Sorry but its a Jatco OD tranny, but you guys are so smart and helpful I thought I would post here as there is really no trans site for Jatco.

Anyway, Ive been building C4's and C6's for years with not too many problems, so I decided to try and build the tranny in my 90 Mazda truck. The technical name for the trans is a 4N71B.

There were no problems, or surprise's with rebuilding the trans. In fact it looks really like a C4 internally.

The trans isn't computer controlled, but it does have two electric shift solenoids. One is downshift, the other is for manually turning off the OD, by pressing a button on the shifter.

After the rebuild it ran and shifted fine just driving it around the block.

The first time I went on a longer trip, was about a 20 miles round trip to the store.

Going to the store all was fine on the highway with driving speeds 55 to 70mph.

On the way back when I got up to 50mph the tranny would start going up and down about 100rpms. The faster I got the faster it went between up, and down. Always just a 100rpms up, then down, etc.

I drove it again another 20 mile round trip. Same thing, on the way there it drove fine, after sitting for about 30 minutes then going back home it did the same thing, just a 100 rpms up and down the whole trip back home. Always in overdrive, but never in 3rd.

My thinking is the trans fluid is too thin I can't think of anything else. I guess it heat soaks when I stop which thins out the fluid. I use Dextron III in this tranny. What do you guys think?

this sounds kinda like a tcc solonoid problem bad oring or a bad torque convertor lock up clutch kickin in and out .

Pull the filter and check it for small fiber like particals. If you find them you will have to change the convertor even if you changed it when you built the unit.
Mac

I also suspect the TC since I did change my old, working TC for a new one.

Before pulling the whole thing out. I drained two quarts of the trans fluid out and replaced it with 2 qts of 30w non-detergent oil.

Guess what, no more funny shifting in OD. I plan to add a good tranny cooler on the front to help a little more.

I do understand that there is still something "not right" about the trans, but for now I'm just gonna drive it.

If it finally starts to shifting funny again I'm just going to pull the whole thing and install my 363w with a C4 and call it done, and to Hell with gas mileage.