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1996 P38A 4.0l P Auto GEMS
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Discussion Starter · #1 ·
1996 P38 4.0l P
The PO had the vehicle converted to coils (TF222) about a year ago, and when I purchased it I noticed a vibration at speeds of 70 kph+ that I took to be driveshaft related because the vibration changed between acceleration and overrun.

I also noticed a vague steering with the tendency to oversteer, and being new to RR I thought no problem, these things and be easily sorted!

In the meantime I had to tackle the usual 'trial by fire' issues such as coolant leakage (heater O rings & radiator hoses) etc.

I had both propshafts removed, inspected & balanced and no problems were found. Phasing was correct. I did notice there was no harmonic balancer on the front diff (there is one on the rear)

I then figured that the poor handling and vibration may have to do with the raised suspension. Passenger side stood at 510mm and drivers sider at 490mm wheel centre to arch) did not seem good to me. I had also checked the wheel alignment and the castor stood at 2 deg. due to the raised suspension. So I went back to the guys who installed the TF kit (well known local Land Rover independent garage) who said "They all do that Sir". I suggested I go back to air suspension and you could judge by their faces that they thought this was naïve madness. They suggested I get the caster adjusted by an 'engineering shop' who slot the diff bracket, twist the axle by the number of required degrees and weld on washers to keep it in place.

So off I go to have that done. Handling is now better but vibration was now off the charts, pretty much making the vehicle undrivable.

So I set off to try and restore the vehicle to standard. The conversion back to air was relatively straight forward. Air springs and height sensors had been tossed some time back, so I got a new set of those, rebuilt the valve block and compressor, installed emergency valves on each of the air spring lines and the air tank line, and soon I was back in business! Air suspension works perfectly and I am pretty happy with the calibration.

But... although the vibration was improved (particularly at low profile) it was not gone.
So, back to the engineering shop to get the castor set back to standard. I witnessed this process and am happy they did the right thing.

Vibration improves again, but still can be felt at 100kph and it becomes problematic at 120. But vibration no longer is dependent on accelleration/overrun.

I measured the front and rear drive shaft angles and the front transmission to shaft sat at about 6 deg and the shaft to pinion at 5 deg. (low profile setting) Both seem too high you read standard drive shaft papers. Maybe LR knew something others did not? Or does the small phase lead do the trick? It's not that easy to measure drive train angles on the garage floor at low profile.

I tried to measure the vibration frequency using both acoustic and accelerometer based iphone spectrometers. Not easy to do reliably. I thought the frequency was about 30Hz which is too low for wheel and about right to transmission at those speeds?

So I decided to get all wheels balanced anyway and replace both propshaft UJ with spicers. This made no difference as far as I could tell.

So, decided to get a Tom Woods double cardan for the front. I remained concerned that it will not resolve the pinion-shaft angle problem, but it was worth a try.

Installed the shaft yesterday and ride is considerably improved. But not gone. Smooth ride until 110kph, but the vibration is still there.

Have I missed anything? Outstanding non standard items are the shocks (it still has the TF gas shocks from the kit) and no front harmonic balancer. Would the air spring/TF shocks create a resonant structure?

Shall I remove the front drive and see if it still vibrates? Problem is I need to drive 100kph+. BTW, VC seems fine, I did the usual test (front wheel up & managed to just turn the wheel by hand)

Do I perhaps have some another drivetrain problem? There is no other indication that there is a problem. Small movement in the front transfer case output shaft when pushed by hand. The front pinion to shaft angle still bothers me. It sits at about 4 deg which is too high for a double cardan. But why is it that high?

Thanks for reading. Any insight is welcome!
 

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I would have thought you should have the harmonic balancer on the front. Very early cars didn't have it, only one on the rear, but there was a TSB very early in the life of the P38 suggesting fitting one to the front and after a short time, they all got it as standard. Standard shocks with EAS are soft and non gas, so damp equally on both compression and rebound but I doubt the shocks would cause vibration, just poor ride. Vibration felt through the seat of your pants at around 60 mph (100 kph) is very often wheel balance on one or both rear wheels. The fact that it is less in Motorway height would suggest something in the propshaft area. Original propshafts are keyed so you can't assemble the central spline joint anything other than correct to give the required phasing, on aftermarket you can. Wear in the splines on the sliding joint can cause it too. You're heading the right way but I know from experience that tracking down a source of vibration can be a real pain. Removing the front propshaft would be my next step too. Only remove and drive the car for testing purposes, don't leave it off as that will overheat and cause the viscous to seize.
 

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1996 P38A 4.0l P Auto GEMS
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Discussion Starter · #3 ·
Thanks for your response. Do you think driving approx 10km mix of slow and faster (up to 110km/h) will be ok for the VC? Obviously go slow on the acceleration etc.

Any thoughts on the pinion - drive angle being 4 deg? I presume some is needed for 'preload' when accelerating, but is 4 deg not quite high?

