A4 B6 CVT seems to downshift. TCM repair didn't fix.
Car's got 165,000 miles and for a month now, when the transmission has had 20-30 minutes to warm up, it will begin to have quick shudders every couple minutes and also occasionally and suddenly the engine RPMs will go up to 6000 to 7000 as if it has decided to fall into a low gear. After maybe 45 minutes to an hour, I've had it get stuck in what seems like a lower gear no matter if I use the manual shifting or not. I let of the gas, sometimes throw it in neutral, and after the engine comes down for a few seconds, the car returns to normal and the PRNDS flashes.
I suspected the Clutch Pressure Plates slipping so I tried to contact a few shops. The dealership changed the transmission oil but it made no difference at all. All the shops were only offering a complete transmission swap. So after the fluid change made no difference, I started suspecting the TCM since the downshifting thing didn't really make sense to me as a symptom of the clutch slipping. I exhausted every forum post and youtube video that I could find on this subject and couldn't find anyone else having the same symptoms, so I figured one I had a wire break in the TCM that had a unique symptom. I've always had a check engine light for output speed sensor but it never caused a problem. For some reason my VAGCOM reports too many errors to talk to the transmission, but the shop was able to get a few transmission errors off of it - nothing interesting.
So I put the car on jack stands and spend a good six or seven hours taking out the TCM, repairing it, reinstalling it, and refilling the oil that came out. Absolutely no difference. The TCM had already been repaired once - so it was already dremeled open with a piece of tape placed over it to seal it, so opening it was easy. The original repair person used what seemed like weatherstip adhesive and it was beginning to peel off, so I didn't need to do much to open it up. There were five broken wires, four near the output speed sensor and one near one of the pressure sensors. I really had my hopes up that the pressure sensor wire was the culprit. I used the aluminum solder kit that I bought on Amazon and repaired the five wires without much struggle. Some of the wires wiggled back and forth much easier than others so perhaps there is still a broken joint in under all the potting goop. The potting is very pliable - it seemed like I could scoop all of it out with my finger.
The clutch pressure sensor wires makes sense for the symptoms, but not for as regular as this is and only after heating up. I also have never read a code that seems related to that problem. After the downshift happens, I got flashing PNRDS and errors about incorrect gear ratio. If I let the car cool down, I can drive it over 65mph and get the flashing to go away.
This problem started after I got the car back from a shop, and they had found some air line that needed replacing on the back of the motor. I noticed the I was getting much more engine power and no lag like I had before. So I drove it a little too hard for a few days.
Since this was around the time the problem started, I figure maybe the clutch getting worn makes sense. But I don't know if the car would downshift in response to a slipping clutch. Also the shuddering is so short and quick that it never feels like a constant slipping. But I generally only see problems when accelerating.So I give up. I'd be curious to know what the issue is, but likely will have sold the car in the next 24 hours. I'll put some tips on the TCM repair below:
- This video helped a lot: https://www.youtube.com/watch?v=KviONp1gjVo&t=266s
- The support member made the transmission drop about 4 inches. You don't need a jack under it but probably will during reassembly.
- I had maybe a quart and a half of oil come out when the rear case was removed.
- I used this solder kit: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
- I recommend pulling up on wires to see if they are broken. I'm worried I wiggled them back and forth too much.
- The potting goop can be scraped off real slowly. It'll booger up and come out eventually. I used a very small flathead screwdriver.
- The soldering kit recommends 450 to 500F for the aluminum. I found it behaved a little better around 575F. The heat gets sucked away and dissipates faster with the aluminum.
- I spent almost two hours trying to get the center bolt for the transmission mount to catch the free-spinning nut on the other side of the small mount hole. Eventually, I went to the store and bought a triple-square set to remove the four bolts that hold this small mount on and then just tightened the mount to this piece and installed them together. You might have better luck than me at catching the nut, but I don't know if you can tighten it without getting to the other side.
- If I remember right, you got 6 18mm bolts to remove the support member, 2 13mm to release it from the mount, 3 13mm bolts to get the cable mount off, 10 45 torx to get the case off, and 3 30 torx to get the TCM out. You'll probably need the triple square though for reassembly - that's 4 10mm triple square bolts.
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