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Weird Gearbox Issue

Discussion in 'Z32 300ZX' started by shadowman, Jul 31, 2009.

  1. shadowman

    shadowman Well-Known Forum User

    Hello again guys,

    Has anyone experienced a case in an auto 300ZX where the transmission will change down into 3rd for no apparent reason?

    What generally happens is this: I will be happily going along at 30 or 40 mph and I will
    be in 4th gear and the engine will be happily purring along. Then I will feel the car change down a gear when there is no real need to and the car will obviously rev higher for the stronger gear it is in. Only if I ease off the gas a bit will it then return to 4th gear again. It happens on the motorway as well. Any idea why this is happening?
    Is it a sensor associated with how far the throttle pedal is down? Started happening the last few weeks. :confused:
     
  2. bandit

    bandit Well-Known Forum User

    take it to an auto gearbox specialist and get it on the diagnostics it could be something simple but a diagnostic will reveal the problem also it may not be anything to do with the auto box it may be engine related and that can affect the way a box reacts i work at an auto gearbox specialists and the first thing we do is a diagnostic check and then work from there but find a reputable automatic gearbox garage we only deal with auto boxes nothing else do you know what box is fitted in the car? and i may be able to help a little
     
  3. shadowman

    shadowman Well-Known Forum User

    Thanks bandit. There is a place in town called AGS who last looked at my gearbox last when the torque converted gave up the ghost. They got it working again but they have absolutely no respect for my car. I took it back for a gearbox servicing but haven't been back since. I don't trust them. I will try to locate someone. Mr F said he would help to try and diagnose the fault.

    I have no idea what kind of box it is. I believe it is just the original unit. No idea what it's model is. The car is a 1992 twin turbo Japanese import if that helps any.
     
  4. bandit

    bandit Well-Known Forum User

    i will have a look at what boes were fitted to your car when i get back in work on monday and let you know mon nightcould be something simple did you have the torque converter replaced when it as you say it gave up the ghost
     
  5. bandit

    bandit Well-Known Forum User

    in reply to your question one of the possible problems could be the throttle position sensor which can cause the gearbox to shift down if the is a fault we have had these sensors cause this problem in the past so would be worth investigating/replacing let me know how you get on
     
  6. twoforty

    twoforty Well-Known Forum User

    I knew a guy that had a Jag that would do this....we spent a lot of time looking into it....turned out to be a slight air leak on the intake pipe to the throttle body....the lean mixture caused a slight missfire at cruise speed which in turn made the box drop down a gear.
     
  7. BIGSSS

    BIGSSS Well-Known Forum User

    Might be worth asking on 300zx-club.com
    Someone on there might be able to help.
     
  8. bandit

    bandit Well-Known Forum User

    a couple of other issues are the wiring harness's to the gearbox there are two and either one can cause problems but are quite easy to replace the other issue is the shift change solenoid on the valve body in the box can pack up or wear and can also cause the same problem as i said in previous post get a proper diagnostic check on it could save you a lot of money in the long run
     
  9. shadowman

    shadowman Well-Known Forum User

    Yeah, apparently the torque converter had developed a fault, I accelarated but didn't go anywhere?? They said they needed to replace the converter and a few other components. Since then no torque problems but it has not been shifting the same. It tends to change up gears rather jerkily/sharply now when before the converter went gear changing was smooth like velvet, also the POWER light comes on if I give the throttle a quick flick where it only used to come on when I really booted it. :unsure: Don't know what's going on there. It's something I've put up with for some time now. Reading the other posts here though I don't think it's related to a converter issue.
    I'm more inclined to believe it's a smaller issue like a sensor or perhaps a leak on the air intake pipe as twoforty mentioned. I hope itis something small! Either way I need to get it looked at as it is one of 2 things that are really driving me nuts! :confused: I will get it checked as soon as. Thanks to everyone for the advice :thumbs:
     
  10. Buckaroo Banzai

    Buckaroo Banzai Well-Known Forum User

    I have suffered this exact same problem, it was the TPS. if its out of adjustment it can result in snatchy gear changes and the constant locking/ unlocking of the torque convertor when cruising at about 40 to 50 mph. Feels like its changing down.
    Try setting the TPS, if that cures it great.
    if it improves it but doesnt cure it I suspect the TPS needs replacing. :)
     
  11. shadowman

    shadowman Well-Known Forum User

    Mate, much obliged to ya. Will get it looked at as soon as!
    Cheers :)
     
  12. shadowman

    shadowman Well-Known Forum User

    How much did it cost you to get yours adjusted/replaced if you don't mind me asking?
     
  13. Buckaroo Banzai

    Buckaroo Banzai Well-Known Forum User

    To be honest I bought a second hand one from a breaker for £20.
    (total false economy, But I'm tight you see)
    It really improved the problem but that ones shot as well and I need to buy a brand new one really. (I will put a post up for Mr F to give us a price)
    The TPS is only a carbon track potentiometer and they just wear out with high miles. it also incorporates a throttle closed position switch inside the TPS and that gets unreliable too leading to high idle problems.
    As for fitting and setting I did that myself, easy to fit but a little tricky to set up if you havnt got the right equipment. If your a bit handy with a multimeter and screwdriver you can have a go yourself.
    See link for details.

    TwinTurbo.NET: Nissan 300ZX forum - How to test and adjust your TPS switch & sensor
     
  14. shadowman

    shadowman Well-Known Forum User

    Much appreciated mate. Thanks for that :)
     
  15. shadowman

    shadowman Well-Known Forum User

    I decided to have the whole TPS unit replaced and I am so glad I did :) The car runs like a dream now. The gear changes are like velvet! I can barely feel the box changing up or down now. No more high revs before snatching into gear which was caused my head to snap back into my chair! I still have the squeak but as Mr F pointed out it does sound like the trim on the wheel itself that's the culprit.

    Just need to fit the coilovers and get the wheels aligned and that will be the suspension nearly done... just some anti-roll bars to go on and then that will truly be it for the suspension! :D

    I had new tension rods, upper link adjustable camber arms front and rose jointed adjustable links for the rear fitted as well. The ride is a lot firmer and feels a lot more planted on the road. I will post some pics up when I get a chance to look under the car.

    Many thanks to Mr F and croeso! :thumbs:
     
  16. shadowman

    shadowman Well-Known Forum User

    Rear rose joint adjustable upper link

    [​IMG]
     
  17. shadowman

    shadowman Well-Known Forum User

    Front adjustable camber upper link

    [​IMG]
     
  18. shadowman

    shadowman Well-Known Forum User

    Tension rods

    [​IMG]

    Apologies for the blurryness. :)
     

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