How interchangeable are the rest ends between an XJ12 and an XJS? My ideal situation would be the whole cage swaps over directly but I don’t expect to be so fortunate. I can get a very good deal on the rear end out of a '79 xj12 with 3.31 and factory lsd, would like to swap it into my '87 XJS. Any wisdom or advise?
I currently have a 331lsd from 1970’s xj12 in my xjs. I did not personally install it but the shop that did said entire cage was identical.
That is amazing news! Thanks. If anyone else wants to chime in with personal experience? How do you like the 3.31? How noticeable was the change, I have been asking and pondering the ratio change for a while but have no experience with changing ratios
I had just installed the 5-speed and dropped a valve seat before driving it much. sent it to the shop for heads and he made me a deal I couldn’t refuse on the 3.31 including labor. having all this done along with bigger cams, I cant really say how much was from the rear diff. but can say with certainty that the 2.88’s are to tall! personally, I would of preferred 3.73’s but that involved much more work/money and 3.31 was ready to install. either way, the 331 is an improvement and it is available.
I believe it WILL swap right over directly, but the springs might be too stiff for
the XJ-S. If you can just swap the coil-over shock/spring assemblies, you’ll
find that very easy. If you want to use the shocks from one car and the
springs from the other, that’ll require a spring compressor – which you can
borrow for free from AutoZone.
Thanks, sounds like more good news! Or at least some confirmation. When you say the suspension will be too stiff will ride height be affected dramatically? I would not complain about a much stiffer ride at all! I will be getting the whole XJ12 minus the engine, interior and transmission, would the front suspension components be worth swapping over for a stiffer overall ride?
Another potential issue just occurred to me, I believe as an '87 my car has the speedo drive in the diff, I can only imagine the '79 will have a mechanical driven speedo from the transmission. What are my options here?
I have a 3.31 ratio in my Series III V12 sedan.
Going from 2.88 to 3.31 is quite a jump. Naturally the acceleration is noticeably stronger and the car feels much more responsive. And, of course, the RPM is is lot higher at highway speeds. These cars are quiet so the difference isn’t objectionable, IMHO…and I don’t do much open road driving these days anyway. For me it was the right decision.
I never checked fuel economy before or after.
You’ll need to figure out how to correct the speedo error.
Verify what you have first. I think the diff mounted system came along at VIN 144xxx or 142xxx or something like that
I don’t know, off hand, how the problem would be handle with the diff-mounted speed sensor. My 85 has the older trans mounted speedo trandsducer. I used a ratio adapter between the transmission and the transducer to slow down the speedo reading
Thanks for the info, I will take a look on my break to verify whether the transducer is on the transmission or the diff.
I’m glad to hear that 3.31 is a significant difference! Most of my driving is on winding back roads and rarely do I get out to the highway. Most places the limit is 80km or below so the top speed and cruising rpm are not likely to be an issue
When you say the suspension will be too stiff will ride height be
Yes. It’ll look like it’s jacked up in the rear. The rear half of an XJ12 is a LOT
heavier than the rear half of an XJ-S, so it’ll sit way high.
…would the front suspension components be worth
swapping over for a stiffer overall ride?
I think the front springs may actually be the same. The front end of an XJ12
is not significantly heavier than an XJ-S. The XJ6 is lighter, uses lighter
There are also “Sportspak” springs available, for the rear at least and
perhaps all around, that are stiffer than the base spring.
Before you jump in, you might want to search the archives here. There have
been several reports of people swapping springs around and ending up with
cars that sat at completely wrong heights. The springs on the XJ-S are very
soft indeed, which means that even a slight change can affect the ride height
greatly. Some refuse to believe it, but losing the gas pressure in the front
shocks will lower the front end about an inch. It’s akin to pushing down on
the front bumper with the same amount of force as the gas shocks exert, and
if you try it you’ll see it’s not hard to depress the front end. As opposed to
the rear end of my foster father’s 1967 AMX, which a friend and I both
climbed on the rear bumper and couldn’t detect that it moved at all.
Get your speed from an app on your smartphone. It’s more accurate
Swapping the rear cover on the diff would be easy, but I think you’d have to
swap the carrier too, which would involve a complete diff rebuild, and it
MIGHT not work with the ratio change anyway. So, your more likely solution
would be to obtain the speedo gear drive and sender for the tranny from an
earlier car, install it, and connect it up. Chances are the indicated speed will
be wrong; you’d have to fiddle with gears to get it right.
