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another 700r4 tranny question

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by scrappyk5, Oct 30, 2003.

  1. scrappyk5

    scrappyk5 1/2 ton status

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    if my speedometer doesnt work becouse of the gear in the
    t-case , keep the torque converter from locking up?

    /forums/images/graemlins/confused.gif
     
  2. outlaw612

    outlaw612 1/2 ton status

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    Nope, it has nothing to do with the speedo. At least on the86 and older, not sure about 87 and newer.
     
  3. dyeager535

    dyeager535 1 ton status Premium Member

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    Yes. All 700's in trucks (so '82-up IIRC) use the VSS (vehicle speed sensor) to determine minimum lockup speed. Since the VSS is driven off of the speedo cable at the speedometer, it not working will never "signal" the TCC box (over your gas pedal) to lock up the converter.
     
  4. Thunder

    Thunder 3/4 ton status

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    Dorian is right! All trucks with a 700r4, even the old ones with the vacume switch on the firewall. Use a VSS to determine minimum lock up speed.
    On the newer TBI trucks the VSS also feeds the ECM info that it uses to compute fuel ratios and timing. Not having a working VSS may result in a slight loss of fuel economy.
     
  5. scrappyk5

    scrappyk5 1/2 ton status

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    thats what i was thinking . 3 years ago i had problems
    where the speedo started going out and was jumping around.
    i never connected the two when i noticed that at the same time my tranny was going in and out of lock up. i took the vehicle to aamco and they told me my tranny was going bad.
    well i had them rebuild it and i still had the problem.
    they were too stupid to figure it out. so all this time i have been trying to figure out why it wouldnt go into lock up and at the same time my speedo didnt work anymore.
    i took the speedo cable off and its good. i took the little gear that goes into the side of the t-case
    and its good. so i assume my problem lies within the
    t-case. now i need a breakdown of my t-case to see how i take it apart with out things flying all over /forums/images/graemlins/eek.gif.
    thanks guys for the answer i was looking for. /forums/images/graemlins/thumb.gif
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

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    If the speedo is bouncing around, its more than likely either the cable or the gauge.

    Not saying the tranny gear isn't bad, but personally I'd say its more rare to have the speedo drive gear fail than the cable or speedometer itself.

    When you checked the cable, did you hold one end in each hand, and twist the inner cable while watching the sheath? The cable should turn freely, no binding, and the sheath shouldn't "twist up" or flop around when you turn the actual cable.
     
  7. scrappyk5

    scrappyk5 1/2 ton status

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    when the thing first was going out , it bounced around.
    then it went completely out. i replaced the cable, the
    little speedo gear that you pop into the t-case , and
    it still doesnt work. oh , the speedo cluster is good too.
    soo that leaves the drive gear in the t-case i assume.
     
  8. dyeager535

    dyeager535 1 ton status Premium Member

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    Ahh...no speedo at all, yeah, sounds more like the drive gear now.

    You can of course check it, just pull the speedometer out and see if the cable spins as the truck moves. May be easier in my truck than most (all the dash stuff is out right now) but it may be worth checking. Just have to see if the driven gear is moving or not.
     

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