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Automatic transmission lock-up problem

I have a problem with the lock-up clutch in my 1998 Jeep Grand Cherokee which has an automatic transmission and request your opinion of the cause and corrective action. The problem is the torque converter lock-up clutch opens (disengages) momentarily and randomly at steady speed (when above the normal lock-up speed of 45 MPH) on both level and downhill roads and almost always at the bottom of a downhill slope, but I have never noticed this problem on uphill roads. The clutch opens and then immediately re-engages (noted by movement of the tachometer and engine sound). The clutch also opens when the gas pedal is slightly depressed above steady state speed condition. Clutch engagement is delayed above 45 MPH if the vehicle is accelerating which is normal. All other functions of the transmission operate normally. To aid in troubleshooting, I have a question that hopefully you can answer: In addition to road speed and engine speed, what other type(s) of signal (e.g. vacuum, electronic, etc.) and source is required to initiate clutch lock-up and disengagement? Thanks in advance for your consideration and I look forward to receiving your response.
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Comments

  • edited January 2009
    Check your brake light switch (On the brake pedal) first. If this switch is not adjusted properly road vibrations could cause it to close which would unlock the converter clutch. Back the vehicle up to a wall, lightly press down on the brake pedal, you should be able to depress the brake pedal about 1/4" or so before the brake lights come on.


    transman
  • edited January 2009
    Thanks for the suggestion!! I didn't think of this before but it could be the solution - I'll try it.
  • edited January 2009
    The brake pedal moves 1/8" before the brake lights come on but there is no adjustment capability for the location of the switch to increase travel before switch actuation. The pedal moves quite hard so I don't think road vibrations would cause the switch to actuate. Do you have any other suggestions?
  • edited January 2009
    later models of some cars prevent shifting into overdrive when temps are too low, for polution requirements. also pressures to torque convertor lock up need checking.
  • edited January 2009
    The problem occurs in both summer (hot) and winter (cold) conditions.

    Where is the TC lock-up pressure checked? Is there a pressure tap for this on the outside of the transmission? Is this something only a transmission service facility can do?
  • edited January 2009
    One of the most common issues causing a converter clutch to fall in and out of lockup is a faulty TPS (Throttle Position Sensor). I have also seen other things such as faulty trans temp sensor and even noise coming from the alternator which would affect lockup.


    transman
  • edited January 2009
    Thanks for your comments Transman. I would like to check-out/investigate some of the considerations suggested so far but I need a good transmission manual/book for guidance. Can you inform me what to buy and where I can get it?

  • edited June 2009
    I assume that you do know that by pressing the brake petal the converter clutch unlocks. I think Transman touched on that.
  • edited April 2010
    My Jeep still has the problem with the torque converter clutch momentarily disengaging or at least slipping on level roads at steady speed. As suggested, I checked the brake light switch in the garage and it was OK and I also checked on the highway by holding the brake pedal fully up with a bungie cord and the clutch problem was still evident/no change. So I have ruled out the brake switch as the cause of the clutch problem. Another condition I noted at steady highway speed was that a very slight further depression of the gas pedal will cause the clutch to disengage or slip. It appears as though the oil pressure engaging the clutch is low when this problem is evident. What regulates this oil pressure and how can this be checked?
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