When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I just got done changing the diff oil in my '84 VDP. A couple questions and notes for anyone who cares.
I was able to get the drain and fill plugs off without taking off C 20651, 'bottom plate" (according to the parts catalog). I was unable to get the fill plug back on because I just couldn't get my hand in well enough to start it properly. Taking off the bottom plate made things infinitely easier, but required the removal and reinstall of 14 bolts.
Q1: I'm not an engineer. But does this thing really need FOURTEEN bolts? Wouldn't the eight that screw directly into blocks, or the six that have nuts to hold them in place, be enough on their own? (I put them all back in, but it just made the job that much longer and dirtier to do. Especially if you're on the ground on your back)
Q2. Does anyone have a "trick" to this? Does the regular Jaguar mechanic really take that plate off just to drain and refill the diff?
Q3. Too late now but I'll remember next time. I see that gear oil now comes in Conventional and Synthentic too (the latter at double the price, or more). Does anyone have a good opinion on whether synthetic is a better way to go?
Q4: Last question and I'll shut up. Frequency of change. Like a lot of people now, this car is my weekend, local driver. As I don't put many miles on it, you can't measure maintenance intervals on mileage (at least for fluids), you're better off by time. Any idea how often I should change the diff oil? Annually? Bi-annually? When I change the motor oil?
Last comment, for what it's worth. I remember having a terrible time filling the diff oil the last time I did this, so I bought an electric fluid pump from Harbor Freight. Cost about $15, runs on two D-batteries. Didn't work at all for me. First, the pick-up tube was solid, not flexible, very long, too big to fit into the bottle of diff oil I'd bought, and too unwieldy to work underneath the car without benefit of a lift. Even though I barely managed to juggle things so it would work, it was still not strong enough to pump the oil up into the diff. It got close but never made it over the final hump. Bought another pump, this time the manual pump (looks like a short bike tire pump with a hose at each end). The hoses were flexible, and fit into the diff oil bottle. Again, took some doing to keep one hose in the diff filler hole, the other hose in the oil bottle, and pump the thing at the same time, but I was able to do it. The manual pump only cost $7 and no batteries.
Next year I'm asking Santa for a garage with a lift.
Last edited by James O; Jan 10, 2021 at 05:27 PM.
Reason: Edited for typos.
Q1: I'm not an engineer. But does this thing really need FOURTEEN bolts? Wouldn't the eight that screw directly into blocks, or the six that have nuts to hold them in place, be enough on their own? (I put them all back in, but it just made the job that much longer and dirtier to do. Especially if you're on the ground on your back)
Don't question. Just accept
Anything on an old Jag that looks like a design flub is actually an "interesting engineering feature".
Q2. Does anyone have a "trick" to this? Does the regular Jaguar mechanic really take that plate off just to drain and refill the diff?
I don't know how regular I am but I learned a long time ago to simply remove the tie plate and be done with it.
Q3. Too late now but I'll remember next time. I see that gear oil now comes in Conventional and Synthentic too (the latter at double the price, or more). Does anyone have a good opinion on whether synthetic is a better way to go?
Lots of opinions on this. Here's mine:
If it makes you feel good to use synthetic, use it. The car really doesn't care one way or the other. And you'll probably never drive the car long enough or hard enough to realize any meaningful or measurable real world benefit from spending the extra money.
Q4: Last question and I'll shut up. Frequency of change. Like a lot of people now, this car is my weekend, local driver. As I don't put many miles on it, you can't measure maintenance intervals on mileage (at least for fluids), you're better off by time. Any idea how often I should change the diff oil? Annually? Bi-annually? When I change the motor oil?
I change mine every 30k miles regardless of the calendar
Next year I'm asking Santa for a garage with a lift.
Its the beer count that really matters with that plate and other little things. Parkbrake pads is another GOOD one.
Think about the fact this is possibly the 1st oil change it has ever had back there, so, you are doing that car a HUGE favour.
Mine get diff drop and refill every 2 or 3 years, AFTER I have done my "back to square one" service.
