Guys, some ideas required:
I have just done an 800 mile round trip. Car, aircon etc perfect, BUT I am getting an annoying low frequency quite heavy vibration up from the floorpan on the overrun at about 70 MPH when I come off the throttle to a steady slight deceleration. This starts up once about 30 miles have been done so I wonder about something like shocks getting hot and giving up a bit. Passenger seat can be seen vibrating and it can be felt though mine. About 3 or 4 beats a second, from the rear as far as I can tell. Definitely not wheel balance, also the entire front end is perfect, steering fine. The vibration disappears on acceleration or cornering.
I am going to renew the cage mounts and rear shocks, more in hope than expectation. If that does nothing, drop the cage and check the diff bolts to the cage.
Anyone got any other ideas? The car is going perfectly, but this is spoiling it.

 

Cage mounts always worth first look with short jemmy/prybar, along with diff bolts and large trailing arm mounts. Also out to catch the unwary, Jaguar specifies many models must have all drive-train joints marked and always refitted to match marks to preserve balance. Any chance they got mismatched?

Having said all that, most oft overlooked are worn prop shaft uni joints, which often start their "wearing out" stage with floorpan & seat shivers like you describe. Perhaps exhaust heat affected after 30 miles? Just as notorious because they rarely fail, were split shaft centre mounts/bearings in saloons that had them.

Best wishes, Greg,

Ken

 

Ken
New propshaft a year or two ago and that cured it for a while. It has just come back. Maybe the driveshaft UJs are getting shot, though always assiduously greased, as the symptoms are like the old propshaft gave. The driveshaft UJs are factory originals and have never been apart.
If the shocks and cage mounts do not do it, I'll do the driveshaft UJs next, and remove the cage and check the diff bolts. A sod these undefinable problems.
Thanks for the ideas.
Greg

 

Greg; getting late so gotta take brain off to bed but . . . what got me thinking, and remembering, was description of low frequency vibration, more felt or seen than heard. Classically, and I hope accurately, referred to as LF "rumble" . . . it is so characteristic of propshaft balance issues or UJ wear. These cars, especially V12s, can be so well insulated from road noise and be so "body quiet" that, paradoxically, it doesn't take much imbalance or wear to upset them.

Not having a "live" rear axle, UJ swing angles are minimized, so such wear in just one year would be extraordinary, but . . . now means we have 2 rear half shafts that are also fitted with UJs and, by virtue of shorter lengths, swing through sharper UJ angles for given bump/bounce displacement. Widens the search area a bit, but since you will be in there . . .

Best wishes,

Ken

 

Hi Greg

Check this out: Top 5 Reasons Your Car Is Vibrating | HowStuffWorks

 

I am pretty certain it is not the prop. When I do the shocks and mounts I will examine the driveshafts. If I have to renew the UJs, I shall require guidance (ie clear instructions) on removing the driveshaft from the hub, and replacing it properly with the new bearings I might as well stuff in as I will be there. I understand this is a tricky setting up operation.
Greg

 

When I did my rear end I replaced all UJ's with solid units which do not require greasing.

Rebuilding the rear is not difficult just dam fiddly, especially that inner fulcrum.

My rear wheel bearings were worn and I had a lot of slop in the rear wheel. You can easily check bearings by jacking the rear up and wobbling the wheel. With 0.001" end float my wheels wobble very slightly, this is difficult to set perfectly as the end float is set with a shim.

 

Thanks warren. I am pretty sure it is not the fulcrums (inner or outer), as I rebuilt these some years ago and when worn the rear tens to lose its superb tracking on long corners. What I am nervous about is removing the splined driveshaft from the hub and setting it up afterwards. Are there rebuild kits with the shims needed, etc?
Greg

 

No rebuild kit I'm aware of.

It's not too difficult just time consuming and getting the right shim.

I ended up sanding the shim to narrow it to get the end float correct.

