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Transmission problem 700 series

posted by  someone claiming to be Fee on  Sun Mar 25 06:30 CST 2007 [RELATED]

Hi, I have a 91' 780 with about 170k. It has an Auto transmission as do all of the 780's that I know of. At least the US spec ones anyway. Anyway, a month or so ago I noticed that on the freeway during normal freeway driving circumstances (no hills/60 mph) the engine would wind up to about 3500 rpm and get very loud. And within seconds it would drop back down to normal rpms.

Lately, it will stay at around 3500 rpm when just cruising and it is way louder than usual. Power seems to have suffered also but the turbo is still active if I give it a lot of gas when it is doing this. I drove the car twice on the freeway yesterday and it only did this on the way back. Once I got off the freeway it sounded fine. I did a trans flush and fill when I first bought the car maybe 3-4 years ago. I might have put 20,000 miles on it since then, but not much more than that.

Is this a normal occurance and is my transmission going out? I am not really looking forward to shelling out a bunch of money on a 15 year old car, but will if I have no choice. Any tips or suggestions?


thanks,
Fee








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    Transmission problem 700 series

    posted by  Aye Roll on  Sun Mar 25 21:42 CST 2007
    Author: [PROFILE] [EVERY POST] [THREADS] [BEST POSTS] RSS Feed     [IGNORE ALL POSTS]
    last visit: Tue Oct 14 21:08 CST 2014
    [RELATED]

    You could have a misadjusted or stuck downshift cable.

    Another possibility, and I've experienced this years ago in a 90 744T, is that there are intake air leaks from the rubber hoses in the lines to the intercooler or off the turbo. The leaks cause the engine to lose some of the boost the turbo is making. Therefore you have to open the throttle more to get the same power.... therefore the trans downshift cable is pulled tighter because the accelerator cable is pulled further... and the trans will shift out of OD at some certain threshold in that condition. So it's doing what it's designed to, in response to the inputs it's receiving.

    Check out those hoses, check the downshift cable.

    Good luck!
    --
    ::: Rob Bareiss, New London CT ::: 92 244 ::: 90 745GL ::: 90 745T ::: 84 242DL ::: 90 745T Parts ::: Used to have : 86 244DL, 87 244DL, 91 244, 88 244GL, 88 744GLE, 82 245T, 86 244DL, 87 244DL, 88 245DL, 89 244DL!








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    Transmission problem 700 series

    posted by  rstarkie on  Sun Mar 25 10:21 CST 2007
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    last visit: Fri Oct 24 10:14 CST 2014
    [RELATED]

    Is the transmission failing to shift into fourth gear (the overdrive associated with the switch on the shifter knob) or do you think it is "slipping"?

    If it is failing to shift into fourth gear then the likely suspects are the overdrive relay, the wiring associated with the overdrive circuit or lastly the overdrive solenoid (not a common occurance).

    Click on the dropdown shortcut selections at the top right of the page and select 700/900 FAQ's. Click on the transmission AW70 and then scroll down to Overdrive Relay and Function.

    Randy








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      Transmission problem 700 series

      posted by  someone claiming to be Fee on  Mon Apr 16 06:08 CST 2007 [RELATED]

      If it is in fact a faulty Overdrive relay, would the yellow arrow light on the dash come on when reaching speeds where the overdrive would normally kick in? Because the overdrive lock-out indicator light is not coming on when I am experiencing these problems. I was thinking of just going ahead and buying a new relay and hoping that this was in fact the problem, but then read that when the OD relay is faulty the light usually comes on. Thanks.








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        Transmission problem 700 series

        posted by  rstarkie on  Mon Apr 16 14:20 CST 2007
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        last visit: Fri Oct 24 10:14 CST 2014
        [RELATED]

        I am not sure what the light does when the relay is faulty; Or at least I am not sure it is consistant in its behavior. By that I mean I have bad relays that failed to extinguish the light when I pressed the button, but I can't say for sure that the light would come on if it drops out of overdrive once it is engaged.

        Sorry, perhaps someone who knows for sure will jump in and answer that for you. If not you might try reposting with the information about the light included.

        Randy








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      Transmission problem 700 series

      posted by  someone claiming to be Fee on  Sun Mar 25 16:44 CST 2007 [RELATED]

      Hi Randy,

      You know you might be on to something. It almost sounds like the engine is slipping out of overdrive into 3rd gear. Would this make the engine rev at about 3500 rpm at highway speeds? Today I drove it twice on the freeway. It acted up when getting up to about 55-60 mph, about the speed I would think that the overdrive would kick in. On the way home it was fine.

      I am not too familiar with the little knob on the shifter, but understand that it takes it out of overdrive for towing and such? Anyway, I usually just keep it in the "D" position and never mess with the other gears, unless using a lower gear if going down a steep hill.

      I am fairly stupid when it comes to fixing cars but at least I can check fuses and such. I have a fairly competent mechanic so I might run it by him if it keeps acting up. Do you think a fluid change would help?


      thanks,
      Fee







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