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P38 gearboxes


GBMUD
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Is there more than one possible gearbox manufacturer for the 2.5DSE? Someone mentioned ZF vs Borg Warner and one being preferable to the other, any ideas? Or is it a transfer box thing?

When looking at P38s, does one have to check out the centre diff/viscous as in classics? What test/tests should I perform?

Cheers

Chris

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As far as I am aware all the autos are ZF boxes either HP22 for the diesel and 4.0 or HP24 for the 4.6.

I suspect the references to Borg Warner is about the transfer box which again as far as I am aware are all the same (but not the same or intercahngeable with any other LR transfer box as it drives to the other side of the car)

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When looking at P38s, does one have to check out the centre diff/viscous as in classics? What test/tests should I perform?

As they are the same VC, the answer to the first question is 'if you check out the centre diff/viscous in classics', then the same checks apply to the 38A.

I don't know what checks people apply to VCs in Classics.

In my experience, VCs generally fail progressively not catastrophically. The failure mode is that they become stiffer, that is the permitted speed differential across the output shafts becomes less.

That said, I have seen a photo of a VC with a split case. This was reported by the owner as a case of extreme abuse (my definition) meaning the period of progression was very short. In detail, he had decided he could not afford to buy five tyres of the same size, so had four 'large' with an old 'small' as the spare. He had a puncture in one of the large tyres, fitted the small spare, and proceeded on his way, on tarmac roads. The resulting persistent speed differential across the VC caused it to stiffen, lock, then break the outer casing (of the VC unit, inside the box).

Under these conditions there is no drive to the front wheels, the VC has no effect, so the centre diff, being open, sends all the power to the front output shaft, so the car stops.

The most obvious tests while not in the workshop:

Look first at the RH rear tyre, if you think it has been in place for some miles. If the tread is feathered it's an indication the VC is stiffer than it ought to be, but it could still be a long way from failing. You will see similar feathering on the LH front tyre, but this can also be caused by a steering misalignment. I would check the RH front wheel to see if the feathering is equal across the axle.

Next drive a 360 slow circle on full lock. I'm not aware that direction makes a difference, but speed distorts the results, so tickover slow is best.

On a loose surface you will get more surface disturbance the stiffer the VC. What is normal, and what is too much, is a matter of judgement.

The same test on polished concrete, as in a warehouse, or multi-story car park, will give more tyre squeal the stiffer the VC. Speed and tyre fitment will affect the result, some tyres naturally squeal more than others.

The same test on tarmac, as on a superstore or pub car park could also create a squeal, but it's a quieter surface than polished concrete, so noise will be less for the same vehicle.

However, the stiffer the VC the more throttle it will take to maintain travel even at engine tickover speed.

With the power assistance available on the 38A it's unlikely you will notice the steering being heavy, but I have had that report of heavy steering from a Classic owner. The steering was so heavy his wife couldn't drive the car into supermarket car park slots. In this case the car also rose on the (coil spring) suspension as it passed along the road. It lowered itself overnight, but never to the normal height. I diagnosed this as a 'very nearly solid' VC causing transmission wind-up, which on tarmac roads was pulling the axles together. The diagnosis was proven by the owner removing the front propshaft. The front wheel span as the propshaft was released, the steering became more normal, and the car stopped rising on the suspension.

The only 100% test is that given in the Classic Workshop manual, of holding the rear output stationary, then applying a measured torque to the centre nut of the front output shaft, and noting the amount of movement.

In my view it should be possible to replicate this measured test at a front road wheel / hub nut, but the presence of the axle ratio and differential makes the calculation a little complicated, and I've never proved my maths by a comparative test.

So the field is open for someone else to take a lead :-)

HTH, if you are still with me!

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Very informative, thanks David. :)

I viewed a RR earlier: I checked out the tyres as you suggested and found some evidence of feathering. There was little evidence of scrubbing on lock though. My examination did reveal a few other issues though, see my other thread...

Chris

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OK. I realise I didn't make clear that I was trying to grade the indications in order of severity, with the tyre feathering being the earliest indication. I haven't read you other thread yet, but I suppose I have to say that when the cars are a minimum of 8 years old then new owners should consider the VC as a maintenance item, and change it sooner rather than later so they get the benefit during their tenure.

I'm not sure where the break even point comes, but it gets dispiriting and expensive having to change tyres before they are truly worn out, because the outside edge has worn prematurely.

Good Luck.

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