Tony,

I have not experienced that over-voltage problem with my XJ, but I did have it with my Ford minivan.

it turned out to be the alternator's voltage regulator gone bad. Replaced the alternator with a new one and problem gone.

That's a great outcome, I am just addressing a known problem with XJ's(from what I've read)of this age. I've read in the forums of posters upgrading to 100 to 120 amp alternators to provide adequate charging. I would think, the engineer's had a grip on the appropriate alternator to install in a 1984, $30,000 car, so before I upgrade, I will repair the poor connection's in the charging circuit and see the results. Voltage regulator replacement, if necessary. My info and pictures are definitely not for Jaguar Vet's, they are already well versed on these subjects more than I. This is for the regular Joe's, DIY's, such as myself, whom might need some Points and Pictures. Hope it helps, Be Safe Out There.

So much time to reflect during these difficult times, I recalled why I didn’t immediately go out and replace my malfunctioning alternator. I don’t have the link, but another poster explained 17.5V rampant in his XJ6. He purchased a reman. on eBay and I believe came across another working unit. That’s 3! All tested working. Installed in his XJ6, 17.5V!..He received info from the forum to clean all positive/negative connections. This was successful. Since I didn’t want to partially remove the sway bar(I caught absolute HELL getting the lower arm to compress to reveal the threads to screw the nut on the link during earlier replacement) to remove unit, I opted for the first solution.

I thoroughly cleaned all the grounding connection's in the circuit, installed a dedicated grounding strap for the alternator. Then, installed a brand new, name brand, voltage regulator($14 online)without removing the alternator from the car. 14.4 volts full time. After performing "surgery" on some spare speedometer's, which I purchased on EBAY, I found that there was too much "DRAG" on the worm gear attached to the step motor in the speedo. unit. The gear was responsible for turning no less than "8" gears in order to operate the odometer and the trip-meter. So, on my unit, I pushed IN the reset stick at the bottom of the dial, applied a couple of drops of superglue, which held the stick in that fully depressed position. This action effectively decreased the drag on the step motor worm gear from "8" gears to "2" gears. Pressing the reset stick retracts the gear rail which disconnects the odometer gears from the trip-meter gears. Drove for 2 days, works, so far, flawlessly. I have not, as of yet, discovered the need for an operational trip-meter in 2020.
[img]219663[/img]
I believe, "resistance" is the more appropriate word to describe the action against the step motor, not "drag". Good luck.

My above post contains a picture of a volunteer “donor” speedometer. On the YouTube channel LIVING WITH A LEGEND, the gentleman video’s the disassembly of a gauge on a SERIES II. He removes the bezel by aligning the tabs with slots located in the case. On my 1984, there are no slots to align, only tabs that must be bent outward. Looks terrible after re-assembly. The black plastic case attached to the back of the unit is attached by 4 screws which are only accessed from the inside of the case after removing the bezel. Go figure! I tried drilling out the screws from the rear, but the plastic is soft, and the screws are hard. Probably needed a drill press. Sooooo, as
mentioned, I just pressed the reset stick, which allowed the “white gear” to spin,’but not engage the red gear. Unit broke at about 88,677 or so. Now...

https://cimg0.ibsrv.net/gimg/www.jag...d0f78b8f2.jpeg
https://cimg4.ibsrv.net/gimg/www.jag...d15fab0ec.jpeg

Fixed Odometer/trip tumblers
 

My Speedo was spinning correctly, but the Odo & Trip meter weren’t moving. Based on the pix below I removed the little side plate roller (2 screws) then the clip on the worm gear motor. If you loosen the motor wires from the frame (don’t disconnect the plug) the worm gear motor can be pulled free. I could feel it was “crunchy” when I turned the shaft by hand so I sprayed “DeoxIT” into the slots in the motor and spun the the shaft , and sprayed, and spun, etc. The motor has steps in it so it will feel like a washboard but just cycle the DeoxIT for a couple minutes to get it through all of the inner workings. THAT’S IT! I put it back together, then loosely installed the speedo and wires (don’t put the glass back just yet). Test Drive to confirm that the gears are working, if it spins then re-install the glass and bezel and put it back in the dash. Easy Peasey. My car lived near the beach its whole life so there is just mild surface corrosion everywhere, I guess it got into the dash too.

Below are pix of speedo innards. One picture show the board w/chip...

https://cimg6.ibsrv.net/gimg/www.jag...ded707c8d.jpeg
Next one is motor and worm gear which spins the “tumblers”.

https://cimg9.ibsrv.net/gimg/www.jag...574937cfb.jpeg

Unscrew the little side plate (it won’t come out until you unclip the motor and shift it out). Remember to put this plate in position as you reinsert the worm gear, it will settle into position as you do this.

I think it’s great that you were able to build on the initial problem and find a solution. It wasn’t obvious to me, at the time, to remove those two screws and delve deeper. If my super glue fix ever fails, I might have the courage to follow your lead, thanks for sharing your solution.