Question for Cliff and other transmission experts
 

I will start at the beginning of the problem so you will have all the info I have. At the end of the 2010 season my rearend let go. It came loose at the end of an 1/8th mile run at 95 MPH. During the winter, and for the most part of early 2011, I was buying parts for a 9" Ford rear. After putting the new rear in the car I found I also had a transmission problem. When the rear broke it took out the trans also. The output shaft was snapped into right behind the splines on the front of the shaft. I took the output shaft from another trans I had and went ahead and overhauled the trans, it had about 1200 passes and 5000 miles or so on it. When I finally got the car to the track I was having an issue with the flexplate hitting the starter under power, and there was a bad vibration in high gear right about the speed trap at the end of the run. No vibration any other time at all, and as soon as I lifted my foot off the pedal it would stop vibrating. I only made 3 or 4 passes and took it home. I spent 2012 in Germany so I didn't get to fool with it till this spring. When I pulled the trans out I found two cracks in the flexplate out near the starter ring. I thought I had found the problem. Well a new flexplate cured the issue of hitting the starter, but first time out this year the vibration is still there. This is with two different converters also. The only time the vibration is present is in high gear at 90+mph and 5000+rpm. With the Ford rear I only have about 1/2 inch of play from the end of the u joint cap to where it seats in the pinion yoke. Could this cause that by the driveshaft bottoming out on the output shaft? Or would you think there is another internal issue with the transmission? It was very violent when that rear broke. Sorry for such a long post, I just wanted to give all the info I could.

Honestly,it could be most anything at this point...

Was the driveshaft replaced after the rear end failure?

How much of the old trans was reused?

From what you said it sounds like you just replaced the broken output shaft,right?

I probably would've replaced the planetaries as well myself...

And yeah,it's entirely possible that the driveshaft is bottoming out too,but that can be a tough one to diagnose.

I dunno,I hate these sorta deals,as they tend to be a real b!otch to work thru.

Bret P.

Yes, I took the driveshaft and had it shortened. They were supposed to shorten it 1.5 inches, but I think someone dropped the ball. It was balanced and has new U Joints. The only thing I replaced was the output shaft, other than seals, bushings, clutches, and steels. All the other internal parts are the same. What is puzzling is that it only does it in high gear at the end of the track, smooth as silk everywhere else. I guess I'll pull the trans again and look for a smoking gun. I have another trans I can try just to see if the vibration goes away. It is not strong enough to leave hard. but it should be alright to run it up to the speed at which the vibration occurs. At least I could maybe eliminate, or prove, the trans is the culprit. I just hate removing and replacing the thing. I'm getting lazy in my old age I guess.

We assume TH350/400 automatic trans?

It's not in the trans, unless it's the tailshaft bushing. Trans should have made noise or come apart after a few passes with that much shaking. I bet the driveshaft or ujoints are damaged, the Ujoint angle is wrong or the driveshaft is bottoming out. It could also be the tires out of balance.

Back in HS a buddy had a '72 Mach 1. At about 90 MPH the car would suddenly develop a violent vibration to where you couldn't see anything in the rear view mirror. If you stayed in it, the car would smooth out at about 98. We never did chase it, but I always felt like it was the U joints or the tires.

Yes th350, new tires (different set from last time out) driveshaft was balanced with new u joints. Pinion angle is at -4 when checked at pinion, last rear was set up at -5. It is weird, I have never had a vibration before the rear broke and damaged transmission. I was hoping someone could say if it was something in the trans that would only do it in high gear. Driveshaft would vibrate in relation to rpm and should do it in any gear, right? It does not care about mph only rpm. Tires would show up at higher mph. Could be tires. Just odd that same vibration with two sets of tires. I guess I will try my other trans and see what happens. Thanks guys.

"The only time the vibration is present is in high gear at 90+mph and 5000+rpm. With the Ford rear I only have about 1/2 inch of play from the end of the u joint cap to where it seats in the pinion yoke. Could this cause that by the driveshaft bottoming out on the output shaft? Or would you think there is another internal issue with the transmission?"

In high gear the trans is 1 to 1. If the slip yoke was bottoming out you'd have torn up the thrust bearings inside the trans in short order.

Since the output shaft was replaced, was a new bushing installed in the front of it where it rides in the rear of the direct drum? Did you check the rear of the direct drum to see if it bent the portion that rides in the nose of the output shaft? New bushing installed in the output shaft to keep the direct drum on center? What about the tail housing and bushing? Any breakage that violent could have damaged them as well.....Cliff

I chased a vibration in my drive line for nearly a year before finally replacing the tailshaft bushing; vibration gone. However, in my case the vibration started at about 45-50 and continued on up through my trap speed. It would go away as soon as I let off the throttle at whatever speed.

That sounds exactly like mine, although it happens at higher speeds, it too stops as soon as I let off the throttle. Thanks for all the replies. At least now I have some insight from ppl who know MUCH more than me. I think I'll try replacing the tailshaft bushing and see if that makes a difference. It sure would be nice if that took care of it.

The other end of the output shaft rides in the rear of the direct drum on another bushing. Quite likely it may have got torn up during the ordeal, and is allowing the shaft to spin off center. I see a lot of direct drums torn up where they drop into the bushing in the output shaft. Most rebuild kits come with a plastic insert in lieu of a bushing for this reason. It's also a LOT easier to install than the bushing. Many builders don't even look at that bushing, or at the bottom of the direct drum when building TH350's.......Cliff

My 2cts. I chased a vibration in the rear of my 65 Lemans back in 77.Drove me crazy. Ipulled the axles and noticed one was tight coming out. I live in the country where ther are hills steep enough to coast at 60 mph down them. I pulled the driveshaft(tried 2 of them) coasted down the hill,yep rear was bent.How that happened I'll never know.

Hillbilly?