NEED 4L80E TRANS HELP!

Okay, here's the problem. When the truck is cold, the trans works fine. Once the trans temp gets up around 150F, it starts to shift really hard, neck snapping hard. Guys on a Chevy truck board suggested I change the fluid and filter, so I did - No change. There's no check engine light on either. The truck is an '00 GMC Sierra 2500 6.0L 4WD. I'm hoping someone here has had the same problem and knows the cause. Thanks, Jim

I had a similar problem in my old '99 C2500 w/ the 454. It would hang in 2nd gear and then shift really hard (enough to chirp the tires with medium throttle). It had about 75k miles on it. As it turns out it needed the trans. replaced (fortunately I had just bought the truck used and got it covered under warranty). The problem was there was a crack in a "shift selection ring" according to what was told to me. (I know they were BS'ing me, but I got a transmission out of it and the problem was fixed.) Good luck!

Was that a 4L80E? Did it only do it on the 2-3 shift? Mine does it on every shift, 1-2, 2-3, 3-4.

Yeah, 4L80E. It only did it on 1-2 for me. That's why I said similar. Did it just start doing this? Does it have a shift kit installed? I'm not expert on slushboxes, but just relaying what happened with mine. Hope it doesn't turn out to be the case for you.

Electronic pressure control solenoid and/or the trans fluid temp sensor is bad.

You really need a scanner to see the datastream out of the trans to find the problem, especially if there aren't and codes present.

Brian

Ranger50 wrote: Electronic pressure control solenoid and/or the trans fluid temp sensor is bad. You really need a scanner to see the datastream out of the trans to find the problem, especially if there aren't and codes present. Brian

One guy told me to go to an auto parts store and they could read any codes that didn't trip the check engine light. Would they be able to do this?

bravenrace wrote:

Ranger50 wrote: Electronic pressure control solenoid and/or the trans fluid temp sensor is bad. You really need a scanner to see the datastream out of the trans to find the problem, especially if there aren't and codes present. Brian

One guy told me to go to an auto parts store and they could read any codes that didn't trip the check engine light. Would they be able to do this?

Location?

bravenrace wrote:

Ranger50 wrote: Electronic pressure control solenoid and/or the trans fluid temp sensor is bad. You really need a scanner to see the datastream out of the trans to find the problem, especially if there aren't and codes present. Brian

One guy told me to go to an auto parts store and they could read any codes that didn't trip the check engine light. Would they be able to do this?

I'm pretty sure that most auto parts stores are just using the cheapie scanners that pretty much just scan the CEL codes, so basically anything emissions related. You probably need to find a shop or a friend that has a nice snap-on or something similar. Or maybe see if a transmission shop will scan it for you. I would think that most of them would have a nice enough scanner to check out the transmission.

I'm near Cleveland. I took it to an Advance Auto store. The guy seemed to know what he was doing (surprisingly). He used an Actron scanner. He said it would read transmission codes, but that I didn't have any. He did refer me to a trans shop he said I can trust, so maybe I'll go there. BTW, my tow mode doesn't work either, but I don't know if it's related or not.

In reply to bravenrace:

If you were close i could scan it. I am willing to bet it has defaulted into a full line pressure mode. Get on the HP tuners forum and find someone close by who can do a real scan of the truck and tell you what the PCM is doing with the line pressure. Charles V8muscle.net

My 2000 did this right after I got it last year. It set a code, but no light. A trans shop where my buddy works pulled the code. Can't remember the number, but basically the code for internal slippage. The trans goes into self-protect mode and boosts line pressure to max, until the truck is shut off for a few minutes, which resets it. Turned out to be a totally abused, cooked, fried trans, including the TC clutch. Not sure if the tow mode is related to this. Tow mode programming sucks anyway...

In reply to doc_speeder:

When I changed the filter and fluid, everything looked really good. The fluid wasn't burned and there was virtually no foreign material in the pan. That make me skeptical that the trans is bad, but who knows. How many miles were on yours when this happened?

In reply to bravenrace:

Only about 135,000 kms. PO had towed heavy, in the summer, with a winter grill cover on...repeatedly. Yes, I got the truck cheap, and it's been solid and capable since the rebuild that the PO agreed to pay half of. Decent guy, just dumb.