4L80E Torque Converter

[HerpDerp1919]

2000 Chev K2500 5.7/4L80E/4x4 with 263k on the clock.
After driving my truck for its longest trip so far, about an hour, it developed an issue that when the my 4L80e is cold, it will shift and work perfectly fine. However, after about 20 minutes of driving, it will drop out of overdrive. The transmission will still shift smooth, no jarring or hard shifts. It just kinda slips out of OD. I have done some research and read that it could be a TPS or a faulty brake switch not allowing TC to lock up. My question is, if it was either of those two parts, would that not also happen when its cold? I'm really leaning towards it may be the clutches in the TC are shot. I'm just curious if I might be on the right track with things. It's been brutally hot here in S. Texas so its been a minute since I've been near the truck so just trying to get into things now that its cooling down. Thanks in advance for any suggestions!

 

[JavaMan]

I've had the same problem for a couple years.
Mine went out in the summer on a trip through the desert. I put the selector in third, and it drives fine. I've hooked up the scanner and got nothing, even in the TCM scanner module.
For me, 4th gear works fine when cold. Can feel the 4th shift as well as the TC lockup. It will do fine whether I'm going 45 or 65 up a 3% grade. But once the transmission temperature gets up, around 90-100 degrees according to the scanner, then 4th slips. It just feels like it went into neutral. When hot and accelerating, I can feel 4th engage and RPM drop for a second, then it picks up RPM like I'm in neutral.
So odd about that temperature range causing such a change.
I've just been driving with the selector in 3rd all the time for the past 20k miles. I occasionally test it in OD just to see if its still the same.

Doing research, I've found some suggestions that it might be the Actuator Feed Limit valve, there's a Sonnax kit to repair it. But I've not been able to find a transmission shop willing to check. They all just want to replace the transmission. Some shops have even told me its impossible to do any valve body work without removing the transmission and at that point you might as well just replace it. Dam lies.

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[HerpDerp1919]

Well I'll keep you posted on if a new TC solves my issue! hopefully it'll be cooler later this week and I can get it swapped out. I wish I could keep my truck in 3rd but Texas has some high speed limits and in most places if you're not going at least ten over they will just run you over.

 

[Erik the Awful]

Some shops have even told me its impossible to do any valve body work without removing the transmission and at that point you might as well just replace it. Dam lies.

Yes, they're lying to you, but I don't think it's out of malice or greed. Transmission shops have terrible reputations, and it's because transmissions are horribly finicky. So are customers. Liability-wise, their best interest is in focusing solely on R&R, that way they have control on what other crap is in the transmission. If you had Joe-Bob rebuild the oil pump last year, and he didn't do it right, and then you pay Bo-James to fix the valve body and a month later the transmission takes a dump because of what Joe-Bob cheaped out on, you're going to take it back to the last person - Bo-James, and gripe that him. Wasn't his fault. If I owned a transmission shop, I'd probably run it the same way.