2500hd 4l80e

[Biasatti98]

I have a 05 2500hd 6.0 with the 4l80e i am having a problem with it shifting out of overdrive anywhere obove 55-60mph. It dosent slip it will just shift out of it. If i am cruising at 50 it will shift into it and hold overdrive all day the fluids good recently changed and truck has about 120000 on it. I tried changing the vehicle speed sensor on the transfer case then the throttle position sensor and my diff fluid everything else looks good. Also i am getting a flashing engine light on the highway but the truck runs great and never misses always has great power. Heard of people doing crank sensor relearns that fixed it. Let me know and thanks in advance 

[davester]

get the codes that are being generated.  I would suggest trying to find a better code reader, to be able to get codes from the transmission computer (TCM) as well as the engine computer.

[harlan]

I had a random engine misfire code (P0300 and flashing check engine light) on my 2004 Silverado 2500 (above 55mph) that was unlocking the torque converter.  Once it kicked out, it would not lock back up unless I shut the truck off and restarted.  Replaced wires and plugs, and ran 2 cans of seafoam and still had the issue.  I would clear the code and drive around for a little while, then same issue.

 

I too read about the relearn, and took it to the dealer.  I explained my issue, and told them to do a relearn.  They did the relearn and a test drive, and they said it was still misfiring, even though the check engine light may/may not be set.  In their experience, they recommended throttle body and injector clean (they were having "special" price).  I said if I get out of the parking lot and it still does it, then I'm coming right back.

 

I'm not sure if it was the re-learn, or the throttle body/injector cleaning but has been 6 yrs and 70,000 miles and has NOT come back.  HTH.....