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4l80E shifts out of OD and 4th upgrade but will not shift back on downgrade

Discussion in 'All Other Transmissions' started by Rex Crick, Aug 27, 2017.

  1. Rex Crick

    Rex Crick New Member

    Aug 27, 2017
    1999 gmc sierra 2500 4x4 6.0l with 247,500 miles. Transmission tends to have this problem when trans temp reaches 170+. Cruising at 75-80 on cruise control will cause trans to downshift normally upgrade, usually to 3rd, but will not upshift on downgrade. Stopping truck, turning off ignition, restarting after a few minutes, and starting up slowly will cause trans to shift properly until next upgrade. If cruise is not used and rpm and speed are allowed to bleed off going upgrade, the trans will remain locked up. Am willing to replace trans, but want to make certain it is trans. Thanks
  2. Jaryd

    Jaryd Oh the sound a 6.5 makes

    Nov 27, 2016
    Hodges, SC
    Welcome to the forum.

    Sorry, I can't help. I'm not a transmission person but I know somebody who is, @THEFERMANATOR


    May 9, 2008
    Scan it for codes 1st. Common issues on those trucks is a dirty maf as all shifting is maf based in them. Most any shifting concern should set a code though. It could be so many different things it's impossible to guess.
    WarWagon and Jaryd like this.
  4. MrMarty51

    MrMarty51 Well-Known Member

    Aug 1, 2017
    Miles City, Montana
    The terminals at the back of the engine on My truck were all corroded. I cleaned them and that ended the problems with the boost system on My truck. Might be the same on Yours too.
    Also, the windshield washer had stopped working, the wipers would go but not the pump system. I pulled the tnk and cleaned those connectors at the pump and they now are working as they should.
    Be worth a try and dont cost a dime.
    I used a dental file to get into the female ends of the terminals, nothing else was small enough.
    Might check with a dentist and see if they have some of those. If not, then I would thingk theyd be available from ebay or amazon.

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