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Vibration story update

917 Views 35 Replies 8 Participants Last post by  Gaubabis
Original post can be found here Vibration... its a kind of a longish story...

Summary: 1996 P38 V8
Since I purchased the vehicle there have been drive train vibrations, to a greater and lesser degree. The original post has some of the details.
The vehicle had been converted to TF coils, so I suspected the source of the vibrations was prop shaft together with the increased ride height etc. I set about reverting to original spec, air suspension, redid the prop shafts, etc. Vibrations improved but did not go away. Next job was to exchange the TF shocks (gas) for Britpart shocks (oil), which I eventually got around to this weekend.

My first test drive seemed to show a significant improvement, its smooth up to 105kph. Previously vibes started at 65, increased up to 80, decreased at 100, increased at 105 upwards. So, definitely improved but not gone, but this has probably been the most improvement I have seen from any one change.

I have been suspecting a suspension resonance problem

The only thing for me is to renew my search for a front diff mass damper, since mine remains AWOL.

The quest continues!
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Very early P38s didn't have the front mass damper but shortly after production started they started fitting it and issued a service bulletin to dealers to fit one if a customer complained of vibration. So you may well be on the right track. I know a P38 without one on the rear will vibrate horribly.
You also should check ujoints. I had a vibration that started at 60 mph. I had to use 2 pry bars to check the joints, there is too much pressure to try to roll forward or back ect. I had less than 1 mm movement in the rear ujoint near transfer case causing terrible vibration. If you dont know how old they are I suggest changing it is an easy job.
You also should check ujoints.
Done that, this was my first port of call. Both shafts checked & rebalanced. Phasing is correct. Then replaced all the UJ's with Spicers and rebalanced. Finally replaced the front PS with a Tom Woods double cardan that they make for the P38. None of this made any noticeable difference. The main difference was lowering the ride height to standard with the EAS and now changing the shocks. The TF coils were riding at about 52-54cm unloaded IIRC!
To add to this ^^ I did a test drive with the front PS removed (vibes still present).
If is not the ujoints then it has to be the tires. Are they over 5 years old. I had death wobble when I first got it but I changed tires,shocks and steering damper all at once so I don't know what solved it. Only vibration now is ujoint wearing out. Wheels just roll on bearings mabey lift the front back and check for play. Good Luck
If is not the ujoints then it has to be the tires. Are they over 5 years old.
Done that.
Original tyres were 4 years old. Had them rebalanced, no difference to the vibrations. Purchased a new set, also no difference to the vibration.
I have checked the wheel bearings for play when I replaced the shocks. All looks fine there.
Wheel bearings have to be really badly worn before you will feel any play as the half shaft is held at the other end so unless you are able to bend the half shaft it won't move. However, I've driven a P38 with a completely shot wheel bearing and there was no vibration, just a horrible drone from it. Vibration at around 60mph (100kph) is usually a wheel out of balance or tyre but as they have been changed, it is unlikely. Low tyre pressures can also cause it, what pressures are you running (should be 28 psi front, 38 psi rear) and are you running standard size tyres?
The original tyres were oversized, but the new ones are back to standard size 235/70/16.
Pressures are as per manual front 1.9bar rear 2.6bar
You must have the 7x16 wheels, the same as I have, not the more common 8x16 which take 255/65x16. Not going to be tyres then.....
If you did all of that then it is time to get the pry bar out and check trans, engine, transfer case, mounts and double check bushings and ball joints. You can switch tires from front to back to see if it changes. Is the vibration in the steering wheel or the seat? Or mabey it is the damper wheel I have one on the front still.
The vibration is general (bodywork), not specific to the steering wheel.
My sense is that the vibration frequency is not speed dependent. In fact I would say I can sense the same frequency at low speeds (sub 60kph) at very low amplitudes and in short bursts, on some of the rougher roads. But I may be imagining things. I have tried to confirm this with an audio spectrometer but there are too many variables (road conditions etc) to get any objective results. The vibration frequency is definitely not engine speed related.
My approach has been to revert back to factory configuration first, check for any worn/abnormal components & rectify those and then take it from there. I may need to get more scientific and attach a number of accelerometers to the main components and log those to see what is 'driving' the vibrations.
Perhaps you've already done so but a check at all four wheels might reveal something. From experience other than with the P38 an initial step was to power the vehicle off ground and observe / measure the lateral runout at each wheel. This only requires idle speed so unless there is some prohibition of doing this with the P38 it could be worth a try. A close inspection of each brake rotor is suggested as well. Replacement rotors vary widely in quality. Any irregularity can likely express itself by creating vibration. Use of a dial indicator should work to find a fault(s). Last thought would be the lateral balance of each wheel / tire. Modern balancing machines typically will have a digital readout for both rotational and lateral balance. It is critical that both of the balances are as close to 0.00 as possible.
Thanks, no I have not checked the rotors, but will do so.

Question: would lateral runout not be picked up on the brake pedal while braking? Or does that only happen when it is excessive?
Radius arms are all good? Half shafts are okay? Wheel centers fit snug to the hubs?
Running it with the wheels off the ground is fine in theory but you will find that one wheel will invariable turn faster than the other so the Traction Control will kick in.
i read you removed the front drive shaft and went for a drive but did you do this with the rear , if not might be worth a try as you have done as much as possible by the sounds of it .
just an FYI i replaced the unis 3 times on the front shaft , that's 3 different brands before i fixed my vibration problem.
Running it with the wheels off the ground is fine in theory but you will find that one wheel will invariable turn faster than the other so the Traction Control will kick in.
I suppose I should be able to check this by hand rotating the opposite wheel as long as I can find an assistant to eye the dial gauge!
i read you removed the front drive shaft and went for a drive but did you do this with the rear
No I have not yet, it was next on my to-do list. Definitely worth doing.
If it is the same with the rear propshaft removed, it could be an engine/gearbox mount problem but that would tend to only be there when under load, so only when accelerating and not at a constant speed. Otherwise, if there at the same speed all the time it has to be something that is always turning at the same speed, so that would point towards hubs, wheels or tyres still. Whether they are being driven or not may not make a difference.
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