4L80e error codes 85, 87

Discussion in 'Chevy Truck Forum' started by randyl2446, Mar 6, 2010.

  1. randyl2446

    randyl2446 Member

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    93 Chevy K2500, 7.4, 4L80e, error codes 85, 87. Runs and shifts great until we get about 10 miles on it. On flat, level road with no change in speed or anything, it downshifts 1 gear smoothly and won't upshift after that. Slowing down or stopping so that it shifts into lower gears causes it to shift extremely hard on upshifts. Worried it might break something. Trans fluid OK and have cleaned wire plug and contacts going into transmission. After it sits and cools down it does the same thing again. Appreciate any ideas. Thanks, Randy
     
  2. oneluckypops

    oneluckypops Senior Member

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    It sounds like a Input shaft speed sensor.
    85 undefined gear ratio
    87 high gear ratio error


    When when the ISS and VSS readings dont correspond the computer sends a signal to the transmission the request Max line pressure which is why it shift so hard.
     
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  3. randyl2446

    randyl2446 Member

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    oneluckypops...thanks for your reply and sorry being slow to respond. (went fishing) I bought a sensor and will install it shortly.

    I also found out I didn't have to cool it down to get it to work. I turned the key off for a couple of minutes and it shifts correctly again; sometimes for a few minutes and sometimes for a couple of hours. I'm assuming turning the key off must reset the controller.

    Randy
     
  4. crabtruck

    crabtruck Senior Member

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    welcome to the forum Randy. Here's another thought.

    Unless I'm mistaken, an 85 will not set if an 87 is already present.
    An 85 will not set in 4th gear, an 87 will default to 2nd gear.
    Both set it for max line pressure, so the hard shift effect.
    Both are supposed to reset with key off/on if "not present".
    Both can be caused by wrong signals from calculations in PCM
    based on in and out speed sensor vs info from pressure switch.

    It might be worth a look to see that the connections are tight
    and there is no visible sign of damage to harnesses at trans.
    Poor connections can cause the same problems as hard parts,
    and of course, looking is free. Good luck.
     
  5. randyl2446

    randyl2446 Member

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    oneluckypops and crabtrucks, thanks for your help. It turned out to be the front sensor. I've run it about a month now and the transmission is working just fine. Didn't want to say anything until I was sure and I am.

    Thanks again,
    Randy
     
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  6. oneluckypops

    oneluckypops Senior Member

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    Glad you got it fixed. The front speed sensor is the ISS (Input speed sensor)
     
  7. crabtruck

    crabtruck Senior Member

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    Glad to hear you got it working.
    Big thanks for posting that result back here.:applaud:

    Nice call oneluckypops.:bow:
     
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  8. thunderduckusa

    thunderduckusa New Member

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    You are right on the money! I had the same problem and spent $400.00 to change the harness because of temp sensors were bad which did not fix the shifting problem and so far all the other codes!
    Thank you and have a excellent thanks giving!
     
  9. chevychase

    chevychase Semi-Admin
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    It is nice when other post back the fix..for others to search from . Glad you got yours fixed thunderduckusa
     
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  10. indiandentman

    indiandentman New Member

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    just joined today to thank all you guys who are good enough to take your time posting this kind of info . found my way here searching codes on my plow truck . sensors did the trick just like above . thanks to all
     
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