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700R4: Torque converter not locking up

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by will89, Jun 10, 2003.

  1. will89

    will89 1/2 ton status

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    Are there any simple explantions or places to check before I tear it apart? I can tell by my tach ( and just feeling) that its not locking after engaging into overdrive.... thanks guys
    will
     
  2. BigBurban350

    BigBurban350 1/2 ton status

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    To tell if its on cruise at like 55 in overdrive and start a light acceleration, then lightly hit your brakes so that it disengages the lockup and youll see the RPMs rise 3-400 RPMs. That means its working.

    Check the electrical connect to the tranny. Driverside of the tranny has the connector, check that the wires arent fraied or burned on the exhaust.
     
  3. dyeager535

    dyeager535 1 ton status Premium Member

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    If your truck has a diagnostic connector under the drivers side of the dash ("below" the light switch approximately) you can force lockup by jumping two pins on it. Never looked for the connector on non ECM trucks though.

    Lockup depends partially on engine vacuum, so you want to be cruising at a steady MPH, and lightly apply the brake to see if RPM's jump.

    The brake switch is pretty easy to check. Can't recall which are combined, but there can be at least three sets of wires to the switch(es)...cruise control, TCC, and brake light.
     
  4. BrianDamage

    BrianDamage 1/2 ton status

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    might be like mine was...the mounting bolts for the lockup solenoid had backed out and fallen into the transmission pan.Then, the solenoid itself had wiggled out of its hole and was handing by its wires. I put some loctitet on the bolts and mounted it back and it works fine now
     
  5. Blue85

    Blue85 Troll Premium Member

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    lockup is controlled by the ECM. The connector or wiring to the tranny could be bad. The brake switch could be misaligned or bad. You could also have a bad solenoid, they do go bad after a while. You can test it by unplugging the tranny electrical connector and appying voltage there to listen for a click. You can change a solenoid just by dropping the oil pan. If you are able to force lockup via the ALDL, then the problem is with some sensor or switch feeding the PCM.
     
  6. blasterD

    blasterD 1/2 ton status

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    In an attempt to wire in a manual lockup switch to a truck I had a while back I left the switch on accidentally and it fried the lockup clutch. The solenoid would engage, but the clutch wouldn't hold. I hear that the stock clutch material is made of some kind of paper.?!
     
  7. HarryH3

    HarryH3 1 ton status Author

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    The stock clutch inside the torque converter has a very small surface area and cannot take much abuse. That's why the factory has it set up so that it will only lock up under very exact circumstances. If you start to apply power, it unlocks. It's only designed to deal with steady state, no-load cruising.
     

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