Tow truck tranny issues, back again

[BTR69]

Good evening. I'm back to working on my Dually tranny issues again. Got some good ideas from here back in 2021 when I was initially working on it, but no luck with them. 1998 Chevrolet 3500 HD, 454 gas engine, 4L80E tranny. I re-retired it in 2021 after throwing the cheaper, affordable options for repairs at it, with no fix. Life has gotten out of the way enough for me to work on it again, along with having a little more money to spend on it. Quick recap... Keeps getting stuck in low gear. Shop referred to it as limp mode. Pulling battery cables, seems to reset the computer and it shifts 1-2 fine, 2-3 shift happens and immediately downshifts to low and stays there.

Things that have been addressed so far.
1. Switch in steering column. Was told about a GM service bulletin that says it would cause a similar issue.
2. Speed sensor. Truck is 4x4 and as I understand it, only speed sensor is on transfer case.
3. Internal wiring harness. Old one was leaking fluid from inside to outer harness.
4. Shift solenoids
5. Rebuilt tranny. Shop did document a couple of internal issues, so I'm ok'ish with the rebuild, but damm thing still acting up.

Tranny shop worked on it further, checking wiring harnesses and other possibilities and can't seem to find anything else that could cause this. Has recommended I find another ECU, as that seems to be the only thing left to swap. Their vendor sent them 2 units and neither were right for my truck. I found a vendor, All Computer Resources in Florida, who advertises they have them, refurbished and ready to program and go. Sadly, they can't seem to reprogram any of their core units to my truck. I'm planning to hit the salvage yards nearby tomorrow. From what I gather, only a 98 year ECU will swap over.

Anyone have any suggestions on a vendor for these units, or anything else I should look at, (aside from a stick of dynamite), to wake the tranny back up again? I'm about ready to bite the bullet and drop it off at a local shop I trust, but I expect that to be an expensive one for me. Hourly rates and an electrical gremlin don't sound cheap.

Thanks and hope y'all are having a great week.

[Steve Grady]

4L80E has a speed sensor that measures input rpm towards the front on the side of the case. The rear speed sensor hole in the case would be plugged on a 4WD with the speed sensor in the transfer case. You should have both sensors.

[Steve Grady]

Have you checked it for any trouble codes or checked the line pressure?

[Jeff Stout]

I know mine is a powerstroke, but brake lights would vibrate off and on and made it stay in low. Don't know why but it fixed it. Things I tried wireless throttle cable gas pedal, 3 pack solenoids, converter The fix was brake light switch and adjusted solved it.

[BTR69]

Originally Posted by Jeff Stout

I know mine is a powerstroke, but brake lights would vibrate off and on and made it stay in low. Don't know why but it fixed it. Things I tried wireless throttle cable gas pedal, 3 pack solenoids, converter The fix was brake light switch and adjusted solved it.

Thanks for the info. I'll definitely change it out. Those aren't so expensive that I can't pay for one and hurt me if it's not the fix.

[Kirk Morgan]

As a last thing to do you could get a stand alone controller for the transmission.

[BTR69]

Originally Posted by Steve Grady

Have you checked it for any trouble codes or checked the line pressure?

The tranny shop ran the codes several times. I don't remember what they were though. I'll see if it's on the paperwork they gave me with it. may take a few days to find that though. They drove it around while hooked up and saw voltage fluctuating pretty badly. New alternator fixed that. Was hoping that was causing the solenoids to fail. Apparently not. They're a reputable shop locally, so I'm hoping the line pressures are where they should be.

I don't remember finding any info on the speed sensor at the front of the case that you mention. I'll look for that, hopefully this weekend.

Thanks..