1993 6.5 shift problem

[lee5]

I have a 1993 4x4 6.5 3500 with a 4l80e trans. It has developed a problem holding the shift to 3 rd an overdrive. It will default back to 2nd. Rebuilt trans because main drive clutches burned up because of this. Replaced the TPS. Wouldn't work found the heater in the fuel filter was causing it not to work disconnected it. TPS worked. Took for a test drive as long as you eased it up the rpm range it would shift but if you drove it normal or went up a grade it would default into 2nd. Within a short time less than 10 miles burnt up the clutches again. Replaced all the clutches but don't want to drive it again till the problem is found. It gives codes 68, 85 and 87 but can't do correct test on it because afraid to run trans again. Could this be caused by the input speed sensor or a short someplace snice there is no code for the speed sensor.

[Robyn]

I am going to say you most likely have some speed sensor issues.

Burnt clutches is either caused by a mechanical fault in the tranny that is allowing oil pressure to leak off or a Throttle pressure issue.

I am suspect that you have speed sensor issues.

This system has an input and output speed sensor and if one or both are flakey then things get real hinky.

I would replace the sensors and then try again.
If you can get the use of a scan tool you can traqce it down fairly easy though.

Hope this helps.

Robyn

[Robyn]

Another area you need to check is all the grounds.
Be sure your battery cables are in good order and the grounds are good and clean. Make sure you have good voltage to the system.
Check the grounds on the right rear of the engine near the back of the manifold to be sure there are no broken wires or poor connections.

Any voltage anomlies can wreak havoc with these things.

Make good and sure all the wiring that goes to the tranny is in good shape with no shorts or broken wires. Be sure the connector to the tranny is good too.

These things are great when they work but old worn poor connections can drive you nuts in a hurry.

[JohnC]

68 is a slipping component. This, of course, is what's burning the clutches. 85 and 87 can also be triggered by a slipping component. I am not an AT expert, but I'd be looking for a line pressure control problem or a crack somewhere...

[JohnC]

I'm gonna suggest you drive it far enough, with light throttle and load, to see if it sets the undefined ratio code first or the slipping component code. If you get 85 first, then it's a speed sensor issue, bit if you get 68 right away, I still would look for a mechanical issue or a line pressure issue. Also, if you can drive with a recording scanner attached, you can likely zero in on the root cause before doing any damage.

Of course, it only makes sense to check over all the wiring, etc. first!

[lee5]

Thank you all for your help. I replaced the input speed sensor, throttle position sensor and the tcm. I also replaced all the battery cables. Checked all the grounds and wiring for a short. This improved the shifts greatly but still have a problem. As long as the rpm's do not exceed 2300 when going through the gears it will shift perfect but when I exceed that it will default but let off the accl. it will go back to 3rd gear but will not go into overdrive. Shut engine down an restart it will shift perfect again. It has something to do with the rpm range but can't figure out why any answers. Could the output speed sensor be the cause or the vsb or is there something I overlooked?

[Recycled]

I had a very similar problem with my 1991 6.2 Suburban with 4L80-E 4x4. I replaced the throttle position sensor, speed sensor and wiring harness and that did not fix the problem. We ultimately found somehow the wiring harness connector wires had been switched in the socket by someone / somehow. This made the transmission shift very erratically. At times it would work perfectly. Then for no reason it would require I nearly redline the engine to get the transmission to shift up to the next gear. After we put the wires in the correct positions the tranmission has worked flawlessly ever since.