Help... I am a pretty good back yard mechanic (been building and working on cars for 35 years) and I can't figure this one out. My daughter has a very nice 2006 MCS with the Aisin 6 speed automatic transmission. The car currently has 81K miles on it. She bought it about 2 years ago with 60Kish miles. I ran and drove great for 6 months or so. One day on the freeway it just randomly shifted into 1st or 2nd gear all on it's own. No warning, no reason, speed was steady and the transmission was in D. She never drives it in SD or manually shifts it. This has been going on ever since for the last year and a half. We have replaced the transmission with a warrantied used unit from a rear end wrecked car with less miles than the original about 8-9 months ago and replaced the fluid at that time. We have had the ecu reset. We have looked at codes and it was saying speed sensor faults so we replaced the speed sensors on all four wheels. The problem still persists. I drove it last night after she drove it over to my house so it was already warm and it works perfectly for the first 2 miles or so. That's when the shifting issue starts. It shifts down very abruptly first then it acts like it can't decide which gear to be in. It jumps all through the range up and down. The ABS light and the DCS lights turn on. Sometimes when you come to a stop at a light and try to start moving again the transmission is in a super high gear and the car will barely move. You have to work the throttle until it shifts down and will start moving. I brought it back home and reset the ECU and took it out for another drive on the same exact route. No lights were on and it drove great until almost the same exact point and it repeated everything. I'm at a complete loss and ready to throw in the towel. The dealer just says "new transmission" but I know it can't be that and we aren't spending another $8K on this thing. I know it has to be some sensor somewhere and something "telling" the transmission bad information. Could it be the ECU has gone bad? I've read they are very difficult and costly to replace. I have searched and searched online and through forums and I see similar issues but no one seems to have a solution that fixed it. If anyone has more ideas please let me know. I can perform most mechanical duties and some electronic ones before just taking it to the dealer... again.

 

Going to move you to the Stock Problems/Issues threads. Should get better coverage. If you have not already done so can I suggest you search that section for this issue?

From past postings changing the fluid in much shorter intervals than what the factory suggest is a must. When did you change the fluid last?

 

Thank you for moving my post to where it will been seen. As I stated in my original post, the transmission fluid was just changed when we changed the transmission. It is not a fluid issue.

 

Was the codes you received specific to wheel speed sensors? There are 2 speed sensors located in the transmission. They are called rational speed sensors #5 and #7 http://www.realoem.com/bmw/enUS/show...diagId=24_1066
If these sensors go bad all kinds of shifting issues will happen.

 

Thank you for the reply Black Forest. The software said it was the rear wheel speed sensors. We went ahead and replaced all four since they are relatively inexpensive and easy to change. I discovered it had been done previously as well because the cable body mounting connectors had been cut and replaced. If it was the internal transmission rational speed sensors you speak of I doubt the replacement transmission would had the same exact failed sensors. There is no difference in what it is doing now vs. when the OE transmission was in the car. That tells me it's something outside the transmission telling to shift erratically. Any other ideas?

 

Are you resetting the ECU or the TCU and how are your resetting it?

There are posts saying you can reset the ECU by disconnecting the battery for a while or going into diagnostic mode in the gauge cluster, but places like WMW and Detroit Tuned have said this doesn't really reset the ECU and you need to do it with software. Also, even if the diagnostic mode reset works, I'm not sure if it resets the transmission control unit, too.

 

I have reset the ECU and did it through the diagnostic mode in the gauge cluster. I had not seen or read anything that said this method wouldn't reset it. I will look into that some more and see if the software we have used has an ECU reset available. I have had the Mini dealer reset one by disconnecting the battery before on a different car so that must work. They told me they have to disconnect it for 30-60 minutes for it to fully clear.

 

Pretty sure that you cannot reset the ECU without proper software and connections.

And remember that just because you get something out of a box does not mean it is going to work. Recent post on a guy that had just that with the blue wheel speed sensor on the front. Thinking you either test the resistance or the voltage coming from the speed sensors to determine if they are bad.

 

Pelican has a good article on speed sensors, see below:

http://www.pelicanparts.com/techarti...or_Testing.htm

 

Thanks for that.

Also, if you go to the bottom of the article you can leave a comment and Nick - our MINI tech - will provide more input for you.

 

According to the huge 90+ page thread on the Aisin auto trans there should be a separate transmission control module under the dash on the driver's side. Here's a post from the thread.

This thread might also help. It has some info on replacing the TCU/TCM with new vs used, and if it can be done or not. Not sure who's right and it would have been nice if there was an update on if a resolution was ever found.

https://www.northamericanmotoring.co...ext-steps.html

 

Kinda sounds like the symptoms of valve body failure...not an 8K dollar job but probably a good $1,200.00 job for a DIY.