I have a 1988 Jaguar XJS. 85k miles. V-12 HE. It has a automatic (believe it's a jag 400. Stock). I think car has 2.88 rear gears. I'd like to go to 3.54 or close. Don't want to spend a fortune either. But do plan on rebuilding brakes (new rotors if needed, pads, parking brake pads, new trailing arm bushings and new IRS mounts). Maybe new springs and shocks too. Depends on differential cost. I'd like to either buy a good used 3.55-3.73 rear end (the "pumpkin" not the whole IRS setup) and just swap it out for mine or buy gears and bearings and seals and have mine rebuilt with new gear ratio I want. What carrier do I have? Can it use the 3.54-3.73 gears? Is it a posi? (Won't come close to tire spin so I don't know). This XJS needs more seat of the pants speed...and a rear gear swap is the most bang for the buck normally. I'd like to go 3.73 if I can...a 5 speed swap might one day happen.

 

Easiest way is to find a complete 3.54 XJS diff and swap it over.

Other options are

Buy 3.54 gears and have a spacer machined to go between the gear and diff centre.
Hybrid the diff with Dana parts.

Jaguar changed the speed sensor from the trans to the diff in the 89 model year which probably started in 1988. Mine has the diff speed sensor so my diff is a hybrid of and XJ40 and my original XJS.

I have 3.58 rear end and with the TH400 the car was turning at 3000rpm at 100km/h, not a very relaxed cruiser as most of my driving is highway 100km/h. I installed a 4L60e 4 speed which fixed this.

 

I always thought the V12 reved quite high at 100 kph... Can't remember exactly but around the 3000 rpm mark sounds right. My old (at the time) C30 did better than that... The XJ8 does a lot better (100 kph is 200 rpm).

 

3000rpm at 100kph is certainly high. In my experience the V12 (at least in HE form with the standard diff/autobox) sits at about 2100 or 2200 rpm when cruising along at 100kph, but certainly rises to well above that when doing the dash from a rolling start to 100kph (which is the highlight of my day when driving home)

 

Then again, my last drive was in 2014... Around about this time of the year... Heck, I miss driving her! Might have the wrong diff or a wrong memory... Dunno quite yet

 

Haha, all good mate! If you've been cruising around in the 3.2 X308, then I definitely know your pain of wanting to get back into the V12. The X308 is no slouch, but for smiles per mile and a much more visceral experience, the XJS cannot be replaced!

 

Yeah, don't get me wrong, that 240 hp 3.2l V8 is a nice engine, but it just isn't that V12. And the ride isn't the XJ-S ride. It is fairly sporty but it lacks the same refinement. The tranny though is nice... Even though I think I killed it. The TH400 is a great tranny too...

 

What's your timeframe? And how much is "a fortune"?

After about a month of frequent scouring of "for sale" sections on various Jaguar forums I was able to find a 3.31 limited slip (from an early XJS) for a good price. I had the seller ship it directly to Coventry West for overhaul and then Coventry West shipped it to me. All in, about $1000.

I would've preferred a 3.54 but the planets seemed to align on the 3.31. With patience you might find a 3.54.

On later XJSs the 3.54 was common. One alternative would be to buy a complete IRS from a later car and get the outboard brakes as well. It isn't a difficult conversion; there are some write-ups in the archives. That might be beyond the scope of your project or budget, though.

Cheers
DD

 

Just to clarify.....

According to Jaguar the change was at VIN 142xxx (I can't remember any closer than that!) which would've been roughly spring 1987 calendar year. For *USA* market cars this would've been shortly after intro of 1988 model year. For legal/marketing reasons not known by me the 1988 model year for USA cars began extremely early. My USA market XJS had a VIN of 139xxx and build of February 1987...but carried a 1988 VIN. In most Jaguar tech literature the same car would be referred to as "1987½"

Cheers
DD

 

I remember my '88 V12 with 2.88 gears yielded exactly 80 mph (129 kph) @ 3000 rpm.....which was exactly the same as my Series III XJ6 with 2.88 gears. Perhaps that's why I remember.

Cheers
DD

 

You got a bargain Doug. I just finished a diff overhaul and I was over $1200 for parts and special tools I needed. I reused my ring and pinion, and the gears I needed came from the self serve junkyard.

 

I know


I got a bit lucky. The seller gave me a very good price to begin with and (I forgot this part) was nice enough to personally deliver it to Coventry West as he was in the area. That saved some shipping expense. The diff itself just needed bearings, plates, and seals. And I didn't have to buy any tools.

In thinking about it could've been much more expensive...and would've been if things didn't click into place

Cheers
DD

 

This is very helpful information. I had thought that after '88, all V-12s had 2.88s, and the sixes had the 3.54.

My question is: What exactly is the speed sensor? Is it a magnet, or some other device that could be added? If I could find a set of 3.33s, could this be modified for my '91? I was thinking about a 5 speed conversion, and a 3.33 rear-end sounds about right, but I will need a speedo, so...

 

use a GPS speedo. much easy install,very accurate(like .01 MPH).

recently some face gages are close to JAG XJ/XJS,its just so simple, and hookup to ignition ON , needle runs a full sweep 0 to 160mph and back to 0, with a quick red light blink!

NO connection to anything in the car, satillite signal ,worldwide!

google GPS speedometers.

i use a 3.73 ratio, JEEP gears, been 22yrs no probs. 700R4 TV cable type, gives in OD 2.60 final drive.

 

Your 91 definitely has the speed sensor in the diff.

When I did mine I used an XJ40 3.58 LSD hemisphere and hybrid it with the original XJS diff to keep the speed sensor. Worked just fine. All the diff centres are interchangeable up to at least the X300. There maybe some small differences but they are all basically a Dana 44 derivative. I had to machine the snout off the XJ40 pinion.

The speed sensor reluctor is part of the diff hemisphere on the RH side.

 

Someone CONFIDENT enough to install/setup the gears that knows what their doing and guarantee NO WHINE OR SLOP/CLUNK. For some reason setting up a Jag diff with the speedo sensor in the diff is rocket science. BTW, I have NO clue when it comes to setting up a diff and don't want to learn @ this point

 

As a general rule, cars with a 3 speed transmission have 2.88, 4 speed with overdrive have 3.54.

I believe all 5.3 cars have 2.88, and the 6.0 have 3.54. The Special XJR-S and JaguarSport/TWR cars may be an exception, I don't know enough about them.

 

Early 5.3L cars had 3.31, I think Jaguar went to the 2.88 with the HE.

It's not the speed sensor that's the issue with rebuilding Jag diffs. It's shimming to set the mesh, this requires removal of the hemisphere to re-shim, a real PITA and time consuming to do.

 

Actually, I think that the final drive was 3.07 for European cars and 3.31 for US cars before the HE.
Both went to 2.88 with the HE in 1982.

Many detailed specs for all the models on this site, you can choose the year, model and country:
1978 Jaguar XJ-S Series 1 Coupe full range specs

 

Yes, you're right about the pre HE cars. I was thinking of the HE versions. XJ Series sedans were like that too, lower ratios pre ~1982 and 2.88 after that.