Occasional suspension vibration on coil sprung P38a

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meles meles

meles meles

New Member
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I originally posted this in the faults forum, but was advised it would be better here, so here goes...

I have a 1996 4.6 P38 HSE which has been converted to coil suspension. Most of the time its road manners are impeccable but very occasionally it seems to get unsettled and develops a quite alarming suspension vibration. Accelerating briefly cures the vibration and it may not then re-occur for weeks. The vibration seems to come on when i'm on a twisty country lane and catch a pothole, though it once occurred as I was joining a motorway as the nearside wheels crossed the cat's eyes and rumble strip. Being so intermittent, it's hard to figure out what is causing the problem. Anybody any clues ? I'm guessing that maybe I should be searching for something not quite tight, or something that can experience a harmonic resonance under certain conditions? I've owned Classic Range Rovers and a Defender but never experienced anything like this.
 
try checking the vcu in the transfer box,numerous people have replaced allsorts including myself to no avail,after checking the viscous coupling unit and found it faulty,i fitted a new one along with a complete new set of u/js i now get no vibration or steering problems at all
 
It will be either the steering damper or the radius arm bushes
Had it happen a few times on coil spring classics - gives you a fright, don't it ! :)
 
The steering damper has been changed (the old one was found to be fine when removed, but replacement was cheap enough), and all the bushes are sound. There's no play anywhere in the suspension, back or front, and the propshafts have no play in the either. All the brake assemblies are fine and the wheels are balanced. No apparent cause anywhere to be seen.
 
andy 0352 said:
try checking the vcu in the transfer box,numerous people have replaced allsorts including myself to no avail,after checking the viscous coupling unit and found it faulty,i fitted a new one along with a complete new set of u/js i now get no vibration or steering problems at all
Click to expand...

I have seen a lot of threads blaming the VCU for problems. I don't quite understand why, ( I accept that the VCU does cause these problems ) the car has a centre diff so surely the VCU is only a means of apportioning power to the front wheels giving a rear bias under normal conditions and then locking to transfer more power to the front if the rear wheels slip? Or is the gear ratio different in the front diff perhaps?? Or possibly the centre diff is a limited slip jobbie with the VCU taking up the the rotational differences???
All the other 4 X 4's I have owned with a centre diff have managed quite well without a VCU.
Any thoughts anyone:confused::confused::confused:
 
It happened three times today - and that's after the new steering damper had been fitted and both myself and a garage had checked out the rest of the suspension.

The first time I was doing about 55-60 mph on a flat, smooth dual carriageway with no obvious surface defect. The vibration started very suddenly and was very severe, shaking the whole vehicle and almost pulling the wheel from my hands. I applied the brakes - which pulled evenly - and once the speed dropped to about 25 mph the vibration ceased almost instantly.

The second time was under similar circumstances an hour or so later.

On the third occasion, i was on a narrow, twisting lane and travelling at around 35 mph. The onset of vibration was immediate and very frightening: again it stopped as I braked and speed fell away. Subsequent inspection reveals nothing wrong with the shocks, springs, steering dampers, tyres, roll bars, propshafts....

*Scratches ikkle head in bewilderment*
 
Can anyone suggest what the problem might be *other* than the suspension ? If there isn't a suspension problem - and neither myself nor three garages can find one - what else might be to blame ? There doesn't seem to be a problem in the drivetrain, but I'm open to suggestions. The VCU has been suggested, but I can't understand how it could cause such an immediate and intense vibration that then disappears...
 
I'll give that a check over next. I've just scared myself silly reading various online fora and finding a known problem called "P38 Death Wobble". It seems to afflict P38s shod with 18" wheels in particular. Guess what mine has. There doesn't seem to be a definitive cure.
 
meles meles said:
I'll give that a check over next. I've just scared myself silly reading various online fora and finding a known problem called "P38 Death Wobble". It seems to afflict P38s shod with 18" wheels in particular. Guess what mine has. There doesn't seem to be a definitive cure.
Click to expand...
change from the 18' wheels?
 
id agree with nathan5346, try and get hold of another set of wheels and try it with them on, even if its just to eliminate the wheels as a problem, might be a wheel buckled?
 
meles meles said:
It happened three times today - and that's after the new steering damper had been fitted and both myself and a garage had checked out the rest of the suspension.