So, let’s go completely off the rails and think outside the box. Take the
pickup out of the '87 diff, look through the hole, and mark one tooth on the
carrier. Turn the carrier around and count teeth. While everything is apart,
take one of the brake rotors and drill a circle of holes with the same number
of holes as teeth on that carrier. Fashion a bracket to hold that pickup right
next to the brake rotor to count holes.
If that works, I expect some serious accolades from this group!
Better idea: While you’re doing all this, go ahead and upgrade to vented rear
rotors. Drill the holes in the hub for the rotor, which is not a wear item.
When the rotor is worn, replace the outer disk and reuse the modified hub.
I could and probably will use an app for acurate monitoring of my speed but truth be told the only reason I need the speedo to work is because it would bother me if it sat at zero full time. From what I have read elsewhere it seems the sensors work on a pulse per rotation, I see no reason why your idea of modifying the hub would not work and even if it does not come to that I will send serious accolades and praise for the idea!
Does my electronic speedo get its signal directly from the transducer or is the signal processed in some way first? I feel like the aftermarket should have sensors that can be mounted easily for electric speedo conversions or manual swaps where transmission driven speedometer is involved. Hell I see a market for an app paired with a plug that connects to the back of the speedometer and gets its signal from there while your stock speedometer displays and appears to function as normal
something like this? (http://www.ebay.com/itm/GPS-MPH-Speedometer-Gauge-Odometer-Black-For-ATV-UTV-Motorcycle-Marine-Boat-/262989713919)
some are using a Sattilite GPS speedos, very accurate , and has other builtin apps.
Dakota digital , and others , EBAY many.
best thing is NO hookups to the car ,except a 12V igntion supply.
I see no reason why your idea of modifying the hub would not work and
even if it does not come to that I will send serious accolades and
praise for the idea!
It’d work mounted out on the hub carrier, too. You could drill a ring of holes
in one of those silly U-joint clamshells. It might even work mounting the
pickup on the subframe itself, as long as it was directly forward or rearward
of a clamshell so it didn’t move closer or farther away with suspension travel.
Does my electronic speedo get its signal directly from the transducer
or is the signal processed in some way first?
I understand there is a “black box” involved. I also understand it fails more
often than the pickup.
Remember, the signal doesn’t just go to the speedo. It also goes to the
cruise control and, if you have one, the trip computer.
I feel like the
aftermarket should have sensors that can be mounted easily for
electric speedo conversions or manual swaps where transmission driven
speedometer is involved.
I used to have a couple laying around. They came with a cheap aftermarket
cruise control that I bought about three of from JC Whitney over the years,
real tiny 3-button control panel. Loved those things, but my current cars
came with CC so I didn’t have to add one. Anyhow, you could wire those
things up to the ignition if you had a M/T, so I usually did that. But they came
with a passle of magnets that you wired onto a CV joint or something and a
pickup that you bolted in place nearby. I generally didn’t use those, but I
kept them anyway. Heck, they might even be in my attic somewhere.
Hell I see a market for an app paired with a
plug that connects to the back of the speedometer and gets its signal
from there while your stock speedometer displays and appears to
function as normal
I’ve been saying for some time that cars need to do away with the dashboard
in entirety, just mount a tablet on the dash. Let it get signals from the car via
bluetooth. You could update the app weekly if necessary.
Doesn’t the XJ-S have ABS? Read the wheel pickup on one rear wheel and adjust the pulse count electronically to match what your speedo is calibrated for.
So I get my parts Monday evening. I may stop into a local Driveline shop, a co-worker has a hot rod they installed a th400 and apparently they were able to drive his electric speedometer off of it. It may be easiest to source a part for this from them if they have experience with this particular issue. They are somewhat of a specialist in hotrods and conversions from what I have been told. I am planing to ditch the trip computer in favour of some gauges so compatibility with that is irrelevant.
My 87 is non-ABS, so that option is out, unfortunately.
So the rear end was delivered today, looks to be in pretty good shape but will definitely get some work before it goes in.
Seeing as I plan to ditch the trip computer maybe switching to a mechanical speedo would be easiest. On that note, I can only assume the drive for the mechanical speedometer would be omitted from the TH400 if it was not specified for the car. How far into the transmission does one need to go to add the drive for the speedometer?
I think the gear drive on some TH400’s is just forward of the rear housing BUT l am not sure the drive output provision even exists in the Jaguar transmission case.