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Thread: been chasing a gearbox issue....

  1. #1
    Join Date
    Sep 2005
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    Default been chasing a gearbox issue....

    for quite some time i've been getting an occasional gearbox 'glitch'.....

    every now and then, it decided to move the shift points of the gearbox up, and not be completely happy, til the car was switched off and restarted.... if it happened an off/restart cycle meant the gearbox continued as if nothing had happened! no 'down time' needed, instant restart cured it.. def hints at electrical, rather than mechanical/hydraulic...

    connecting the test point to get the blinky/flash dash indicators showed nothing.....

    as i had another spare gearbox ecu (actually got 2 spare!) I swapped that in, but it eventually showed the same issue! hmm not that then!

    i was going to ask on here or Prime, but as the usual advice is 'put in a manual' or 'it's not a proper NSX', i didn't bother!

    eventually i did it again, and this time i thought to check the diagnostics with the engine still running... sure enough we have a fault code! hurrah! 3 lovely flashes!

    this points to the gearbox ecu losing the throttle position signal (permanently or momentarily makes no difference, once it's lost, it defaults to lost until it's reset!)

    this tallies up with what i see at the fault condition, the computer moves up the shift points as it doesn't know what the throttle level is, and loses kick-down and torque converter lockup!

    so now i know it's the red/yellow wire, i checked as per the manual, levels where expected,changing with the throttle position, disappearing when the throttle pot is disconnected so i know i have the right wire......

    connections checked and cycled, trying to replicate any loose connection, alas no

    on the way to goodwood we had to brake on a particularly rough surface at some traffic lights.. after that the gearbox was not happy... again 3 flashes in the OBD test condition.....

    moving away from the ecu, tracing the wiring loom, i find the connectors between the engine loom, and the 'floor' loom.... mounted on a weak mount and in fresh air behind the drivers seat.... hmmm that could be subject to vibration.... so the connectors oiled and cycled a few times, and then supported with some foam.....

    fingers crossed!

    IMG_20180715_115421 by jon sutherland, on Flickr
    Last edited by britlude; 20-09-2018 at 10:31 PM.
    aka Jonathan!!

    '92 charlotte green auto.... as a daily
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  2. #2
    Join Date
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    Default

    Very impressive diagnosis and hopefully rectification there Jonathan, your problem solving is like rocket science compared to my recent battery problems.
    I hope you've cured it.

    What did you think of FOS this year???
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    1992 My beloved Red/Black manual NA1.😎
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  3. #3
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    Default

    fingers crossed its sorted, it's the first time i've fiddled in that area, when various other investigations/swaps/changes had had no effect, so promising! just got to wait and see, the problem with intermittent faults!

    i'll start a FOS thread with some pix!!!
    aka Jonathan!!

    '92 charlotte green auto.... as a daily
    '37 Ford Y street rod......... something for the weekend!

    photobucket patch if you use chrome https://chrome.google.com/webstore/d...fegnfnflicjjgj

  4. #4
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    Default

    So, it's still ongoing, it was ok for 2 weeks then returned, so another cycle of the connections and all was well for a few days, then the same again....

    So next step was to put a jumper wire between the TCS computer and the gearbox controller.... this worked briefly, but didnt cure it.... soo bypassing all the connectors, I linked the wire coming into the cabin and the link wire i'd added between the computers..... Hmm still occasionally going into fault mode....

    So... What is actually going on? I've tried a different controller and get the same error.... Back to first principles... Start at the top... Is it the throttle position sensor? So unplug it and drive off... Instantly I get engine management light, then TCS, while the gearbox is going into its fault state.... Right so the sensor error trips all 3 computers, but my normal fault is JUST the autobox controller... That narrows it down to just the connector to the computer and it's associated 3inches of wire (before the bypass link.... ) all the other in cabin connectors are bypassed and we know the signal is good at the TCS computer... Think we're getting close!!!!
    aka Jonathan!!

    '92 charlotte green auto.... as a daily
    '37 Ford Y street rod......... something for the weekend!

    photobucket patch if you use chrome https://chrome.google.com/webstore/d...fegnfnflicjjgj

  5. #5
    Join Date
    Dec 2007
    Location
    Switzerland
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    Default

    Quote Originally Posted by britlude View Post
    on the way to goodwood we had to brake on a particularly rough surface at some traffic lights.. after that the gearbox was not happy... again 3 flashes in the OBD test condition.....
    The "rough surface" caught me here. It looks likes the intermittent error is pronounced by vibration. Maybe you'll find a way to monitor the voltage signal of the throttle position signal while driving.

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