W126 auto not changing up

Discussion in 'Engine' started by Llewelyn, Jul 20, 2017.

  1. Llewelyn

    Llewelyn Hardcore MB Enthusiast

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    The other day, when I'd been travelling some distance, on pulling off from a standstill the 'box stayed in low gear until about 3500 revs, despite relatively modest boot applied - sounded like it was in first (switch was in S mode at the time, so it would've used 1st). Checked the fluid level which looked a tad low, so gave it a bit more.

    Today (with it in E mode) it was fine going to town (~10 miles) and around town, yet coming home it stayed in 2nd all the way, regardless of throttle or road speed.

    Looking at it the cable is still attached to the throttle linkage at the engine end, an pulling the cable it feels like it's attached at the other end as well (besides, if it was broken or detached, it'd have the opposite effect of changing up at minimum revs).

    So... any clues?
     
  2. OP
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    Llewelyn

    Llewelyn Hardcore MB Enthusiast

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    Thanks, I'll have a look. One thing did occur to me: kickdown switch under the throttle pedal. It appears intact, though. The symptoms kind of match being stuck in kickdown.

    What happens if I disconnect it? just no kickdown? I almost never use it.
     
  3. OP
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    Llewelyn

    Llewelyn Hardcore MB Enthusiast

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    I guess an oil and filter change isn't going to hurt as a first step anyway - I've no idea when it was done on this car. The oil doesn't smell bad but could easily be worn out :)
     
  4. grober

    grober MB Club Veteran

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  5. OP
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    Llewelyn

    Llewelyn Hardcore MB Enthusiast

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    Yes that is a good document. It's possible there's a fault in the kickdown system, or, since so far it only happens with the transmission hot, the oil is out of spec due to age and gets too thin when hot, leading to low pressure - that document seems to say low pressure can cause delayed or absent upshift.

    My first point of call will be changing the oil, still, as it's more or less certain to be due, and can't hurt in any case. While doing that I can check the kickdown solenoid. I'll take it up the road later and see if it behaves when cold.
     
  6. OP
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    Llewelyn

    Llewelyn Hardcore MB Enthusiast

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    Took the car up the road to see if it was different when cold, but it still doesn't change up from second. However, if I force it to start off in first, it does change up to second, just not any further. Selects reverse, neutral and park apparently normally, and behaves normally moving off: in E mode, it pulls off in second, and in S mode with a bit more boot it pulls off in first as I would expect.
     
  7. exvolvoman

    exvolvoman Hardcore MB Enthusiast

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    A bit off topic really but years ago I had a Jaguar XJ6 4.2, one of the tests on the 'box was a stall test...handbrake & footbrake hard on, in D and floor the gas pedal...then note how high the engine revs go. Is this still done?
     
  8. grober

    grober MB Club Veteran

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  9. OP
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    Llewelyn

    Llewelyn Hardcore MB Enthusiast

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    This morning in a lull between the rain, I tried disconnecting the kickdown switch under the throttle pedal. Took it up the road and it worked normally - although I'll reserve judgement on whether I've found the fault until I take it on a longer run tomorrow.

    BTW this doesn't have a cat, it's a 1990 W126 - I beileve the 124 has the 722.4 'box, while this should be a 722.3 although I've not actually checked. Since it has no cats I would imagine it doesn't have the delay thing, although it's risky to assume anything. The 1984 W126 280 I had before this had basically the same 'box but this one has an additional S/E switch for example, and that in some way affects the shift pattern.
     
    Last edited: Jul 22, 2017
  10. OP
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    Llewelyn

    Llewelyn Hardcore MB Enthusiast

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    Update: finally got a dry day to change the transmission fluid. Drain plug in the sump immovable, but that's OK since I was changing the filter as well, so loosened the sump and let the oil sneak out the gap.

    the state of the oil and filter lead me to think the last time it was changed was somewhere between quite a long time ago and never. Oil dirty and brown, filter shows signs of breaking up so there's a chance that bits of filter are in the works, time will tell.

    Converter drain plug came out OK, just hope it stays put now I put it back, since I didn't have a new one to fit. I did tighten it well though.

    Took it up the road after putting about 7.7l of oil in, still had issues engaging 2nd. Backing off the power, 2nd engaged and after that was fine. Tried this by putting the lever in 2nd, same thing: moving off from rest, slow to engage and then works normally once engaged: it doesn't start slipping again.

    Then tried first with the lever from standing start, apparently normal. Having done that, 2nd also seemed to work in auto mode. I'll take it for a run tomorrow, see how it behaves. Topped up the oil to level when I got back just now.


    I don't think there's a knackered clutch in there (yet, anyway) as with the transmission cold, it was behaving normally even before the oil change. Only got the slow engage of 2nd when moving off once it was warmed up.
     
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  11. OP
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    Llewelyn

    Llewelyn Hardcore MB Enthusiast

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    Quick update on this. It's done that trick a couple more times since, where it doesn't take off properly in 2nd. Each time, manually selecting first for the next take off has cured it. I'm suspecting some small crud in the works somewhere from the crappy filter that was in it, and selecting first in some way washes it out of wherever it's causing this issue.

    Aside from that (I think it's 3 times now, counting that first one) it's working great.
     
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