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Vibration at 55 mph

Discussion in 'Non-Mopar Tech Support' started by core, May 29, 2016.

  1. core

    core Member

    Joined:
    Nov 1, 2012
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    Hello
    I have been chasing down a vibration on my 1975 valiant. Here is some back ground information.
    360 block turned into a 408 stroker with a 727 three speed.
    The vibration starts around 50 MPH and 2500 RPM.
    I have balanced the tires and drive shaft twice.
    Installed new u- joints, 3000 B/M hole shot converter and B/M flex plate.
    Verified that the block is correctly balanced.
    Removed the tires and brake drums and returned the car to the correct ride height. Brought the speed up to 50 and the vibration was still there.
    Today I went for a drive keeping the tranny in 2nd. I brought the speed up to 50 then 55 no vibration. Maintaining 55 I shifted into 3rd and the vibration immediately appeared. I repeated this test several time with the same results.
    I also tested out the RPM. In 3rd at 2500 rpm I had a vibration and in second at 2500 rpm there was no vibration.
    I'm thinking it something to do with 3rd gear in the tranny, but i have spent hours looking online and can find nothing.
     
  2. ImperialCrown

    Level III Supporter

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    Slipping clutches can shudder. This can be from low line or frt/rear clutch apply pressures?
    If it doesn't vibrate at 2500 RPM in 2nd, but does in 3rd, then it doesn't sound like an engine balance issue. The 360 is externally balanced.
    If driveshaft and wheel balance has already been verified, then things point to the transmission.
    Can you feel the shake in your seat or through the floor by your feet? Does it feel like it is from the rear of the car?
    If the vehicle is jacked off the ground by the axle (to duplicate the driveline angles the same as going down the road), can the vibration source be pinpointed under the car?
    This must be done with safety in mind with the rear wheels spinning at 55 mph. A drive-on lift at a shop to keep the car level and all wheels off the ground would be recommended:
    AR14_NV_2014.jpg
     
  3. core

    core Member

    Joined:
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    The shaking can be felt in the seat. Hard to pinpoint. The motor itself shakes and causes its own vibrations.
    I'm sending it back to the transmission mechanic next week to have a new stall converter installed. The car has chewed up the snoughts on the last two. The dowel pins on the block needed to be replaced so I think.
    I had a cheap converter installed just to see if the problem was fixed.
    I'm thinking that the transmission could be causing both.
     

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