issue was clearly identifiable as a downshift clunk/bang in the higher gears (6/5 and 5/4) that was worse the hotter the car, whereas the vacuum leak caused numerous part throttle issues that would otherwise make you think the transmission was possessed and was worse when the car/air temperature was coldest. While my car is driving pretty well right now, I still need to try to smoke test it later this year to make sure I'm not missing any other leaks that might be degrading the transmission behavior. Part of me hopes the exorcism will one day be complete.

On a related note, does anyone know if there's been any work done on an aftermarket TCM flash? In my opinion, the single greatest flaw in the drivability of the 955s is the wonky transmission control (at least on my turbo). It's just too lazy. And it's way too sensitive to vacuum leaks... I'd pay good money for a flash that fixed those problems -- an extra .5 mpg is not a priority for me.

Good luck.

Brainz

One other thought. After I fixed my vacuum leak(s), I had to use Durametric to clear the transmission adaptations. Much improved after both steps were completed. Not sure if it would have otherwise adapted fully over time, but the Durametric reset was definitely noticeable.

khowell

Thanks! I hadn't thought of the vacuum connection......

Brainz

And another idea: Check your torque arm strut on the top US passenger side of the engine. Mine was shot at 35k miles. It made some difference, but the vacuum issues dwarfed it.

My vacuum leaks included the infamous plastic tee at the top back of the engine against the firewall (completely broken, now replaced with metal), the brake booster vacuum connection at the back of intake manifold on the passenger side (was not fully seated), and the oil separator cap on the top of the drivers side valve cover (I inadvertently didn't fully seat it correctly). I also found some vacuum plumbing under the throttle body where lines had been chewed up by the water pump pulley, but not sure they were leaking.

TT Turbo04

Thanks guys. I've been having this same problem on my 04T, In addition to the driveshaft. This gives me a better description to share with my mechanic next week.
Brianz.... what costs were involved with your fix?

wrinkledpants

I have an 04 CTT, and it's buttery smooth during low-speed traffic. Feels exactly like any other car with an auto tranny. Have you checked or replaced your tranny fluid? Sounds like a symptom of low tranny fluid. Might also check your t-case stepper motor and software. Those are the two things that come to mind.

Brainz

I fixed my vacuum leaks myself, but I assume a shop could rule the same out using a smoke machine or boost tester for a few hours worth of time.

AGARubberDuck

I have not replaced the tranny fluid yet, but think it's coming up in one of the milestone services.
As far as the transfer case stepper motor, I am getting the 4-wheel drive fault, more and more lately. Thanks for the tips!

hahnmgh63

'06 CTTS w/75K (new driveline in the last 5K), also tranny flush 10K ago w/Redline D4. I've owned this one since 20K and it has always had a fair amount of driveline/drivetrains slop/lash. My old '05 CTT had the same. I have a lift and if you jack the car completely off the ground you can feel it when rotating a wheel or reaching up to the driveline and twisting it. Is it cumulative or mostly from one component (tranny, transfer box, differentials)? I haven't looked into it.

Corvetteboy6988

I agree. It seems I have noticed this more recently on my 05 CTT, usually in second gear starting when already moving, and you get on or off the throttle in low speed, 2nd gear, mainly during parking situations, if you hit the throttle just right, you get a clunk as the lash gets taken up. I have replaced the tranny fluid Mobil 3309, did all the diffs and transfer cases with the proper oil in synthetic, have an EPS center bearing fix, so everything is solid. Higher speeds nothing, and in normal driving, I dont get the clunk, rather only when on and the off and then on the throttle, at a speed where things arent meshing well (not everything in the tranny, engine, wheel speed is the same.
I feel its in the transfer case, but I might be wrong, and think its normal. My old Toyota Lancruiser did the same thing, its a function of having the low range gearbox for the 4WD, a lot of moving parts. I imagine with age and wear it can get a little worse. I bought mine with 43K in 2017, now have 63K, cant say its any different, I think I am just paying attention to it more now.

I believe the vacuum leak impact as well, as I might have one as it its worse when cold, and ever since I had the coolant pipes preemptively replaced, it seems to be off just a bit, so either the intake maniforld gasket didnt seat completely, or a pipe/hose is loose or damaged (although I and the dealer have check visually and found nothing). Will need to do a smoke test next to tell for sure.

Brainz

Yes vacuum leaks will accentuate the sensation of drive lash. My theory is that the engine vacuum is an input to the trans controller and hence the wrong intake vacuum (due to a leak) will confuse the trans controller and/or adaptions.

I think what actually causes the sensation of extra lash is the torque converter slipping or unlocking (and relocking) at the wrong time. When the PIG is running right, the converter only slips at idle and very low rpms in 1st or 2nd gear. After that, the converter should lockup and only release/engage briefly around shifts. If you let off the gas at say 2000 rpm, and your rpms drop more than the proportionate decrease in speed/gear (i.e., the converter slips), then the transmission controller adaptions need to be reset/retrained.

You might even try resetting the TCM adaptations first (before spending time lookink for leaks). After the reset, the trans will relearn to shift as you drive, usually getting better in about 30 mins. Accelerating moderately up to highway speeds and slowing moderately to a stop multiple times is best for the inintial training. Sometimes, for whatever reason, such training doesn't work right, and the car will not shift optimally - - if this happens, reset the adaptations and try again. If it's still really bad, look hard for vacuum leaks and then try again.

BTW, the above approach to resetting the TCM also fixed my "rumble when coming to a stop" issue that is commonly blamed on engine mounts. While engine mounts may also play a factor, my observation was that my rumble was caused by the torque converter lockup NOT releasing in a timely manner and was lugging my engine - - a TCM reset eventually made it go away.

Petrolhead_007

Even my manual 957 V6 has drivetrain slack, and it's more pronounced in sport mode. I usually don't use sport mode for that reason, makes the throttle too sensitive and too much drivetrain lash/slack between gearchanges.