I have been experiencing a vibration in my 78 F100 302 C6 that I can feel in the seat, floor and steering column. It starts around 25 mph and evens out around 50 mph, with the roughest being 29 - 40 mph. I know I need rear brakes and drums, pulled the driver side off, not much braking action going on, the drum has a small chunk missing from the outer lip (3/8"w x 2" long) and probably a 1/2" play in the axle. The u-joints are in phase but do not know about the eng/xmsn to rear end. Yesterday I picked up a 1/2 yard of top soil, fully expecting problems before I got home but to my surprise there was absolutely no vibration. I am planning on getting the wheels balanced today, could my problem be in the rear end? Weight equals pressure on wheels to tighten up slack? I am at a loss on this one.

 

maybe the topsoil "muffled" or "dampened" the vibration. Possible that it is your rear end pinion. Alost check the tranny mount

 

I have been dealing with my vibration issue now for 5 years so I know how you feel. (Excuse the pun) A few questions to start with,

Does your vibration go away if you put the transmission in neutral? Also if you have a 4X4 does it go away if you put the transfer case in neutral? And if you have a 4X4 does it go away if you remove the rear drive line and operate in four wheel drive?

I managed to get 90% of mine smoothed out but I still suspect a rear differential problem or something going on with my transfer case. Here's some things to try to help troubleshoot the cause of the vibration that has worked for me.

1. Replace the rear drive shaft.
2. Check the pinion bearings. Hold on to drive shaft and see if you have any play in the rear end, you shouldn't.
3. Balance your wheels.
4. Replace brake drums
5. Tighten everything on your drive train including tranny mounts, cab/bed mounts and cross members.
6. Replace U joints
7. Take the driveline out 90 degrees out of phase and check your results.
8. Rotate tires

Good Luck!

 

You have a whole 1/2" of play in the axle?

I'ld pull that axle and either get the collar pressed back into place or replace it. One day you're going to be loaded good and walk that collar off the sweet spot as the wheel comes out so far and either the truck just stops when splines are not engaged .... or the axle snaps off.

I'm guessing a 2wd F-100 ..... likely has a cenmter carrier bearing? Check the U-joints .... all three if you have 3 ... and remove the bolts holding the carrier bearing collar and drop it enough to spin it in place and feel for roughness.

Also jack and rotate each rear wgheel looking for a bent wheel or axle.

Yes, a good load will dampen vibrations.

 

My 78 F100 is 2WD and I an not sure of which rear end is under it (I think Ford). The vibration goes away when I put the truck in neutral and diminishes some when I go down hill. I am beginning to suspect my brakes are a big part of this because when I brake to slow down and get below 15 mph the vibration picks up and I am starting to feel some pull to the right. I am going to jack it up this weekend and remove the rear wheels (put back on and tighten lug nuts) put it in gear and see if I still get the vibration. I am trying to go about this using the order of cheapest/easiest repair to expensive/hardest repair, but somehow I always end up using the shotgun effect until I fix it or break it.

 

And it does not have a carrier bearing (thank whoever for small miracles). There is a clunk sound and hard shift sound sometimes when I go from park to reverse and the xmsn has problems engaging drive (rev engine until it goes or drop into low and rev engine).

 

I would say that it sounds like you have a failed universal joint at the pinion. Often times they will go quiet under load. You may have roller bearing dents in the caps and cross or in a worst case....the joint may have ground the rollers to dust. That also explains the clunk from reverse to drive and such. The noise diminishes when coasting and down hill as the pressure is swapped to another side of the ujoint cross. Often they hammer roller tracks in the "load" side of the cross.

Pull those ubolts out of the pinion and carefully remove the available caps to take a peek. If one or both have questionable looks....very little grease....metallic flakes...blue discoloring in the steel or even VERY subtle roller tracks, change the joint. It takes very little roller mark to make an obnoxious noise. When you go to remove the driveline there... apply the parking brake and check the joint for vertical , horizontal and rotational play. Put the transmission in neutral for this test so the transmission doesn't hold the joint under load. You want the drive line free to slop around when testing the joints.

You may also wish to pull the plug on the rear end to check for large volumes of steel in the oil or severely burned rear end oil. If the rear end has been overheated it may have chewed the ring and pinion gears causing excessive backlash that leads to excessive side loading of the pinion as the gears are wiped off. This leads to pinion bearing failure and occasionally carrier side bearing failure. Overheating will destroy the hard chroming on the bearing races and rollers making for a rear end growl and ultimately a gear shudder.

Again, this problem often goes quieter under load as it chews the gears to death and makes more heat. It often gets quiet under deceleration and down hill as the load is transferred to the back side of the gear.

 

After reading his last two posts, I'ld agree ..... universal joint.

Universal joints that are worn will often "self center" under load like when power is applied going up hill or when gearing down using engine drag to control speed .... but in lightly loaded conditions like traveling on a level roadway or slight down grade the excess room in between the bearing caps and trunions of the cross allow it to flop letting the drive shaft swing in an off cenmter arc like a jump rope. I've seen more than one do it .... seen them still in service long after rollers were dust.

Still, that 1/2" play in the axle isn't normal by my way of thinking, I'ld be checking on that as well.

 

Finally got around to removing the drive line to fix my vibration. The rear u-joint was okay had some galling on it. But the front u-joint was totally gone. To say it was dry and lacking grease would be an understatement. My next question is how do I reinsert the drive line into the transmission? I thought I kept everything lined up but when I go to insert the yoke over the shaft it gets stuck and I have to force it off. The manual said to apply grease inside the yoke (which I have not tried), maybe this would help? Any suggestions will be greatly appreciated!

 

And there is no 1/2" play in the rear end. Must have been my wife chirping in my ear that distracted me when I was measuring it.