The first time I was doing about 55-60 mph on a flat, smooth dual carriageway with no obvious surface defect. The vibration started very suddenly and was very severe, shaking the whole vehicle and almost pulling the wheel from my hands. I applied the brakes - which pulled evenly - and once the speed dropped to about 25 mph the vibration ceased almost instantly.

The second time was under similar circumstances an hour or so later.

On the third occasion, i was on a narrow, twisting lane and travelling at around 35 mph. The onset of vibration was immediate and very frightening: again it stopped as I braked and speed fell away. Subsequent inspection reveals nothing wrong with the shocks, springs, steering dampers, tyres, roll bars, propshafts....

*Scratches ikkle head in bewilderment*
Click to expand...

Coil sprung P38 running on shocks designed for Air suspension? Could simply be that the shocks, although good, are not suitable for the coil springs, something to do with frequency response or the like I believe.
 
That's a few more things to check out. I was contemplating changing the shocks and springs anyway, just in case I ever ventured off road in the P38. Unlikely, as that's why I own a Series IV, but maybe, just maybe, one day....

Any suggestions as to what would be a good shock / spring combination ? Would a 2" lift cause any problems ? What would be a good replacement for the current 18" wheel / Scorpion tyre combination - primarily for on road use?
 
Loads of standard 16" wheel and tyre combos for sale on egay going cheap from owners who have have put bigger wheels on
I have a coil conversion running on 16" rims with 255/65/16 tyres and standard shocks - I dont have any steering wobble......................................:)
 
I suspect the steering box will be up amongst the first things checked come the weekend. Does anyone have any thought sre tyre sizes if I change to 16" rims ? There are plenty of tyres for 235 x 55 x R16 but rather fewer 255 x 65 - is there anything to be gained by going to the larger size ? They'll be for road use only.

(you can call us 'badger' - meles meles is just our Sunday best name)
 
Well 'oomans, the next stage in the investigation was undertaken today. The rear wheels were re-balanced and swapped to the front. The front wheels were re-checked and found to be still properly balanced though the observation was made that there was close to 980 g of lead weights on one of them and 700 g on the other. It seems 18" rims on a P38 are often like that.

It don't cure the problem. The vibration still occurs intermittently. To call it a vibration is perhaps a little misleading. One moment the vehicle is cruising serenly along, a microsecond later it feels as though the propshaft has become detached at one end and is flailing its way up through the floor at the same moment as a team of goblins takes hold of each corner of the vehicle and attempts to pull the wheels in different directions. The only course of action to take is emergency braking: once the speed drops below about 15 mph the vibration vanishes again.
 
meles meles said:
Well 'oomans, the next stage in the investigation was undertaken today. The rear wheels were re-balanced and swapped to the front. The front wheels were re-checked and found to be still properly balanced though the observation was made that there was close to 980 g of lead weights on one of them and 700 g on the other. It seems 18" rims on a P38 are often like that.

It don't cure the problem. The vibration still occurs intermittently. To call it a vibration is perhaps a little misleading. One moment the vehicle is cruising serenly along, a microsecond later it feels as though the propshaft has become detached at one end and is flailing its way up through the floor at the same moment as a team of goblins takes hold of each corner of the vehicle and attempts to pull the wheels in different directions. The only course of action to take is emergency braking: once the speed drops below about 15 mph the vibration vanishes again.
Click to expand...

Try radius arm bushes & swivels pins & pin bearings..
 
Well, some progress has been made. A complete new set of bushes and adjustments of the steering box and swivels has tamed the worst of the judder. Now, if I hit a slight sinusoidal bump - subsidence on a motorway is a good example then a judder sets in right away in the suspension but that only lasts a second or so and dies away right from the onset. Previously, the judder would have grown rapidly over several seconds until the vehicle became unmanageable - emergency braking was the only option.

I still believe the suspension is prone to some kind of harmonic vibration that the current changes have only tamed rather than totally eliminated. Next step may be new shock absorbers. Any suggestions ?
 
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