I just last week purchased a beautiful 1986 XJS V12 coupe with only 74,000 miles on it...and the transmission blew out after putting less than 100 miles on it! I'd loathe to think that the seller somehow bamboozled me, knowing full well that the transmission was dysfunctional, however the car drove so flawlessly during the test drive—and for the first 4 days afterwards—that I can't imagine how an impending transmission failure could possibly have been covered-up.

Regardless, I need to replace the transmission, and since I am fairly experienced working with cars, I plan to do it myself. That said, one of my biggest gripes about the vehicle (though I knew full well in advance that this would be the case) is the gas mileage. It is my understanding, however, that post-91 XJS's came from the factory with GM 4L80E 4-speed automatics, replacing the old TH400 3-speed. Seeing as I'm going to go through the time, effort, and money of replacing the transmission anyway, I was curious as to whether or not a 4L80E would be a relatively straight-forward swap into an early XJS.

If possible, I'd like to determine the following, and if anyone here could answer any of these questions it would be greatly appreciated:

1. Would the shifter on the 4L80E come out in the same place as it did on the TH400? Or was the console moved forward/back in the later XJS's, and/or was the engine moved forward/back?

2. Since the 4L80E is computer as opposed to hydraulically controlled, would setting up the ECU for the transmission be a terrible headache—or for that matter, is the 4L80E controlled by the *ENGINE's* ECU?

3. Is the 4L80E for the Jaguar different in any way from 4L80E's for other (i.e. GM) vehicles? In other words, would I have to find a 4L80E pulled from a salvaged 1991+ Jag specifically, or would any 4L80E work? Are the bellhousing/bolt patterns the same?

4. Would there be any mounting issues/differences, i.e. are the rear mounts in a different location?

5. Lastly—has it ever been done? It seems to me that if it IS in fact a straightforward conversion, that it would be rather common. I am not looking for a project that will require custom designed CNC-machined parts, dozens of precision TIG welds, custom re-programmed ECU's, complex home-made mounts, or a re-engineered center console (nor a shifter that pops out of the glove box). If it would be an extremely complex project, I'll probably just replace the transmission with the original TH400.

Any insights would be much appreciated!

 

I've decided to go with the JCI kit. It is EXACTLY the type of set-up I was looking for. Thanks for all the info...if it weren't for this forum, I would probably have never even found that kit!