Dax Jag XJ12 IRS rear - Power-lok diff confirmation

[CobraPaul]

Looks like there is a square drain plug directly on the bottom/underside of the diff.
Haven't got the ability to get it in the air yet, but I will do soon.

[Dek]

Thank you sir.
I'll crawl underneath now that I've bought the car and check for drain plugs.

When you've got the rear jacked up mark one wheel and the prop shaft. Turn the wheel one revolution (the opposite wheel should turn in the same direction confirming it's a PL diff) and count the number of revolutions of the prop shaft. If it turns 3 3/4 turns it's a 3.73 for example.

[CobraPaul]

No drain plug its a Dana. If a drain plug is present its a Salisbury. 4HU is Salisbury casting.
The internals could be different for the ratios which is two numbers 43/15 2.87 (ref to as 2.88) to give you your ratio. You could always count the crown and pinion teeth with the cover off.

Thank you sir.
I'll crawl underneath now that I've bought the car and check for drain plugs.

[JON1D]

No drain plug its a Dana. If a drain plug is present its a Salisbury. 4HU is Salisbury casting.
The internals could be different for the ratios which is two numbers 43/15 2.87 (ref to as 2.88) to give you your ratio. You could always count the crown and pinion teeth with the cover off.

[CobraPaul]

So, stamp on diff is:

CO4HU-004-U30

Indicating a PL diff. Haven't had in the air to confirm wheel rotation yet tho.

Can anyone confirm Salisbury or Dana, and the ratio, from this?
Internet search suggests the stamp is very similar for either.
Thanks!

[CobraPaul]

Thanks all - fantastic and informative replies.
Glad it's very likely to have an LSD. Will have a crawl underneath and check for markings to confirm.

[CobraPaul]

With LSD diff you need to have the diff de-rated for a lighter vehicle like the cobra which lowers the power transfer with the internal clutch plates. A near 2 ton vehicle which the diff comes from, has a higher rating on power transfer across the axle. Not having the diff de-rated make the steering feel heavy and the rear end pushing you into the corner.   

As this is a car that has previously been on the road (albeit with the v12 engine) I'm hopeful that this has already been done. Will have to check the paperwork though I guess.

[JON1D]

 All XJ12 5.3 will have a power lock diff. It will be 2.88. Look for the tag 2.88 OR THE RATIO X/XX LSD . The other tag might be "Use LSD oil" on the diff cover.
If no drain plug is present you have a Dana 44 not a Salisbury diff. Jaguar were running out of Salisbury diffs due to short supply and installed the Dana diff on XJ12 5.3 vehicles . Both are good diff's but with no drain plug you have to drop the cover to drain or suck the oil out to replace. Check the pads for wear and disc for thickness and run out. Easy to change the oil and pads plus handbrake operation and pads while the unit is out.

To check the diff draining the oil and check the magnetic plug will show you wear. Check the drain oil for any metal swarf showing wear issues.

Check the back lash and wear pattern of crown to pinion. Also check for any seal leaks.

With LSD diff you need to have the diff de-rated for a lighter vehicle like the cobra which lowers the power transfer with the internal clutch plates. A near 2 ton vehicle which the diff comes from, has a higher rating on power transfer across the axle. Not having the diff de-rated make the steering feel heavy and the rear end pushing you into the corner.   

[GDCobra]

From research, I believe all of these cars were equipped with a 3.07:1 ratio Power lock / Sailsbury 4HU limited slip differential (LSD). I'm also led to believe that rotating one rear wheel and observing the other rotating in the same direction confirms this is the case.

Yes, open diff' wheels will go opposite direction.
I thought XJ12s were 2.88 but not much difference to 3.07, either way it's high so will help make it a motorway cruise rather than giving good acceleration but if the engine is lively enough should be OK.

Is there anything else I can to do confirm this on visual inspection?

You could crawl underneath and check if there is a tab marked PL attached to one of the rear cover bolts, of course this could have been lost so not a definitive test.  Should also be a tab showing ratio there too.

If I have no record of an oil change on this differential, I'd assume a change would be wise? If so, any recommendations for the replacement oil?

Would always be a good idea to change the oil but certainly if there is no record. 
Obviously making sure you can open the filler before the drain :-)
Can't remember what I put in mind, I'll have a look in the garage tomorrow.

I'll need to check out the discs and pads (inboard rear) but I'm hoping I won't need to do too much to this part of vehicle other than possible a pad change (which can be done via the access hatch from within the cabin?)

Sounds like a good idea, hopefully discs will be OK as changing would be a bigish job.
Makes me so glad I converted mine to outboard.

[CobraPaul]

Hi Folks,

I'm buying a Dax Tojeiro that had a 1978 Daimler Double Six v12 as a donor car. I understand these to be equivalent to the Jag XJ12 - Series 2.

From research, I believe all of these cars were equipped with a 3.07:1 ratio Power lock / Sailsbury 4HU limited slip differential (LSD). I'm also led to believe that rotating one rear wheel and observing the other rotating in the same direction confirms this is the case.

Is there anything else I can to do confirm this on visual inspection?
If I have no record of an oil change on this differential, I'd assume a change would be wise? If so, any recommendations for the replacement oil?
And any other checks I should do to ensure this is in good shape?

I'll need to check out the discs and pads (inboard rear) but I'm hoping I won't need to do too much to this part of vehicle other than possible a pad change (which can be done via the access hatch from within the cabin?)

Thanks