 

How's your rear rear engine/ transmission mount Greg? Specifically the cotton reel.
I spent much time and effort years ago chasing similar problem in my xj12 and worked on just about all the subassemblies mentioned and then more (including half shaft and diff output shaft bearings) in an effort to cure it.
Turned out to be worn out bushing in rear mount.

 

Rear hub bearings.....

A bit abbreviated but this gives you a sense of task:


You might find a kit with bearings and seals but probably not shims as there are just too many sizes.


All that being said, my gut feeling is that worn hub bearings wouldn't cause the symptom you describe. My experience, FWIW, with worn hub bearings is that they eventually rumble and complain but don't cause a vibration. I think the UJs and trans mount, as alrady mentioned, are more likely suspects.

Sometimes a UJ is so bad that you can actually see free play if you use a lever against the shaft. In these cases you can often also see metallic dust or debris in the area. All sure signs that the UJ is self destructing. However, lack of these indicators does not prove that the UJ is good. For that you'll have to remove (or at least disconnect) the shaft and manipulate the UJ through its full range of movement in all directions. It should feel as smooth as a baby's bottom. If you feel a 'catch' or stiffness the UJ is no good....and can definitely cause a vibration.

Cheers
DD

 

Baxtor, thanks for the suggestion. I did tighten up the gearbox mount a bit recently and packed out the foam bushing a bit to give it more resistance. The bushing was fine but I felt it was not doing much damping of the gearbox tail. That did remove some vibration. So I will take another look at it and see if it has too much looseness again. If it does, I will try to fab up something more solid and then try the car before ripping into the axle! The difference this time though, seems to me to be that the vibration is on the overrun, or gently trailing throttle, whereas the gearbox induced vibration was when I was on the throttle slightly.
Time will tell! I have been feverishly reading up about the hub float etc!
Greg

 

Thanks Doug, that is a help. I do not think the bearings are bad either, it just is that as I have to get the driveshaft out of the hub carrier to do the UJ, I though I might as well renew the bearings. Maybe I will not if they are OK, which simplifies the reassembly as the end float etc should not have altered, I guess?
I will do the tests you mention on the UJs, as well as the diff fixing to the cage. We will see, I hope.
Greg

 

Right, you can pull the half shaft out of the hub and then simply reinstall it. None of the 'settings' will change from doing so.

I agree to not change the bearings unless actually needed....or unless you have some sort of irresistible urge to go thru the process for personal satisfaction or edification. It ain't a 15 minute thing

Cheers
DD

 

I'm with Doug, don't replace the bearings unless they are bad.

Getting the cup out, I used a screwdriver and carefully knocked them out from behind. To re-install the new cups I ground the old cup smaller in diameter and used that up against the new cup to tap it home as I didn't have a press.

 

Other thing to check is the exhaust knocking against the cage of the IRS or the transmission pan? Space in tight under there.

 

Honour is due to BAXTOR. This morning I took a look at the gearbox mount, which seemed to be too easy to move up and down, even though all assembled as per OEM.


I removed the mount and added a cut-down polybush to the top of the gearbox spigot, then re-assembled with the normal OEM squashy cotton reel bush. The effect of the polybush is to compress the OEM cotton reel more than normal. thus, I hoped, making it damp the spring movement more firmly.
A fast half hour test revealed NO vibration, at least so far. Therefore, fingers crossed, I am spared rebuilding the rear axle UJs etc etc.
Pic attached of the mod. My sincere heartfelt thanks to all who made such helpful suggetsions.
Ggreg

 

happy for you Greg.
I recall when looking for the problem l mentioned l changed out universal joints on halfshafts, output shaft bearings, rear wheel bearings and swapped out the entire propshaft all to no avail.
I was running a manual trans in the car at the time and remember one day resting my hand on the gear lever during a drive and noticed the vibrations disappeared with just the slightest of downward pressure.
This helped isolate the area concerned and in turn led to the cotton reel. That was probably 30 years ago but still remember like yesterday.
All the best.

 

Well done that man.
Well spotted; good fix; good explanations and well shared.

Cheers,

Ken

 

Bad motor mounts will cause vibration. Jack up motor a little an see if vibration goes away.