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 is 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 cant 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.

This message has been edited. Last edited by: BTR69,

Only thing I can offer is to maybe look into a stand alone transmission control to run just that. I know they are available to install that transmission into older non computer vehicles.

I think every 4L80e has two sensors. I agree on a k series the TOSS will be in the transfer case. The other (TISS) is located just above the shift linkage shaft on the left side of trans. I had the same issues with a 2wd and found the connector greasy. Cleaned that, changed sensors and perfect shifting afterwards.

quote:

Originally posted by bry-war:
I think every 4L80e has two sensors. I agree on a k series the TOSS will be in the transfer case. The other (TISS) is located just above the shift linkage shaft on the left side of trans. I had the same issues with a 2wd and found the connector greasy. Cleaned that, changed sensors and perfect shifting afterwards.

Thanks.. I'll look for the speed sensor I missed. Probably swap it out, just to take it out of the equation. May be a few days before I get to play with it.

Do you have access to a good scan tool that can read transmission codes?

quote:

Originally posted by Eman:
Do you have access to a good scan tool that can read transmission codes?

Not one local to me. I'm going to check the speed sensor that I didn't know about before, then check another possibility. Someone told me their diesel dually had issues like mine. found it to be the brake light switch. Seems odd, but I'll check that out as well. After that, if it's still a problem, I'll likely drop it at a shop nearby to scan and leave it for them to fix. Haven't been able to get to any of our salvage yards to see about another ECU yet, but that/s still on my radar as well.

Beg borrow or steal a scan tool or you're just guessing. Brake switch would usually only effect torque converter clutch. I can drive my 98 with a 4L89E and if I touch the brake pedal while in 4th with the TCC locked I can watch the tach and see the TCC disengage and engage.
The frt. speed sensor is for input speed and it compares input and output speeds in the PCM mostly for TCC engage and modulation of the pressure.
Does the check engine light come on?

quote:

Originally posted by Eman:
Beg borrow or steal a scan tool or you're just guessing. Brake switch would usually only effect torque converter clutch. I can drive my 98 with a 4L89E and if I touch the brake pedal while in 4th with the TCC locked I can watch the tach and see the TCC disengage and engage.
The frt. speed sensor is for input speed and it compares input and output speeds in the PCM mostly for TCC engage and modulation of the pressure.
Does the check engine light come on?

Good evening. The check engine light doesn't come on. I've been considering taking it to a shop near my house to have them scan it, but I expect to have to pay for that. probably worth the cost. Good folks there, and have treated us well on other needs in the past. With everything I have going on, I should simply bite the bullet and have them get this thing working for me so I can work on other needs, like the race car, which has tranny issues as well. LOL. Tranny for it is currently sitting inside the house waiting for an input shaft.

I did a quick ride around the neighborhood tonight after work. Trying to keep from letting it sit for more than a week or so between short drives. This time when it shifted into 3rd, it stayed there, but when I stopped and took off again, it wouldn't shift out of low gear as usual. Typically as soon as it shifts to 3rd, it drops down to low right away.

With how this week is looking, I won't get to spend any time on it until Sunday.