Possible Transmission Problem when shifting from park
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HT4100 4.1, 4.5, 4.9 Discussion, Possible Transmission Problem when shifting from park in Cadillac Engine Technical Discussion; I have a infrequent problem with my 93 Deville. I have 99800 miles on it and have had a problem ...
  1. #1
    behind-bars is offline Cadillac Owners Connoisseur
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    Possible Transmission Problem when shifting from park

    I have a infrequent problem with my 93 Deville. I have 99800 miles on it and have had a problem occur about 4 times over the last year.

    The problem happens after I start the car and apply the brake to shift from park to drive. Sometimes after I shift and take my foot off the brake the car doesn't move at idle, if I press the gas the engine just revs and the car stays motionless. If I shift back to park, then press the brake and shift into drive the car will act like normal and coast once I take my foot off the brake. I checked the fluid and while a bit old and needing to be replaced, it isnt black at all and the level is good. As I said this rarely happens, but none the less raises a few questions in my mind.

    Does anyone know a likely cause for this sort of behavior? Does it sound like transmission or something with the braking system? In all my ownership of the car I never recall setting the parking brake so I dont think thats a likely problem. I think my ISC is flaky but dont think it would be the entire cause of my problem either

    Thanks for any input

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  3. #2
    HUF's Avatar
    HUF
    HUF is offline Cadillac Owners Connoisseur
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    Re: Possible Transmission Problem when shifting from park

    What about misadjusted PRND (neutral safety) switch?

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    behind-bars is offline Cadillac Owners Connoisseur
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    Re: Possible Transmission Problem when shifting from park

    Quote Originally Posted by HUF View Post
    What about misadjusted PRND (neutral safety) switch?
    I have seen this mentioned on the site before, In those cases I think I recall it setting a code. I will try and remember to check codes this afternoon when I drive it again, however no "service x soon" lights have come on yet.

    Thanks for the hint, That is one thing I didnt think about that I will definitely look at.

    The fact that its rare occurrence and the car shifts and runs strong sort of led me to think it was more of a electrical problem as opposed to a full transmission failure. Hopefully its the prnd switch and problem solved.

  5. #4
    kdcing is offline Cadillac Owners Fanatic
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    Re: Possible Transmission Problem when shifting from park

    ...hmmm...I believe the neutral safety switch only affects the ability to start the engine (i.e. engage the starter) in park or in drive). I do not believe that it would affect the ability of the transmission to engage a gear of the transmission.

    My '93 Eldorado 4.9L has exhibited the same problem for about 2 years now. That is, the engine starts fine, however, when the gear selector is moved to "D" and my foot is removed from the brake, the car simply will not move. If I press the gas pedal, the rpm's increase similarly to if the transmission was in Park or Neutral, however, the car does not move at all.

    I typically park in a parking spot with a cement parking bumper, so I usually need to reverse out of the parking spot after starting the car.....I have never had a problem w/ the car getting into reverse. As a matter of fact, when I do park on the street and need to go from "Park" to "Drive", I usually move the gear selector from "Park" to "Reverse"...and then to "Drive". This method works 100% of the time...i.e. the transmission gets into "Drive" (and the car will move forward) as long as I put the gear selector in "Reverse" 1st.

    I may have come across a possible root cause...my Idle Speed Controller/Motor was giving me problems recently...so I simply forced the ISC to retract and then I disconnected it. My idle speed correspondingly decreased from the normal 650rpm down to a lower 450rpm. I have noticed that the transmission has a very good success rate of engaging in "Drive" after I let the engine settle down at the lower 450rpm With ISC disconnected, the throttle plate is slightly more closed. With the throttle speed slightly more closed, there is an increase in vacuum (i.e. negative pressure) at the throttle plate.

    There is a vacuum line that goes from the throttle body to a module on the transmission....I believe that the module is the vacuum modulator. If my theories are correct that the "slightly more closed" throttle plate causes higher vacuum to the modulator....and if the increased success of the the transmission getting into gear is due to the higher vacuum...then I would suspect that the vacuum modulator is going bad or there is a vacuum leak in the line going to the modulator.

    I have a pretty good understanding of engine operations, however, I do not have a bunch of experience w/ transmissions. My theories as indicated above may or may not be of much help to you, however, I hope my observations may be of some assistance. If you find a solution for your car's problem, please let me know as I have been stumped on my car's problem for some time now.

    ...btw....I do not recommend driving w/ your ISC disconnected as the engine has a tendency to stall at such a low rpm (i.e. not all that safe).

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    behind-bars is offline Cadillac Owners Connoisseur
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    Re: Possible Transmission Problem when shifting from park

    Thanks for the reply. I checked diag codes tonight and didnt get any at all. My problem is so intermittent which makes it hard to fix. My ISC is messed up, or atleast mis adjusted. In park I idle at over 750 rpms I believe. That seems high. Maybe your theory about the transmission not wanting to shift at higher RPMS has some validity. Ill have to look into it. if it happens again soon and i remember i will go into diag and look at the rpms before shifting back.

  7. #6
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    HUF
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    Re: Possible Transmission Problem when shifting from park

    kdcing,

    You can readjust minimum air screw to get higher RPM with ISC motor disabled (to avoid stalling).

  8. #7
    carnut is offline Cadillac Owners Master
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    Re: Possible Transmission Problem when shifting from park

    The hydrallic pump within the trans does not function in park, so when the lever is moved to reverse or drive the pump then circulates fluid engaging that gear. If pausing in reverse helps to engage drive then the pump is a little weak or the trans filter is possibly restricted. I would think an idle below spec would make the engagement more delayed rather than quicker. I dont think an ISC problem would affect gear engagement. If it were mine and I didnt know when the trans filter had been replaced, I'd change it as a matter of course. Cannt hurt, might help. Also the fluid level is critical. Must be between the add and full marks and never above the full mark hot. Also the cable that attaches to the trans lever is adjustable at the trans. Could be wasted motion in the cable not fully moving the lever. The factory manual describes how to verify/adjust the cable.

  9. #8
    kdcing is offline Cadillac Owners Fanatic
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    Re: Possible Transmission Problem when shifting from park

    Huf,

    Yep, adjusting the idle speed screw would fix the stalling issue. In my case, I forced the ISC to retract/extend to a point where the idle was @ a normal 600-650rpm...then I disconnected the ISC. Of course, this was done after finding out how heavy our cars are to steer without the assistance of the power steering pump. It was a bit easier to trial-and-error the ISC to the correct spot then to walk all the way to my toolbox to find the correct screwdriver or torx driver to adjust the idle speed screw!

    Carnut,

    Thanks for the tips. My trans filter is probably due for a change. Do you know if the fluid goes thu the same section of the filter if the trans is in drive vs. if the trans is in reverse? On my car, the success of gear engagement in Reverse is so good, and the success in Drive is so poor, that the problem really seems related to a signal for the trans to engage (i.e. the vacuum signal from the throttle body to the vacuum modulator...or the modulator itself).

  10. #9
    carnut is offline Cadillac Owners Master
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    Re: Possible Transmission Problem when shifting from park

    The flow thru the filter is the same. Verify no wasted motion in the linkage. Reverse is close to park whereas drive is 2 detents down. If there is play then that could delay drive.

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