I decided to repair the shielded wire from the ignition amp to the ECU. It was a couple of inches longer than it needed to be at the amp end so I open up the shrink tubing near the end of the wire and there is a black wire included in the loom connected to the shield. I checked the resistance between the black wire and a chassis ground (earth?) and the resistance is quite high which surprised me as I thought it would go directly to a good ground. Does anyone know where the other end of the black wire connects?

 

To a ground in the ECU, I think. I say leave well alone!

 

You can download the 1989 Electrical Guide at the link below, courtesy of our members Gus, who hosts the website, and motorcarman (Bob), who provided the file: I think the circuits should be close to your 1987.

Jaguar XJS Electrical Guide 1989

ECU harness shields are shown to connect to ground at point G6, and two black wires from the ECU connect to ground at point G10. It might be worth cleaning those ground points. See the diagram in the manual for ground point locations.

Cheers,

Don

 

Shield must only ground one end.