If anyone is below their vehicle maybe can they check what the driveshaft angles are.(y)
 

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1995-2002 Range Rover P38A
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Does the vehicle have its original rims? If they have been replaced with those from a LR3 or RR Sport, then they will need spigot rings to fit properly. If they don't have spigot rings, they will vibrate badly.
 

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1996 P38A 4.0l P Auto GEMS
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Discussion Starter · #5 ·
That's useful to know. These are the wheels I have. Apart from the PO painting them gangster black, are these originals?

Tire Wheel Car Automotive tire Motor vehicle
 

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They are the original Tudor Rose style wheel which are 16 x 7J so correct tyre size for them would be 235/70x16. You've gone up by one size but that shouldn't be enough to make a difference. Can't help on the drive angle and I've no way of accurately measuring it on either of my two cars. Obviously the angle is going to be greater in High or Standard than in Motorway heights but as it is designed to have such a wide variation, I suspect it is correct.
 

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1996 P38A 4.0l P Auto GEMS
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Discussion Starter · #9 ·
No, I have not ruled out tyres.

I did have them removed, inspected and balanced which did not help.
But they do have a reasonably coarse tread when you can detect at very slow speeds. Sometimes uneven wear might result in vibration frequencies at multiples of wheel rotation speeds.

I might take your advice and change them, but that is a costly exercise.

Does anyone have any further advice of the risks of removing the front shaft and going on a test drive?
 

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The only risk is putting strain on the viscous coupling which can damage it. But if you are only doing it for a short time to identify, or rule out, a problem at the front end, it won't do any harm. Rear is a different matter. Despite the parking brake drum having the screw to hold it on, it will work loose if that is all that is holding it in place so you need to put the nuts that held the propshaft in place to hold it on. You don't really want it coming off and flying down the road.

Tyres are a possibility but then it wouldn't matter what height the suspension is in, it would be the same all the time.
 

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One of my Admin privileges is that I can merge users so should be able to merge both accounts into one. Never tried it though so no idea what will happen......
 

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Blimey, it works! A pretty simple job too. Ballots now no longer exists and everything he ever posted is now attributed to you.
 

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No, I have not ruled out tyres.

I did have them removed, inspected and balanced which did not help.
But they do have a reasonably coarse tread when you can detect at very slow speeds. Sometimes uneven wear might result in vibration frequencies at multiples of wheel rotation speeds.

I might take your advice and change them, but that is a costly exercise.

Does anyone have any further advice of the risks of removing the front shaft and going on a test drive?
You could either buy a set of good used tires or if you can find someone willing to lend you a set of tires.

In my experience tires have often been the source of many issues, harsh damping, vibration, strange sounds at low speeds, etc.

Sent from my MAR-LX1M using Tapatalk
 

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1996 P38A 4.0l P Auto GEMS
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Discussion Starter · #18 ·
Thanks, you may well be correct......

This morning I removed the front drive shaft and guess what.... the vibration is still there.
Should I be happy or sad? Well, its a P38 so I am always happy :)

I was always under the impression that the vibration is of a constant frequency independent of speed. Speed affects the amplitude. But this has been difficult to verify/quantify in any scientific way.

I took the engine up to 3000rpm while stationary and in neutral to check engine vibration and that's all fine.

I think next I will rotate tyres back to front to see if anything changes.

BTW: I did a more accurate set of measurements (I now have an inclinometer meant for these measurements) on driveshaft angles (only at standard ride height):

Rear:
  • diff pinion to driveshaft: 0.4 degree
  • driveshaft to transfer box 0.3 degree
This implies the transfer case shaft to diff pinion is within 0.1 degree

Front:
  • transfer box to driveshaft: 8 degree. The double cardan should take care of this.
  • driveshaft to front diff pinion: 6 degree. This should be close to 0 if using a double cardan.
This implies the transfer case to pinion angle is a whopping 14 degree!
 

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Although you think the angles are excessive and a double cardan is needed, don't forget that, as long as the mounts have been put back where they were originally, that is how it was designed and works. With the amount of travel available on the suspension, the angles are going to be high, particularly with the suspension on the High setting. By driving without the front propshaft with no change to the vibration would suggest to me that the propshaft angles aren't relevant. so not the cause Your lack of a harmonic balance weight on the front may be relevant though, as said, only very early cars didn't have it and it was recommended to fit one on any cars with vibration issues.
 

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1996 P38A 4.0l P Auto GEMS
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Discussion Starter · #20 ·
I agree with you 100%. Hats off to the engineers who got this to work.
They might have found a double cardan is not the answer, instead opting for a standard driveshaft (offset by some phase) plus harmonic balancer did the trick. They seem to have broken a number of 'rules' along the way so it must have been some doing to get it right.

Just to be sure there is not something else in the driveline causing the vibration, is there any harm in disconnecting both driveshafts then running the vehicle up to 120kph whilst stationary? I presume I may get a ABS error, but I suppose this can be reset.
 
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