My car is making a pulsing vibration under steady acceleration in drive only. Does not do it in 1st or 2nd. 340 with torqueflite. Not there when coasting, only under acceleration and only in Drive. Could it be the torque convertor or driveshaft out of balance? This is a good one.? ? ? ?

same load (in hg) for the steady accelleration in all gears but acting up in Drive only? Everything as far as you know is the same? WAG, slight fuel starvation. EDIT or tire/wheel balance/wheel bearing issue, loose etc

Do you feel it in your hands or your butt?

TMI
TMI
TMI
TMI
TMI

My tire man asked me that when I was tracking down a vibration. turned out to be a front ujoint getting ready to let go (& it did)

Has the rear suspension been updated or altered in any way? Incorrect pinion angle will cause a harmonic (pulsing) vibration if the u-joints are not properly phased.

Hands = front end, steering etc. Butt = drive line or rear wheel area.

Butt. It's like I'm sitting on top of the vibration. I already replaced U joints.

too much shaft end play causing not enough front yoke spline engagement in the trans (I had that causing a vibration for me once). worn extension housing bushing. worn trans mount. throwing out potentials.

What is the correct length for the driveshaft in a 69 GTS 340 auto?

The best way to check the length of the shaft is to push in into the trans until it bottoms out and then measure the distance to seat the U joint in the rear end yoke. That shouldn't be more than 1.5"

What Stump said, you may be OK on that dimention as is. (have it loaded)

Is there any way that it could be the torque convertor balance? You are basically at a 1 to 1 ratio in drive. Could I be feeling an out of balance convertor in drive where the vibration is lost in first and second.

I think any converter imbalance would be felt more in neutral and worse at higher RPM. Have you checked the trans mount etc?

Just for giggles, drop the belts off your engine, I swore I had a converter or drive line issue, was the alternator!

Convertor out of balance will be out of balance in all gears ... and in neutral .... moving or standing still ...

If it wasn't acting up before you had the "U" joints replaced, THAT is the first place to look and most likely culprit provided that nothing else was messed with.

Check/replace the transmission to cross member mount. A bad mount/worn rubber will drive you crazy chasing other components for vibration problems.
Ron

flex plate cracked?

What's the verdict?