'93 4L80E downshift/upshift problem

[3500GMC]

Truck in signature. It has an annoying 4-3 downshift under load. This is with shifter in OD(4)... Rolling along, either with cruise set or with my foot doing the work, doesn't matter, in 4th gear lockup, start asending a grade, the TC will unlock then the trans will shift into 3rd no problem... Problem is: This is where it stays 'no matter what' until I either:

a) shift into neutral while rolling, shut down engine, restart, shift back into 4th and go.. The trans is now in 4th lockup like it should be. Or.

b) get to the next stop and shut off key/engine and restart. Same same.

Other: I can duplicate this 10 out of 10 if I just barely ease into the throttle (cruise also), but if I use a little more sudden pedal movement, maybe a 1/4 inch, it won't downshift- just pulls until speed is reached.

I suspect a noisy TPS... or.. ??? Trans computer is getting a headache?

 

[bk95td]

I had a bad ecm /tcm in my 92 do that exact thing. It would loose the throttle signal . I got very good at shutting it off and restarting on the roll . I had to drive with the tech 1 hooked up to see what was happening. after the restart it would have the throttle signal again. I checked the throttle position sensor and it tested good. I replaces the tcm and life was good. I was lucky enough to have a spare tcm from one I junked. Check your local junkyards. New remans are around $130 last I checked at R/A.

 

[WarWagon]

Computer is seeing something it doesn't like and locking out OD. I had a hard problem, OD sprag failing in the trans, do this OD lockout.

Check codes with paperclip as the computer doesn't have any codes that trip the SES light for the transmission.

Check voltage at TPS and wiggle test harness around TPS. TPS at WOT may be dropping out.

TCC may be slipping too much under load and limping the ECM to 3rd and No TCC lock. Worn valve body TCC bore causes this.