Like many, I'm fighting the dreaded rear end oscillation in an older truck. Note : I'm having this in both my 05 Ranger and my 05 F-150. The Ranger is by far the worst.

I ask about the differential, because it's about the only rotating part of any weight I haven't looked to deeply at. At interstate speeds, 72 to 78, I get an oscillation if the road has and irregularities in it. If it's a smooth road, it's there, but you would take it as part of various road vibrations if you didn't recognize the particular frequency.

History - Balance and rotation have absolutely no effect. Check for bent axles comes up negative. I've had U-Joints go bad in the past and this is nothing like it. The frequency is to low.

That leaves me with the differential itself. I have worked on just about everything else on a vehicle, but I have NEVER actually worked directly on one. I know how it functions, or how it's supposed to function. The LS clutches are out in the Ranger so that may be a contributing factor in it's problems. But the F-150 is not a LS differential. I did have a carrier bearing go bad about a year ago, but other than the osculation, I get no indication of any problems with the rear end other than the whine under power that appeared after the bearing was replaced. Something I've heard before in other vehicles when the wear pattern is off between the ring and pinion.

So am I on the right track and it's worth looking into, or just tilting at windmills?

 

I seriously doubt the differential is causing this. Are your shocks worn out? Check the temperature of the brake rotors with an infrared thermometer. A caliper may be sticking causing the rotor to drag.

 

Your driveshaft may be out of balance.

 

All new shocks and struts all the way around. I have one rear disk that is not far from neeidng to be turned, but it's not consistent enough to be a rotor / caliper problem. I've experienced that in the past on a different vehicle and it's not even close.

Weird part is that it comes and goes totally dependent on the quality of the road surface. Three hour interstate drive from Omaha to Kansas City. I've made it often enough that I know exactly what sections of road are going to set it off. Within a hundred feet of hitting smooth asphalt the major oscillation goes away and is reduced to the background vibration. A decent pothole or chunk of missing asphalt will momentarily set it off, but then it settles right back down.

I wouldn't think the differential could cause it either, but I've went through everything else.

I thought about the drive shaft, but this is a very course oscillation. More reminiscent of the old Twin I-Beam front suspension pogo, or a badly poured section of concrete highway. I've experienced a bad U-Joint giving the out of balance feeling so I'm familiar with the feel of that. It's been a while, but memory says it was a much higher frequency oscillation.

 

Personally, I think you're just hitting a harmonic frequency in the bad road surface. I doubt your truck is at fault at all. Something out of balance in the truck would be dependent only on speed, not the speed and road surface. If you're not getting the oscillation on smooth roads, then it's the road.

- Jack

 

It's 'there' even on smooth road surfaces, but just in the back ground. Like's whatever is just barely out of balance. A bit of rough surface is just enough to aggravate it. And it ALWAYS comes from the rear.

 

Expensive since it's off warranty, this is part of TSB 04-25-7:

https://www.fordpartsgiant.com/parts...-5d008-aa.html

 

Interesting. I don't have a problem in the front, but a search of the TSB had a link to another forum and they were discussing the exact problem I'm having. Seems the E-brake, which I never use but who knows about the tire shop. Seems it can hang and cause all sorts of problems. Take them apart, clean and verify function, reassemble and the problem goes away!

Not going to happen in the next few days as I'm a shade tree mechanic without the tree. Cold and wind and a bit of snow is NOT conducive to working on rear ends. But I will DEFINITELY be looking into it. Who knows, same type of setup on my Ranger, could be a two for one solution!

 

A rear wheel drive vehicle has an inherent harmonic vibration built in that can not be designed out. Most vehicles do not have it enough to notice. The Fox body cars that Ford used to sell had a weight attached just above the pinion to change the speed at which the vibration was felt in order to get it to a speed that isn't normally driven so it wouldn't be noticed. I had an 81 Granada that had this vibration around 55 to 60 mph and it annoyed us for the life of the vehicle. I drove the Ford dealer shop crazy trying to get them to fix it. The best they could do was to balance the tires on the car and that would eliminate most of it. At that time, an engineer coworker had a new full size Buick that had the vibration. He had the tires replaced and did lots of stuff trying to get rid of the vibration but couldn't.

 

I'm sure there are lots of us scratching our heads on this. My biggest bug-a-bo is that it's come on in the last 50,000 or so. Highways I've driven since the truck was new now give it fits. In some ways the barely out of balance causes it's own problems when it will just be in the background on smooth roads. Will be interesting to see what I find and what happens when I pull the rear drums and clean the inner E brake system.