I've got a 73 F100 LWB with a 360/c6 and a 2piece driveshaft. I'm not getting any help from the local driveshaft shops, so I'm trying to see if I can sort this out myself.

My issue: Low speed wiggle right off idle to about 15-20mph, and a random vibration at freeway speeds. I've spent some time studying the driveshaft, and I noticed something that doesn't make sense. The driveshaft where it comes from the transmission is bent down a bit on it's way to the carrier bearing. This doesn't make sense, I would expect this joint to be pretty close to straight. the drive shaft joints both behind the carrier bearing and at the rear axle have nearly no angle whatsoever. Surely this is not the way it should be. any thoughts on where to go next would be greatly appreciated.

 

All the old trucks I've owned, the carrier bearing or "center support" bearing is always completely trashed, and should be replaced. Any rotating assembly needs to have vibration minimized, because otherwise it will beat the hell out of everything it's connected to, needle bearings, oil seals etc.

Also inspect for maybe balance weights that have flown off, or "out of phase" assembly. One trick I've never had to use, install a large radiator hose clamp on the d/s, and clock the screw adjust in different positions and see if that helps. Vibration can be tricky to locate sometimes. Tires can cause trouble too of course, separated belts. Can try rotating tires around. Even if it seems like it's the d/s, it all adds up.

 

Have you had the shafts balanced yet? Is the vibration being induced by rear suspension travel?

The connection points of the shafts need to be stable too. That means that your transmission output shaft/yoke, the carrier bearing, and the differential pinion/yoke, all have to be stable with no play or movement.



Tedster mentioned phase, and what that is (I think) is the angles of the connecting yokes. The angles must be the same where your front driveshaft drops down from the trans, and then where the opposite yoke 'levels out' at the carrier bearing.

If the trans is dead level, then the yoke which goes into the carrier must be dead level. Not sure what the max angles are here, but the closer you can get to straight, the better. The driveshaft itself doesn't have to be level, or longitudinally straight, as long as the driving and driven shafts, which it connects to, are level, and parallel in the longitudinal plane as well.

Then of course, the rear driveshaft must adhere to those same standards. It's a little more simple with a one-piece driveshaft, since you can correct pinion angle by raising or lowering the transmission mount - and hopefully not having to rotate the rear axle in relation to its mounts...


If you do have a pinion angle issue, hopefully you can correct it by simply altering the height of the carrier bearing or transmission mount.
If it's the rear driveshaft, then adjust the carrier, and follow up by adjusting the trans height to the carrier.
If it's the front shaft, then adjust the trans height.


There is also the issue of shaft balance in conjunction with the yokes. When you disassemble a Fox Mustang driveshaft replacing U-joints, instructions will tell you to mark the front and rear yokes, so that they can be reassembled in the same orientation. This means that you might be able to fix the issue by unmounting the driveshaft and rotating it 90º, and remounting it. That's a bunch of work, especially with two driveshafts.


You could temporarily mount a camera under there and see if there are any visual signs while you drive the truck. If you still can't find the source of the vibration, you might have better luck taking your truck to a place that works on bigger trucks, which are mostly all working vehicles, which really need to be fixed.

 

Dont forget motor and trans mounts.

 

Thanks for the thorough response! The local driveline shop basically told me to kick rocks when I drove the truck up there. I concluded the reasoning for that was the angle to the carrier bearing, which upon further inspection the bracket it was bolted to appears home made. The rear phase of my shaft looked about equal, in order to have corrected the angle to the carrier, I would've had to lower the transmission or raise the carrier, both of which did not seem feesible.

I decided to take a drastic measure, I found a one piece driveshaft 7 hours round trip from my location, and proceeded to go get it last night. It's a big joint shaft, so I intend to install new joints in it, and I got the corresponding larger yoke for my pinion. The rear axle has some bearing noise, but hasn't developed play in the input, so I'm hoping the one piece shaft will correct the vibration before I get the differential and axle bearings rebuilt.

Thanks for the input everyone, I will update this thread once I get the one piece on later this week. Incidentally, does anyone happen to know the torque spec for the pinion yoke nut? Or would I be better off with a combination u joint until the rear end gets addressed?

 

I'd settle for the combo U-joint as an interim solution, just for the sake of not opening up another can of worms. Save that rabbit hole for later.

 

Thats what I'll do, Ill save the yoke for the rear end guy when I get that done. Thanks!

 

As promised update: I installed the 1 piece driveshaft with ac ombination joint, vibration is gone. Ironically on the way to the shop the sector shaft seal decided it was done after 47 years so I have a bluetop on the way. Thanks for the guidance everyone!