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I have new pictures of my old mechatronic unit that better illustrates the damage that occurred. One can see the melting and the scoring of the hard plastic. The picture of the bottom part of the unit clearly shows how the plastic particles contaminated the fluid. The heat had to be substantial.
Just got the car and other transmission home. Now when work allows I have to get everything set up. Car raised, disassemble the donor transmission, flush my old lines and cooler very well, then reassemble. I will try to take pics along with the process. I also want to take the old circuit board apart and see if there is any hint within as to what went wrong.
Please note the Audi markings!
Last edited by Olddragger; 04-19-2022 at 04:01 PM.
Yes, looks like the ATF was too hot. That spot is where the fluid exits from the clutch packs, and is the hottest fluid temperature. That's why they put the clutch ATF temp sensor G509 there....which is partially melted.
An aside observation: The Mechatronic valve body castings have Borg Warner logos on them, the DL501 must have been a joint design effort between VW/Audi and Borg Warner, but built at a VW factory.
Agree with all said. Refreshing the background story. I never had any transmission symptoms except one time. This is a daily driver car, usual type of suburban with some interstate. 66000 miles, transmission service done aT 40k miles, transfer case replaced at 59K miles approx. 8 months before this occurrence. Approx 2 months before the event I did notice a hesitancy after stopping for a red light, then the clutched grabbed and away we went. I said too myself that that wasn't normal and I did a "pdk reset" at home. I don't know if this does anything or not but afterward there were no problems. It shifted great.
I have never launched the car by the way. I was running a Cobb tune, but I never really explored the full performance of this car. I liked the tune because of the responsiveness in everyday driving. I never lugged it, never towed with it and to tell you the truth I was pretty easy on the drivetrain.
My wife and I took a small trip to North Carolina, the family has a mountain cabin there, and it did snow. Maybe 4-5 inches. I got up the mountain without spinning a tire...approx 1 1/2 miles. The cabin has about a 200 feet long driveway that is basically level, maybe a 3-4 degree incline at the end of it. We arrived and I wanted to turn the car around to make it easier to get out...it case it snowed more etc. I got a little stuck in doing that, nothing big. Tires did spin maybe a turn or two during the entire process. I said the heck with it and put chains on. No more spinning. During the last manuver, it went into limp mode...and no reverse thing popped up. What the hell? I let it set for 20 minutes and it was ok. The next morning I drove down the mountain, very slowly...had chains on. I took those off at the bottom and we started off and the warning came back on. We continued to drive and shifting started to become erratic. It went into limp mode once again. I stopped, waited and when I tried to get going again there was nothing. Then suddenly the clutch would grab so hard it would kill the engine. I nursed it back to the bottom of the mountain, parked and haven't driven it since. This was over 2 months ago.
I am very interested in what caused this. Perhaps the ,heating of the plastic caused the coolant lines to clog and it went into limp mode? Perhaps the temperature sensor went bad and it did the same... I just don't know. Oil and coolant temps during all of this was what they normally would be. To top it off the dealership only found one code/fault and that was a low clutch pressure one. More to come!
Last edited by Olddragger; 04-19-2022 at 07:46 PM.
About to dive into this, have my car in position with Jack stands....found the factory Jack stand points under small parts of the underbody panels....never knew about them before.
Have the donor transmission in position to access the mechatronic unit, fluids drained etc. Looking closely at what position to place a temperature sensor for gauge and independent cooler Trying to place sensor into the pan with a bung but space is a premium and clearances are small between bottom of pan and the mechatronic unit. Will figure this out. Also looking at possible better underbody cooling in the pan area. The panels may be restricting air flow some?
In concern about the additional cooler, I will prefer a divorced system with its own pump and thermostat BUT first I will monitor the temps for data to ensure another cooler is actually needed. But I am getting ahead of myself. Hopefully will have pics and get unit in tomorrow. I do have new information to share with all
New things available. The mechatronic unit from the used transmission (2017 base) has some changes compared to my 2015. The solenoids are different part numbers but that may not mean anything. Have a lot of pics but will post only a couple. Also found out that there is not a tremendous amount of flow through the external filter...it is too restrictive. The transmission coolant lines do directly behind the drivers side catalytic converter with only a thin heat shield separating them. It appears that there is not much airflow going on in that area either.
There is more green insulation on some of the connections in the 2017 unit How close trans line is to convertertrans coolant lines continue on across rear of engine Tran coolant lines continue on across the rear of the engine
Why do you think there is not much flow through the external filter/cooling lines?
I think the added green insulation is actually a rubber "booty" seal to keep ATF fluid out of the solenoid windings. I think the ATF corrodes the copper winding wire, or the push-on contacts between the black board and the solenoid connection pins.
As a side note, PDKs in Base models may have lower ATF fluid temperatures, as the 4 cylinder engine only has one downpipe/catalytic converter which is on the other side of the engine, not right next to the ATF cooler lines.
The filter itself is very fine and very thick. I put 20 psi of pressure to it and it only flowed a small amount. It does not have a bypass valve. I am not the expert on the fluid flow from the trans to the cooler....but I do know that if the fluid going to the cooler is dependent on what flows through the filter then it is not flowing a lot.
After the repair is finished and it is running may be time for a laser temp measurement from that area. Not only is the hot coolant from the trans to cooler passing that area the cooled fluid Ed is also passing that area.
04-19-2022, 03:52 PM
