1993 Isuzu NPR overdrive shifting problems

Discussion in 'Mechanic and Repair' started by 123hotdog, May 8, 2013.

  1. 123hotdog

    123hotdog LawnSite Senior Member
    Messages: 485

    I have a 1993 Isuzu NPR. All of a sudden it quit shifting into overdrive. While in drive at 35mph or so, if you stomp it (not necessarily to the floor) it will shift into overdrive, however, if you lift off the accelerator or the truck levels out based on where you have the accelerator, it immediately drops back down into drive even if you are going 65mph. I am at a loss. The transmission shop (the best and most reputable in my area) said it is not internal. The truck service company that normally works on the truck says they don't do transmission work. I am so stuck and this is the backbone of my business and just spent $3000 in repairs at beginning of the season for new turbo exhaust ect. Forced me to park it and go back to a truck and a trailer.:hammerhead:
  2. Ray_Lawns

    Ray_Lawns LawnSite Member
    from NE AL
    Messages: 125

    I had the exact same problem in an old Bronco II I had. There was a vacuum modulator on the transmission that went bad. Super easy fix if you have one on yours which I am unsure of. You may also have a bank of three modules I believe on the passenger side back where the cab rests at and if one of those lines is off it will do the same thing.
  3. windflower

    windflower LawnSite Bronze Member
    Messages: 1,111

  4. larryinalabama

    larryinalabama LawnSite Fanatic
    Messages: 19,647

    I think that set up is electronic, and doesnt have a modulator. Its possibe if it has a throttle cable its buinding, I dont think those have throttle cables and it may bt the TPS sensor. Those transmissions also have solenoid problems which is a real big deal.
  5. herler

    herler LawnSite Fanatic
    Messages: 5,139

    How I learned to respect the automatic transmission.
    Automatic transmissions can cost thousands to rebuild.
    We know little about them, those who do know about them can easily take us to the cleaners.
    They can charge us fifteen hundred for a rebuild, replace a broken rubber grommet, wait three days to
    call us and tell us it's fixed, collect our money and send us on our way.
    You and I, we wouldn't know the difference.

    First rule of automatic transmissions, we don't test suspected faults by bearing down on the throttle.
    You can let OFF the throttle to see if it will shift, but do NOT push it.
    If an automatic transmission exhibits signs that lead us to believe something could be faulty...
    No matter how minor it may seem, I would strongly advise to drive it GENTLY until such time the fault can be addressed.

    Second rule of automatic transmissions, we change the fluid and filter every ten thousand miles, religiously.
    If the fluid and filter hasn't been changed since who knows when, that's the first step.

    It needs to be done, I don't troubleshoot machines that are outside of their periodic scheduled maintenance.
    That is the correct order of operation, first the periodic scheduled maintenance gets done, then we see if there exist other problems.

    Failure to adhere to these rules is asking for a failure.
    Last edited: May 8, 2013
  6. 123hotdog

    123hotdog LawnSite Senior Member
    Messages: 485

    Following up on an old post. I can't explain it but the truck did this for a month or so and fixed itself and never had this problem again.

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