In June of this year (2006) we purchased a 2002 Mazda Tribute. About a week later we was trying to back out of a parking lot on a very slight incline and it would not backup at all. It seemed to go into reverse but it would not backup the RPMs would rev but the vehicle would not move. After putting into drive and inching forward we was able to place it in reverse and backup. For several weeks we had no trouble with this again. I figured that like some manual transmissions they will get suck in between gears, or something, and you have to drive forward then you can put it in reverse, kinda like a motorcycles do. So a few weeks later we were trying to back up and this time on a gravel road going up a steeper incline and the clunking sound that was coming from underneath the vehicle was tremendous, like we were doing neutral drops or something. That was the last straw. I spent 30 min an empty parking lot making sure that I was able to reproduce the problem every time. It turned out that if I backup like one would from a parking spot at the mall there is no trouble. But if for any reason I have to back up quickly I get the clunking sounds and it will slip into neutral. We took the Tribute back to the dealer ship and said hey this isn't right, and explained how to reproduce the problem. They had a look at it and said it was one of the transmission mounts. So we had them replace it, but it didn't solve the problem. They call their 2nd level support and they said that a new transmission was needed and gave a price a couple of used ones. We were with in 1 day of moving from Oregon to Missouri at the time and were not about to postpone the move due to this and the Mazda dealership was not going to fix it, (to explain why they would not fix it would make this story even longer). Now our 4-year-old Mazda, with 67,000 miles on it, is working on its 4th week in the shop. The transmission specialty shop that it's in now has rebuilt the whole thing and we still have the problem. After talking with the mechanic at great length it turned out there wasn't anything broken in the transmission at all. There was a few warn parts that he replaced but nothing that would lead to this type of unique issue, he even tried a new Neutral safety switch with out luck. So after he researched the issue the only things it could be was the valve body or the ECU. He tried a new valve body and that didn't work. Now we are down the ECU. At this point he has the transmission back apart to double check his work. He will be having a computer hookup to the ECU specifically to monitor when the cycloids are engaging be the theory is that the cycloids are engaging in reverse when they should be. After that the ECU will be re-flashed.
The mechanic working on it now has my complete trust for various reason AND he is not charging and arm and a leg that I would suspect out of an unscrupulous mechanic.
So I present this story to the Internet world in hopes that somebody out there has a different theory, the Internet research I have conducted to this point has proven inconclusive.