Persistent vibration

S

Seawarp

New Member
I've not been able to get under the car for a while but the physio seems to be working and I can now continue my search for the mysterious vibration. I haven't yet changed the upper link bushes but I doubt whether that's the problem anyway and the main contender now seems to be the propshaft. I have fitted a ZF box and the propshaft was modified and I was assured that it was balanced. However, the car did have serious vibration problems at the time of purchase and I have no way of telling whether it had been disassembled previously. So, the car is a P6b transition model, lhd, French from new and the diff casing has a serial number that appears to be K 0000 2. The question is, what degree of offset should the splines have? 107° or 144° ?
Following on, whilst under the car should I shim the new diff mounts by 3/8" to bring them up to 1.7"? The new ones didn't make any noticeable difference to the vibration.
 

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Can't help on offset. Check out all the universal joints by removing components and checking for movement in all planes, if you find stiffness in any joint replace it.
 
I would examine all 6 UJs (both propshaft and half axles). They can do pretty much what you describe. I've come across UJs that look and feel fine only to find that at least one bearing cap is full of rust and broken bearing needles.
 
The propshaft offset is built in, so you need to find the arrows (sometimes quite difficult) and align them. The 144 offset applies from suffix B diffs, so that should be most cars - mine is an E.
 
If the propshaft has been modified, most likely shortened, I don't know the details of the conversion, there's a good chance that it hasn't been done to maintain the correct alignment.
 
In that case the only option might be to try shifting the alignment one spline at a time and see if the vibration changes. Starting with an alignmment as close to 144 as possible. Note that 144 is 40% of a circle - if there are 10 splines in the joint, its 1 spline off a half circle; if 20 splines, 2 off half way.
 
cobraboy said:
Can't help on offset. Check out all the universal joints by removing components and checking for movement in all planes, if you find stiffness in any joint replace it.
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I did all that and renewed all the u/js anyway. I've also replaced the wheel bearings and serviced the de-dion tube. The tyres are all new and the wheels and discs checked for true. The engine sits at the correct angle and the tracking is correct. Not a lot left to eliminate. (The diff mounts which I replaced were actually all in good condition and I can't help thinking that maybe they are the problem having been fitted previously without any shims: hence my post)
 
If dropping the prop try using a digital angle finder on the gearbox and diff flanges and get the angles to being the same from the vertical as possible.
Doing this, and finding one stiff driveshaft joint eliminated the vibration I had after a 5 speed install.
 
jp928 said:
The propshaft offset is built in, so you need to find the arrows (sometimes quite difficult) and align them. The 144 offset applies from suffix B diffs, so that should be most cars - mine is an E.
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Thanks. My diff doesn't have a suffix letter though, unless the number 2 could be seen as a Z ? The car is European export 1971 so has features from both series. First things first I suppose will be to get the propshaft off and see what we've got, and then if by any chance it looks correct, shim up the mounts to the 1.7" specification.
 
Where did you look for the diff serial - bottom of the rearmost part of the casting? My diff rubbers dont have any shims that I know of? The book lists 578080 rubber mount as 1.7" high, but without shims. I replaced mine a few years ago.
 
jp928 said:
Where did you look for the diff serial - bottom of the rearmost part of the casting? My diff rubbers dont have any shims that I know of? The book lists 578080 rubber mount as 1.7" high, but without shims. I replaced mine a few years ago.
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Yes, as in the photo I attached. I ordered the mounts for a1971 P6b, chassis number******* , and what I got back were the same as the ones on the car - not 1.7" so I assumed they were correct. Subsequent research suggests that apparently the slimmer ones can be used, but with shims to take the distance out to the 1.7" as specified.
 

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Thats not the Diff number. The correct number is at right angles to that one and further in toward the axle line. they are not well stamped and may well need some brushing to get the rubbish off it so it can be read. The important part is the letter at the end - sometimes stamped a little beyond the numerals to the right.
 
mikecoombs said:
Thats not the Diff number. The correct number is at right angles to that one and further in toward the axle line. they are not well stamped and may well need some brushing to get the rubbish off it so it can be read. The important part is the letter at the end - sometimes stamped a little beyond the numerals to the right.
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OK, good to know, I'll look further. (This number isn't well stamped either ! What's it for?)
 
Maybe the diff has been reconditioned at some time and the reconditioner has ground the original number off and stamped in their own one
 
The parts book shows 2 different heights of support bushes with no mention of shims. There are 2 different support plates listed so maybe the bush change is relevant there. The book also lists different length bolts , presumably because of the different height rubbers. My 74 D chassis has the 1.7" rubbers.
 
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