View Full Version : Transmission Computer

July 26th, 2004, 08:52
I have an 88 XJ that the Tranny Computer is acting up. I was going to buy a junk yard tested one but don't know if you can get from any other years. Specifically has anyone tried a 90 or 89 tcu in an 88 XJ? (4.0L 4X4 AW4 w/242)

Bounty Hunter
July 26th, 2004, 10:55
You sure it's the TCU? Could be the TPS or speed sensor or even shifter. What's it doing exactly?

July 26th, 2004, 13:18
It is engaging sometimes and not others and the challenge is finding the pattern, which I haven't. Sometimes it will seem to slip at higher rpms and others it will slip until you hit higher rpms. I have adjusted the cable on the throttle body and have checked my fluids and have replaced my TPS and speed sensor (HESCO enhancement kit). The transmission was rumored to have about 115 k on it when I got it from the salvage yard and I have put an additional 50 k on it since. Sometimes it will not shift from first and seems to jump to third at about 40 mph. Very confusing, I wanted to buy a used TCU to just test the theory as they are going for about 75 bucks around VA junk yards, but no exact 88 models just 89 and 90's. (When I type slip, I mean that you give the jeep gas and you get no acceleration, to match the rpm's which are functioning just fine with no engine lag.) Worst case I am ready to buy my way out of this, as it is making me crazy, but my skinny wallet makes me work for it to be fixed by my hand if it can be.

July 26th, 2004, 16:00
I have a '88 TCU in my '90 that works just fine. A long time ago (1995 or so) I was having what I thought was transmission problems but turned out to be the TPS. I swapped in the '88 TCU back then and have just kept it in there and now have my original '90 as a back up.
So I'm pretty sure a '90 will work. Good luck.

July 26th, 2004, 17:33
My feeling is that the computer would not cause the NO accelleration at all issue.

July 26th, 2004, 18:19
i dont believe it is the trans controller, but if you want to rule that out, remove the inline fuse in the harness next to the controller, then drive it. it wont shift unless you do it manually though. so start out in first, then shift to 2nd manually etc. i'm leaning toward a tps problem. did it do this before you replaced the tps, is it set correctly? if your not sure i know of a dealership in arlington that can check this for you.

July 26th, 2004, 19:55
I had problems with my 87 about 2 years ago. Changed every sensor that had to do with the tranny even had the thing rebuilt at the cost of $2400 (i think I got taken). Turns out the computer was the problem, it keep loosing power intermittently causing the tranny to stay in 2nd or 3rd gear with no upshift, only cost me 20 bucks from the junk yard.
But if yours is accually sliping, the problem is probably inside the tranny and not an electrical problem

July 26th, 2004, 22:46
I had the same problem and the inline fuse holder to the TCU had a bad solder point in it and it would go on and off and screw up the shifting. $2 fix.