I just had the transmission rebuilt this past week with a new torque converter. it is a c4 automatic. went for a ride yesterday and noticed a vibration under the floor that was not there before. Could the torque converter be causing the vibration or something in the transmission itself or none of the above? I won't be able to talk to the mechanic until tomorrow. I don't want to drive it until I know what the problem is.
Thanks

 

Check the bolts on the rear mount, maybe they're loose.

 

I believe some C4s had a vibration damper bolted to the tail housing. Did your transmission have it and is it there? It would not be the first time someone left it off.

 

The C4 vibration dampener looks like this. They are bolted to the rear of the housing.

 

Things that rotate cause most of the vibration in a vehicle...I would look at the part that changed (torque convertor) first and then the flex plate. If the flex plate was replaced along with the torque convertor, it may be that the wrong weighted plate was used. Possibly the plate could have been bent or the bolts not properly tightened as well. I am not sure what Ford had in mind with that vibration dampener...I know a number of people who don't install them on their C4/AOD transmissions and don't see any difference in how the transmission operates. The engineer in me says that the dampener is on the tailshaft to compensate for driveshaft vibration.

 

Does the vibration vary with speed, or RPM?

 

You can eliminate the torque converter by shifting into neutral while the vibration occurs. If the vibration goes away then it is not the TC.

If the vibration occurs at float (not on acceleration or deceleration) then the Ford weights will probably help.

My 8.8" rear axle had a set of "monkey *****" mounted on it too. I kept them as once I get the truck on the road I'll see if I need them.

 

Make sure the convertors drain plug is in the correct position on the flexplate .
It is possible to bolt it up without it poking through and will cause problems .