Whatever oil is on special has served me well for too many years.
You MUST have all the tie plate bolts! The tie plate fixings do two things:
brace the bottom of the cage to make it rigid, otherwise it would be a floppy open-topped box (albeit upside down).
it ALSO fixes the bottom of the diff to the (now rigid) cage. The diff is otherwise only fixed at the top to the top of the cage; if the bottom was not bolted to the tie plate, there would be huge levering forces applied only to the top part of the cage under acceleration or braking. The bolting of the bottom of the diff to the cage bottom, means these bending loads are applied to the entire cage structure.
Even given this, for racing applications, and for some actual factory models, further bracing was applied between the cage bottom and the body of the car to add extra strength against acceleration and braking forces.
How much diff oil does the rear end take? And anyone have a preference as to what kind and weight of oil. I'm in Florida; never gets colder than 50 degrees here.
About 2 quarts, perhaps 2¼. Keeping adding until it runs out of the fill hole; the correct level is bottom of the fill hole so, ideally, you'd check with the car level.
I use 80/90w, whatever name brand is on sale that day
I wonder if the 14 bolts have anything to do with people jacking up the rear of the car at that point , albeit with a block of wood to spread the load?
I wonder if the 14 bolts have anything to do with people jacking up the rear of the car at that point , albeit with a block of wood to spread the load?
I do not think so, as under the jacking compression load the bolts are not doing anything. It is really 8 bolts to attach the plate to the diff (actually to the dogbone castings that take all the suspension loads), and 6 to attach the diff/plate combo to the axle cage. You need the 6 on the cage to give adequate rigidity, as neither the cage nor the plate are very stiff on their own, and you need two each end of the dogbone casting (the four sets of two on the diff) so as to ensure adequate strength and twist resistance to help distribute the cornering, braking, acceleration and suspension loads.
When you think about it, those 4 (each side) small bolts attaching the diff bottom castings and thus the lower wishbone to the cage bottom do not seem over-large when you throw the car into a corner at 80 MPH, or screech to a halt to avoid a clown car! That is part of what is distributing the rear wheel cornering loads into the bodywork, by contributing to the strength of the diff and cage!
Last edited by Greg in France; Jan 19, 2021 at 07:02 AM.
I haven't looked at this yet but I am going to look into drilling a two inch hole into the spare tire wall closest to the differential itself. Going to be close considering the fuel line pipework and the heatshield. Big grommet and duct tape.
I bought 75W-90, simply because the 90 weight was the one called in the Jaguar book. I don't know what effect, good or bad, 140 weight would have on it.
I emptied out the dirty oil in the diff last night and only about 1 1/2 quarts came out. Was concerned, but then I could only pump 1 /1/2 quarts back in before it started spilling out the fill hole. So I'm going with 1 1/2 quarts for the amount. At least on my car.
Now it's on to the center bearing. That will be a bigger job.
This seemed a good discussion about this job so, after doing it today, I thought I would post a few pics to illustrate what people are talking about
This is the diff plate. Mine was a little battered. All the bolts are 1/2 inch. 8 bolts and 6 nuts and bolts (3 at each end)
An electric ratchet makes things easier, especially if you are on the floor.
After clean up and panel beating
Its feasible to get onto the fill plug with a 15mm socket, and if it hasnt been tightened by a Mongolian arm wrestler get it out (or come up from underneath with a spanner). Both are fiddly and the hard part is getting the plug started again as that hole is smaller than a man sized hand. If you go too hard with the 15mm its also possible to round the plug, its an OK fit but not perfect.
Once the plate is out you have plenty of the access, and as Doug said you can clean the accumulate muck away.
i've just taken off the plate and i see the square headed drain plug as per your picture however, my fill plug is not square headed. mine appears to need a square headed allen type wrench. i might have to sacrifice an allen wrench by grinding it to a 4 sided tool. once it get it out i will change it to match the drain plug.
Changing Diff Oil in the S3
