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Flex plate: what does it do? why fail?

Yesterday the flex plate on my 2000 Toyota 4Runner (w/ 155K miles) was replaced. The symptom was a "ticking" sound under certain conditions. Expensive job! Just what does a flex plate do? What are the likely causes of failure (so maybe I won't do it again)?



Thanks for your wisdom.



George
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Comments

  • edited June 2009
    The flex plate replaces the flywheel in automatic transmission cars. It bolts to the end of the crankshaft and the transmissions torque converter bolts to it. Usual failure mode is the crankcase bolts or the torque converter bolts work loose, allowing the parts to start moving in different directions, the clicking sound you heard. Failure is a matter of luck and is beyond driver control.. At 155K miles, I would have had the transmission rebuilt while it was out..
  • edited June 2009
    The flexplate in an automatic is the same part as a flywheel in a manual. The torque converter is bolted to the flexplate, and so the engines rotation is delivered to the transmission through it. The flexplate also has gear teeth on it which the starter engages to start the engine.

    A failed flexplate is pretty much an unlucky event under normal driving conditions, nothing you can really do to prevent it.
  • edited June 2009
    A flex plate seldom ever fails. Have you owned the truck since new or is it known if the transmission has ever been out before?

    Sometimes when an automatic transmission is removed the installer may make a mistake and not get the torque converter seated on all of the splines.
    When the transmission is in place and the trans mounting bolts/torque converter bolts are tightened the flex plate is distorted and under a lot of pressure.

    Upon startup the converter may then seat (or not) and the weakened flex plate may crack shortly afterwards or it may go months or years before giving up.

    I had a Subaru into the shop one time in which someone had made this mistake and the entire center of the flex plate had peeled clean out followed by the car ceasing to move one inch further.
  • edited June 2009
    Thank you all for your comments. I appreciate it. Would using the engine to help brake on hills contribute to failure (described as a cracked flex plate)? We live in the Sierra Nevada mountains and going anywhere requires a lot of up and down.

    Thanks again.

    George
  • edited June 2009
    I don't think engine braking would be an issue. Otherwise many of your neighbors would be having the problem too. I do however know of a flex plate cracking on a car that had an engine miss for an extended time. That was a 72 Ford Torino.
  • edited June 2009
    While I've never heard of engine braking cracking a flex plate, I'll go out on a limb and say that I think that it's possible.

    That spinning converter is heavy and is applying a jolting force to the flex plate when the shifter is dragged down into a lower gear. A few times may not hurt anything but doing this on a regular basis might cause a tiny fracture which could snowball and become noticeable.

    Back in the 50s, there were several catastrophic crashes of the new British Comet airliners over open ocean. It was determined from recovered wreckage that these planes came completely apart in mid-air due to a fracture so small that it required an electron microscope to even detect them.
    If an airliner can break up from something so tiny then why not a flex plate too.

    If the transmission has never been out of the truck before then my guess (and that's what it is at this point) would be that chronic slamming of the converter mass against the flex plate bolts could be behind this.
    Careful examination of the old flex plate bolt holes may reveal something. If the bolt holes are slightly shiny or wallowed a bit that could point to downshifting being the cause.
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