E38 1998 728i gearbox problems

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  1. E38 1998 728i gearbox problems 
    #1
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    rollcage's Car Details
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    Hi there,

    My newly acquired 1998 728i seems to have some sort of strange gearbox issue in that it works absolutely fine in all gears in steptronic mode, but when left in auto it seems to have a mind of it's own.

    When driving along much above 40/50mph it seems to have a hard time deciding which gear to stay in, and hunts around between (I think) the top three gears. When driving home in the rain tonight it was doing this for the first few miles, and then after a while the orange DSC light came on and then it seemed to want to stay in fifth gear but didnt seem to want to change down for hills and certainly didn't want to kickdown at all - just builds spedd slowly.

    The changes all seem to fine, no sign of slipping. It just cant make up it's mind when it wants to do them!

    I don't seem to have any gearbox warnings on the dash, and the car has done 55k.

    Does anybody have any idea what might be causing this? I'll get it put on a scanner later in the week to see if there are any fault codes stored, but in the meantime, any ideas would be useful. It feels to me like it's some sort of electronic issue somewhere, but my knowledge of Auto boxes is limited!

    Cheers
     
     

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    #2
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    TheEnd's Car Details
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    Check the tyre sizes all round, there's a chance you've got the wrong size on there somewhere.
     
     

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    #3
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    From memory, it has a matching set of Pirellis, but I will check later on. I had read up about differing tyre sizes causing problems.

    Thanks
     
     

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    #4
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    Just been and checked the tyres, they are the same size and load rating all round. There is a bill for 260 for a recent new pair, I forgot that! The fronts, which I believe give the road speed signal are both evenly worn and the same tread depth.

    Just been out in it this morning, and it really doesn't know what to do with itself in D. It will pull away in second, feel a bit lethargic, plunge down into first then change back up into second, then third, and sometimes back down into second again. I just knock it into Step mode and change manually then, no problems. Changes down ok slowing down in manual mode, stays in second at a standstill, wont go into fifth at low speeds, etc.

    It's just the auto mode that is playing up.

    I've ordered a diagnostic machine and hopefully it will be with me soon, and I'll be able to shed some light on it all.
     
     

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    Still waiting for my machine to turn up - however, I noticed to day that when I've been on a decent journey, ie 30 miles or so, and everything is up to temp nicely then it behaves itself completely. Let it cool down for an hour or so, and it starts faffing around again. Most strange!
     
     

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    #6
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    Right, got my little code reader hooked up, and the DTC for the gearbox is "9A CAN Accelerator pedal setting".

    I'm guessing this is something to do with the drive by wire throttle at the pedal end. This would make sense - live values showed that it does seem to show strange values when at idle , either 100% or 23%. Give it a little throttle and it picks up and shows 7% or so.

    The car also has a very poor idle when cold, to the point when it will stall.

    Google doesn't throw much light on these issues - anybody have any knowledge here?

    TIA

    RC.
     
     

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    It's a new one for me, but it does look like you have a problem with the pedal position sensors.
    Try and find out what engine you have fitted, whether it is the M52 single vanos, or the M52TU dual vanos.
     
     

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    rollcage (17-10-2013)

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    Just been for a bit of a drive, after reading and extinguishing all logged fault codes. Surprisingly, it drove perfectly until about the last mile or so, whereby it reverted to holding 1st for ages, and general faffing.

    No lights lit anywhere, but a couple of codes stored in the engine - "Oxygen sensor control limit, cylinder 4-6" and "Throttle valve potentiometer". Haven't had chance to see if these codes have returned yet.
     
     

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    Quote Originally Posted by TheEnd View Post
    It's a new one for me, but it does look like you have a problem with the pedal position sensors.
    Try and find out what engine you have fitted, whether it is the M52 single vanos, or the M52TU dual vanos.
    It's a single Vanos unit, 1998 car.
     
     

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    OK, it's likely to just be a worn TPS, which can be easily changed by itself.
     
     

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    rollcage (17-10-2013)

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