91 GMC 4L80E Transmission question

My van has a freshly rebuilt (8500 miles since rebuild) 4L80E
transmission & now tends to stick in second or third without shifting,
or will stay in first & disengage. When in first & stuck, I accelerated
hard & it shifted very harshly into second. It lit up the tires, & this
transmission is healthy. The problem is intermittant. Both speeed
sensors are now new as is the van's ECM brain. Wire harness suspect.
Any other ideas from similar experience appreciated.

Check the trouble codes.

It's possible the TPS is bad. When worn, they can can occasionally go
open-circuit momentarily, in certain positions, causing the computer to
go into "trouble" mode. An analog ohm meter with clips well fastened to
the disconnected TPS plug will work best for testing the switch.

I had this on my '91 6.2L. (Dealer misdiagnosed it for about $100.00.)
The tranny would only shift up to 3rd (no converter lock or 4th) and
would not shift down unit stopped. Shutting of the engine would "reboot"
the computer and get it working in normal mode again. The situation got
progressively worse unit I fixed it.

Good luck.


[email protected] wrote:
> My van has a freshly rebuilt (8500 miles since rebuild) 4L80E
> transmission & now tends to stick in second or third without shifting,
> or will stay in first & disengage. When in first & stuck, I accelerated
> hard & it shifted very harshly into second. It lit up the tires, & this
> transmission is healthy. The problem is intermittant. Both speeed
> sensors are now new as is the van's ECM brain. Wire harness suspect.
> Any other ideas from similar experience appreciated.
>