Renewing the Rear Axle Oil (Differential Fluid)

1990 XJ-S Coupe (US Spec, 5.3L V12)



Ramps or other safe means of raising the rear end

½” Drive Socket Wrench and ½” Socket

½” Box Wrench

Grease Cleaner/WD-40/Paper Towels

Rust Treatment and Black Spray Paint

Medium Flat Head Screwdriver

Long Needle-nose Pliers

Small Funnel and Hose

7 ounces Limited Slip Gear Oil Additive

48 ounces 75W90 Gear Oil

A little RTV Silicon Sealant

Grease Gun/Grease

Anti-seize Compound


According to the XJ-S Service Manual “Maintenance Summary”, the Rear Axle oil (also known as “Differential oil/fluid”) should be renewed (changed) every 30,000 miles.  Not knowing when the previous owner had last changed it (and having put 9,000 miles on it since I’ve owned it), I did it as part of my Winter 2002/03 work on my ‘90 XJ-S Coupe.


Picture of Rear end of XJ-S on RampsI backed the car onto ramps – no ramp extensions needed as the stock tail pipes where not in the way.  I first took some time to clean the differential area up with “Greased Lightening”, WD-40, and a bunch of paper towels.


I took off the “center reinforcing plate” (8 bolts and 6 bolts with lock nuts).  A ½” socket for the bolts and a small ½” Box Wrench for the nuts.  I’ve read that the Plate does not need to come off, but I am sure I got a much better shot at cleaning everything under there, including the rust/grime on the topside of the Plate.  Also, finding the right combination of sockets/extensions/universals needed to get to the fill plug without busting knuckles would have probably taken longer than taking off the plate.


After using a wire wheel on a drill press to clean up the center reinforcing plate. I painted it with Permatex® Extend® Rust Treatment (79DA Item# 81849) then plasti-kote® Universal Black No. 203 engine enamel.  Classic Jaguar sells an all-aluminum plate (they call it a “billet IRS tie plate”) for $115, but I saw no reason for that.


Center Reinforcing Plate (After Being Cleaned-up)

(Between the rear wheels.  Rear Axle Drain Plug In Center, Front of car at lower-left)


Picture of Center Reinforcing Plate covering Differential


I put a suitable container underneath (you’ll probably get about 2 quarts), removed the fill plug (using a ½” drive socket wrench without a socket), then removed the drain plug and let it go for about an hour (see picture below).  Man, does it stink.  You’re supposed to have it warm before you drain it (driving around a bit) so it flows out quicker, but the winter weather and the fact that some other parts of the car weren’t quite completely back together prevented that for me.


Both the drain plug and fill plug are magnetic on the surface that is internal to the differential to collect any wearing metal inside – clean them thoroughly.


Replace the drain plug.


In the ‘boot” (trunk), I removed the spare tire and got the fuel filter and fuel hose holder out of my way.  Using a flat head screwdriver, I pried around half the edge of the second-from-right side “inspection hole” cover then was able to pull it out with the pliers.  I put a small funnel into a piece of clear hose (3/8” I.D by one foot long), put the hose into the inspection hole, grabbed the hose from underneath (with long needle-nose pliers) and guided it into the fill hole.


View into Trunk (“Boot”) with Spare Tire Removed





View of Differential from Underneath with Reinforcement Plate Removed


Location of Drain/Fill Plugs on Differential



According to the Service Manual, the rear axle capacity is “approximately 1.6 litres (2.8 pints, 1.7 US quarts)”.  Well, which is it?  2.8 pints is 44.8 ounces, while 1.7 quarts is 54.4 ounces – a considerable Valvoline Gear Oil High Performance SAE 75W-90difference.  Since 1.6 liters is about 1.7 US quarts I assumed that is correct (54.4 ounces).  The manual says the differential should be filled “up to the bottom of the thread in the filler plug hole”.  With the car on an angle due to the ramps, I couldn’t do that – nor could I easily get under the car when it was flat on the ground…


When changing the oil (as opposed to topping it off) you’re supposed to use an additive.  The “Trans-X Posi-Trac Limited Slip Gear Additive” I bought was 7 ounces ($7) and the instructions said to add the entire contents.  I poured it in, followed by slowly adding the first bottle of Valvoline “Gear Oil High Performance SAE 75W-90” (32 ounces - $5 a bottle).  I then added another 16 ounces of Gear Oil from a second bottle (55 ounces total).  I never added more than the funnel could handle, and had no drips out of the filler plug hole.


I tapped the inspection hole cover back in with a light hammer and then went around it with some RTV silicone sealant.


I replaced the fill plug and Center Reinforcing Plate (using anti-seize compound on the bolts), and then took the car for a drive (with some figure-eights/sharp turns to “exercise” the differential).  Don’t forget to grease the rear end while you’re right there.  Next project…


Questions/comments: Email me.