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Thread: 1989 GMC Trans Problem

  1. #1
    Junior Member
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    Cool 1989 GMC Trans Problem

    Hello,

    I own a 1989 GMC 4x4, 193,000 miles, 350 (with only 10,000 or so on it), overdrive trans (original trans) and I was wondering if anyone had ever had a problem with their truck shifting into overdrive. It will shift once in a while on it's own but sometimes it's like it's in race mode. I have replaced the kick down cable, the filter, fluid, lock up selenoid for the torque converter, I'm just running out of ideas..... I also have been very religious about changing the fluid and filter once a year and adding lucas trans to help with the had use it sometimes gets. I have disconnected the computer, to make the computer relearn everything (braindead as mechanics like to call it), and it works great for a while and then goes back to this hard shift pattern ( 25-30 for first, then around 40 or so for third and then if the conditions are right overdrive at highway speed) wondering if there's a sensor I am overlooking and/or an electrical connection I should check because I know it isn't the bands since I still have a good strong reverse, which is the same as od....well hope u have some knowledge that can help this shade tree mechanic....thanx

  2. #2
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    Well for starters, the trans is braindead. the only input from the computer would be for the lock up converter. that's an old school 700r4. Still shifts from the TV cable, and a possible modulator. (can't ever remember if the 700r4 needs a vacuum source). I'd say make sure the cable is adjusted properly. AT WOT, the cable should be pulled tought. Modulator if equipped should be swapped. Probably the start of a tranny rebuild though.
    Joe
    89 K2500 Chevy, Wee-Oh-five 294K miles
    1962 Scout 80 parts I guess
    1966 Scout 800 resto job
    2005 Chevy 2500HD Short cab long box DURAMAX
    1996 P30, 6.5 NA Mac Mobile
    whole lot of buicks

  3. #3
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    bradrhale's Avatar
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    start with readjusting the tv cable. i cant remember if 89 had a tps on it but a faulty tps will make it shift odd ball like.

  4. #4
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    Well, I was reading in another forum to try going into reverse with changing speed from fast to slow to get some varnish out of the valves, I tried it and it seemed to have worked....it's shifting close to what it was before all this happened....I will change the fluid here again after a little while and make sure all the garbage doesn't get sucked back in.....

    My Trans is computer controlled, so it's not the modulator....89 is one of those years that they just started with computer control, complete that is

    It still shifts a little strange, I will have to check and see if it has that tps on it, I don't really know, but I think it does.....since it is throttle body, but that should affect the way it runs so I'm not sure, worth checking into....

    The cable is adjusted correctly, I had a friend of mine who is a mechanic check that after I put it in....
    Thanx

  5. #5
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    The TPS will not affect the tranny shift points. its the TV cable only. only input from computer is converter lock up. when the 700r4 became computer controlled it became the 4L60E. That was around 93-95. Otherwise, its braindead. no disputing that. Who ever told you otherwise is wrong. plain and simple.

    varnish in trannys is a bit of a myth. ATF has alot of detergents in it. keeps the transmission very clean on the inside. You can take a high mileage, blown transmission and it will always look new inside. not like an engine where its covered in grease, ****, and junk. Always look brand new, especially if you have been relegious about flushes as you say you were.
    Joe
    89 K2500 Chevy, Wee-Oh-five 294K miles
    1962 Scout 80 parts I guess
    1966 Scout 800 resto job
    2005 Chevy 2500HD Short cab long box DURAMAX
    1996 P30, 6.5 NA Mac Mobile
    whole lot of buicks

  6. #6
    Junior Member
    Join Date
    May 2007
    Posts
    4
    Well, an update on my trans. it is working just fine now, it is shifting normal and there is no sign of problems now. shift points are back to normal, just put about a hundred miles on it and hasn't skipped a beat, don't know what is up but it is working good now...

    No sign of burnt oil or smell, so I don't know what's up, guess if it goes it goes, but I'll get some more miles out of this one I guess....

    thanx

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