I bought a Smiths RVC (voltage pulse sensing) tach for an '84 Jag XJ6 because it was for a 6-cylinder, the right size, and I knew I could alter the Jag's calibration to match the GT6 face. Since the case size is the same, you can move the bezels back and forth between the GT6 and Jag tachs depending on whether you want the black or chrome bezel.
I had to add a large hole to the back of the Jag tach's case to support the Mk1's turn signal indicator lamp. Later GT6 have 2 jewels that should line up with those used in the Jag tach so this step would not be required for owners of Mk2/Mk3 cars.
In all cases (Mk1 thru Mk3), to use the Jag tach in the GT6 you will need to drill out the brass bayonet posts on circumference of the Jag tach case and you will need to drill the back to add threaded mounting studs. I used 8-32 round head bolts. To use the GT6 face you will have to drill 2 new holes in it to line up with the face mounting screws used by the Jag's movement. To get the calibration right you will need to add a 300 Ohm adjustable trimpot in series with one of the two wires going from the Jag's circuit board to the mechanical part of the tach movement. That will allow you to adjust the span of the needle movement.
An easier option for you Mk2/Mk3 owners is to use the 1970s XJ6 tach without cosmetic modifications if you are not worried about originality. The 1970s Jag tach will have round jewels instead of the rectangular ones shown in my picture below. You could repaint the needle if you wanted to as I show in my second picture below but that is a choice. Beyond cosmetics, you would have to remove the bayonet posts and add the threaded mounting studs, but apart from the different redline and needle, the Jag tach will look close to correct.
The Jag tachs are surprisingly inexpensive. I paid $40 for mine including delivery and it came with the Jag wiring harness. If you don't mind the lower redline and different needle... it is a simple task to modify the Jag case for mounting in the Mk2/Mk3 GT6 and using it "as is". To keep the look, I went further in my conversion. So far the results seem to be worth it... way better than the 4k Max RPM from my last conversion. Based on my bench test equipment, the converted tach is accurate within 100 RPM across a range from 500 to 6k RPM. That's pretty darn good, especially when you consider that the GT6 face was laid out to work with a mechanical tach movement and not the Jag's electronic movement.
Doug L.
Responses