Vibration right off idle

Transmission and Drivetrain Tech

  1. plumkrazee70

    plumkrazee70 Well-Known Member

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    I just finished my 70 Dart with a 340 and 42RH conversion.

    I took it out for a test drive, however the vibration is so bad, I had to take it home. I can feel it idling in drive or reverse and continues right off idle. I brought it up to 35 mph and the vibration/noise increased with speed. I took it home, but am un certain where to look.

    The driveshaft, is new with joints (wasn't a shortened core, but built) the converter is new and supposedly the trans has been rebuilt (only real unknown) The 8 3/4 is also brand new.

    My first thought was the converter is externally balanced, but I didn't see any weights and I used the 340 flexplate. I am afraid to drive it as I don't want to mess something up.
     
  2. George Jets

    George Jets 1967 Dart 2 Door FABO Gold Member

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    What year 340 ? The later 340s were cast cranks with external balance front damper.

    Early 340s were internal balanced with forged cranks.

    If you have a late 340 with and early internal balance damper on it, that will vibrate.

    Here is a 72 340 external balance that uses the counter weighted damper on the front.
    Screenshot_20210401-203129_Gallery.jpg
     
  3. George Jets

    George Jets 1967 Dart 2 Door FABO Gold Member

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    What is the 42RH conversion ?

    Edit:
    OK looked it up on Google 42RH is a Dakota od auto trans.

    Want to find out if your 340 is an internal or external balance to begin with. If it is a '70 would think that would be internal balance.

    Have to see if that 42RH was set up for internal or external balance from the Dakora it came out of.
     
    Last edited: Apr 1, 2021
  4. plumkrazee70

    plumkrazee70 Well-Known Member

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    It is a '68 so its neutral balanced. I just double checked the converter I got was for internal balance as well. 42RH or A500 Overdrive Transmission.
     
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    • George Jets

      George Jets 1967 Dart 2 Door FABO Gold Member

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      Do you have a Lock Up Converter on that 42RH? Or is the converter non lock up.

      If it is a lock up converter that may not be wired up right causing the vibration ??
       
    • plumkrazee70

      plumkrazee70 Well-Known Member

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      It is a lockup converter. I didn't think that would matter. I believe the lockup just comes on or it doesn't based on 12v. It's worth a look.
       
    • 92b

      92b FABO Gold Member FABO Gold Member

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      Does it vibrate in park with engine rpm or only when the vehicle is moving?
       
    • plumkrazee70

      plumkrazee70 Well-Known Member

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      I can feel it in drive and reverse idling and at speed. However I do not feel it in Neutral or Park.
       
    • George Jets

      George Jets 1967 Dart 2 Door FABO Gold Member

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      Tying to think how that lockup works, could possibly need power to disengage it and no power to engage it.

      There are 2 pin or 3 pin wiring plugs on the trans to help identify what it is.
       
    • 92b

      92b FABO Gold Member FABO Gold Member

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      I think you can rule out engine balance then.
       
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      • George Jets

        George Jets 1967 Dart 2 Door FABO Gold Member

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        This overdrive trans came out of a '90 D150 4x4 it's a 2 pin electric coupler.

        20201027_110952.jpg

        20201027_111140.jpg
         
      • George Jets

        George Jets 1967 Dart 2 Door FABO Gold Member

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        Should be able to rule out drive shaft and rear end too if vibrating when idling in drive not moving.
         
      • George Jets

        George Jets 1967 Dart 2 Door FABO Gold Member

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        Did you get an aftermarket converter for it or did you use the converter that came with the 42RH trans?
         
      • plumkrazee70

        plumkrazee70 Well-Known Member

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        Yea it's a 3 pin circuit. The middle pin is 12v and the other two are OD and LU
        I just bought a factory lockup converter for a 92 Dakota. I wanted to see how it performed first. The two pin is non lockup.
         
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        • George Jets

          George Jets 1967 Dart 2 Door FABO Gold Member

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          Screenshot_20210401-213241_Google.jpg

          Screenshot_20210401-213318_Chrome.jpg

          Some info here from online.
           
        • George Jets

          George Jets 1967 Dart 2 Door FABO Gold Member

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          OK, sounds like you have everything right for your setup then.

          Just need to figure where the vibration is coming from.

          Hope they sent you the proper Lockup Converter to fit your trans.

          _________

          I tried to fit the 90 overdrive converter from this pictured trans into my '76 D100 727 trans, no way it would fit, was a neutral balance converter that I needed to go with 90 318. Had to cut the weights off the 727 converter to make it neutral balance then it worked fine.

          Mated up the '90 318 Roller engine to the 727 trans.
           
        • plumkrazee70

          plumkrazee70 Well-Known Member

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          I'm going to try and put the front on ramps and the rear on stands and then run it in gear. Then look to see if I can see any wobble anywhere. I'm not sure what else to do.
           
        • MOPAROFFICIAL

          MOPAROFFICIAL FABO Gold Member FABO Gold Member

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          Is the transmission touching the floor pan under load?
           
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          • plumkrazee70

            plumkrazee70 Well-Known Member

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            I had that thought, however it seems like the vibration increases with speed. Maybe the driveshaft is touching?
             
          • George Jets

            George Jets 1967 Dart 2 Door FABO Gold Member

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            Check your tranny fluid on the stick on a white paper towel to see that it is nice and bright cherry red yet, and not any fine filings in it.

            Just wondering if that new converter is fitting in the front pump right.

            Worse scenario is you drop the pan to check for any metal or bearing material, and make sure the filter looks clean yet.

            Even before dropping the pan you could unbolt the 4 flexplate bolts, then rotate the converter by hand to make sure it is turning freely.

            _______________

            So here is the question? Was it a good working used transmission to begin with. Or a rebuildable core that you rebuilt.

            Did the trans come with it's own converter and you decided to just get a new one?

            ______

            Just trying to figure if it was a good working transmission, what changed.
            The New Converter ???
             
          • moparmat2000

            moparmat2000 FABO Gold Member FABO Gold Member

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            Another member mentioned vibration issues with poly mount. Installed a rubber mount, and vibration went away. 42RH torque converter should be neutral balance converter. You need to figure out of your 340 is external balance or internal balance. If its internal balanced a regular A904 flexplate bolts right up
             
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            • George Jets

              George Jets 1967 Dart 2 Door FABO Gold Member

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              So yeah those overdrive transmissions push back and crowd the tunnel, so definitely first check there for clearance problems.
               
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              • George Jets

                George Jets 1967 Dart 2 Door FABO Gold Member

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                If it is not vibrating in neutral or park, then that kind of says the new converter is rolling in the front pump OK.

                So looking like more than likely that the tail overdrive unit is rubbing on the tunnel.

                I have heard that people actually have to cut open and modifying their A Body tunnels for the Overdrive transmissions cause they fit so tight.
                 
              • Phreakish

                Phreakish Well-Known Member

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                Has this engine ever run before? Or is this a 100% new build?

                All the trans recommendations make sense, but don't forget the basics. Verify your firing order? We all screw it up at least once and it can cause some odd issues with the right cyls switched.
                 
              • plumkrazee70

                plumkrazee70 Well-Known Member

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                So this has been an 8 year build.

                1. The trans was purchased used (supposed to be a good working unit). It did not come with a converter. I purchased one.
                2. My 340 is internally balanced, using 904 flexplate
                3. I have a rubber trans mount.

                So I'm leaning towards something hitting the floor. Or the upper hoop. I'll take a closer look tomorrow after work.
                 
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