[kansan]

I have a 1968 C20 Chevy pickup with a 396. The transmission shop took out the turbo 400 and put in a 700r4. The transmission works perfectly in the first three gears.
When if shifts to fourth and the converter locks up, it will vibrate under a load, but at a level speed it does not vibrate. It must have something to do with the converter., it does it at any speed from 40 to 100 plus. But only when it is locked up and you are mildly accelerating. . The shop has replaced both the converter and then the transmission, with no change. When in overdrive and you kick it down, it works perfectly.
It runs at 1000 rpm at 40 and 1650 rpm at 70.
From trouble in Kansas. Thanks.

[snydski]

The angle of the pinion to the driveshaft must be the same as the angle of the output shaft to the drive line check out this link your answer may lie there. tom woods 4x4 http://www.4xshaft.com/driveline101.html

[kansan]

Thank you, I have a two piece driveshaft with a carrier bearing. The truck spent two days and over $600,in a frame shop, having them check out the angles.
I also had a new shaft made with new bearing and ujoints, no change. If I can find a turbo 400 that I can rebuild, I am thinking of putting it back in. Getting tired of the problem.

[rick 427]

Those rpm numbers are sure low....it's got me wondering and I'm thinking out loud here....with the converter clutch on,you are direct with the motor,like a stick shift car....with that said,if i'm in 5th gear in my truck,and i have to slow down to say 30 mph,the rpm's drop, and if i then accelerate lightly while still in 5th,the truck vibrates,because the clutch can't handle the load at that low rpm,,then i downshift to bring the rpm's up and everything is fine.....and with a 396 tugging on that converter at those low rpm's,a little more gear in the rear end??...just a thought...

[kansan]

You have a good idea, I changed from a 4.56 to a 3.73 when the trans was changed., Maybe a 4.11 would work. Thanks.

[SSedan64]

X2^^, with that low rpm & the TCC locked up, you may be feeling the Engine firing pulses, a large Cam will make it even worse.
The type of Friction material used on the TCC can also makes a difference, PWM & Normal lockup types.
TCC = TorqueConverterClutch
PWM = PulseWidthModulation

This should smooth out at higher rpms tho'.
Have you checked Flexplate runout?
A misaligned Stator Support in the Pump can also cause Vibes, + Bushing, Seal wear & Pump problems etc..

[73elcorod]

I had the same problem then i put a 700R4 in my elcamino. it was fine on the street but as soon as i put the taller drag slicks on the back it would vibrate like heck in 4th. i put lower gears in it and now it is fine with both the street tires and drag slicks. 700R4's just cant handle that ultra low rpms hope this helps