Potential 4l80e Problems

Newmember2020 · July 10, 2020

I recently bought a 04 Sierra 2500HD with a 6.0. The guy that sold the truck said that his grandpa use to use it for towing and had someone put a tow tune on it. Currently the truck does not go into 4th which the previous owner said is because of the tune. I know that overdrive going out is a problem with 4l80es though. Could his story be true? If so is there a way to remove the tune? I called a GM dealer to see if they can flash it, but they said it wouldn't help. If it's possible that it is tuned could i just get it returned, possibly with a handheld or Black Bear Performance? I knew about the problem when i bought the truck and I got the truck cheap enough that even if it is the transmission and needs a new one I'm well under resale value. Any advice would be great.

Edited July 10, 2020 by Newmember2020

davester · July 10, 2020

This is all "most likely", not "for sure".

The tuner would have to be pretty incompetent to tune out 4th, so the guy lied to you about the tune not doing 4th, but rather the transmission has lost it for whatever reason.

The dealer likely meant that they could flash the truck back to stock (which I would recommend doing, just so you have a known starting point for how the truck is supposed to work), but that it was not going to get the transmission to go into 4th.

Rebuild the transmission land you should be good to go.

Some other things to do:

-ask the P.O. (if possible), if they have installed a pump-rub fix in the transfer case (if the truck is 4x4). If not, or unknown, I would install a pump rub fix in the transfer case while doing the transmission fix, as you've got to get the TC out anyway, which is the hard part.

-check if all the exhaust manifold bolts are present. They have a history of breaking off, and can cause exhaust leaks (or enable air to be drawn into the exhaust) making the engine use more gas.

Newmember2020 · July 10, 2020

That's what I thought.

The dealer told me that flashing it would not remove the tune. I never told them about the transmission problems.

davester · July 10, 2020

"The dealer told me that flashing it would not remove the tune"

That's crazy, as that is literally what flashing the truck is (or should be). They (the dealer) downloads the stock programming and tuning information from GM's servers and loads it onto your truck.

Newmember2020 · July 10, 2020

That is what I thought but didn't want to wast the $110 pus tax.

davester · July 11, 2020

Hopefully it's just the service writer who is that stupid, and the technician who does the actual work knows more. I would make sure the technician actually does know better before paying them to do the job...

sefiroxx · July 15, 2020

Take it to a tranny shop for a diagnostic. They'll install a monitor into the harness. Via the monitor, they can verify:
- the 3-4 shift is commanded
- the various solenoids are working

carkhz316 · July 15, 2020

I agree with Davesters points. The only thing I have to add is that if the ECM is locked (from the tuner) then you are out of luck and you'd need a new ECM. That'd be the only instance of not being able to reflash it. I would verify that it is indeed OD that is not shifting into, and not just the TCC locking up. The TCC in the 4l80 through 2006 are known to have problems with the clutch piston cracking. Also, any type of misfire will cause it to unlock for 10 minutes at a time, so if it's randomly misfiring (doesn't have to be real either; can be due to crank angle relearn which is required after a reflash on these trucks), it'll do it. I chased my tail trying to diagnose a rogue P0300 for awhile after tuning my '03 I had. It turned out I had to do the crank angle relearn. The issues presented after I did my tune.