James71Cutlass

From 35mph to just under 40mph and regardless if I’m speeding up or slowing down I get vibrations in the car. It feels like I’m shifting from 1st to 3rd gear if I was driving a manual car. Is this because my rear end gearing is 2.56?

I’ve got a ‘71 Cutlass S car 307 4bbl with cam, lifters and a turbo 400 transmission.

Vintage Chief

Has this always been an issue? Did this just begin occurring? Did you do something recently to the engine and/or the transmission?

My 1971 350 4bbl TH350 w/ a 2.56 shifts firmly & smoothly - no issues whatsoever.

Is the Automatic Transmission Modulator line hooked up to the engine & the transmission? That's just a first start question and a simple question/issue to check before proceeding with additional diagnostics.

James71Cutlass

It has always done this. I figured it was the gears as like I said before it seems like your shifting from 1st to 3rd. I’ve never had this in any of the cars I’ve owned.

Vintage Chief

I can assure you it is NOT because you have a 2.56 gear. Now, to the other questions I asked?

Vintage Chief

The other very simple thing to check is this. With the vehicle rolling between 35 mph - 40 mph (during the vibration event), put the vehicle in NEUTRAL. Perform this test several times. Does the vibration STOP or does the vibration CONTINUE? If it continues, have you been down below to examine the integrity of the U-Joints? Faulty U-Joints can have a propensity to demonstrate vibration during a particular speed range before they go completely South. Might not hurt to go underneath and check them (if the vibration continues during free-wheeling w/o the transmission engaged - in NEUTRAL).

James71Cutlass

Sorry, I haven’t checked that as of yet. I will, thanks for the suggestion.

Vintage Chief

Additional simple stuff to (suspect) check before moving into more worrisome/bothersome stuff would be to address the tires/wheels. Tires/Wheels which have gone out-of-balance can present a speed-sensitive issue. (1) Has the vehicle sat for an EXTENDED period of time on OLD tires which may have created flat spots - and, as tires increasingly age and sit extended periods, sidewalls do lose their rigidity (unequally) i.e. tires themselves could be suspect; (2) Balancing of the wheels. Have you lost one or several weights off the wheel(s) causing an out-of-balance condition? (3) Check the integrity of the tires themselves - can you identify any cupping on the treads? (4) Rotate the wheels/tires - vibration still occur at the same speed range?

Sugar Bear

Check all mentioned above plus the transmission mount and the transmission crossmember to frame bolts. Look closely at the seals for the bearing cups on the universal joins for any signs of rust.

Good luck!!!

James71Cutlass

A new set of tires were put on the car and then it sat for some time. I don't know exactly how long. I will have to check them. Thanks for the suggestion.

James71Cutlass

I will try that this weekend, weather permitting. Thanks.

James71Cutlass

I just had a new trans mount put in. I 'll check the crossmember bolts. I'm having a set of 3.42 gears put in this winter so I think I'll just have the u-joints replaced as well. I'll take a look at them to see what condition they are currently in. Thanks for the suggestions.