Finally had a chance to spend some quality time with my 1984 XJS this weekend. I had already checked Pin 18 at the ECU connector in the trunk (after unplugging the white wire at the amplifier), and did have ground continuity (a short somewhere in the circuit / wire). Usually this points to the shielded white wire up front.
Today I isolated the shielded white wire from the harness all the way over to the right fender. All the wires were in nice shape with no sign of heat damage or deterioration. I cut the white wire over by the right fender, carefully stripped the end of the shielded wire going back to the ECU (rear of the car), checked for ground continuity, and everything was fine (no continuity - no short). I then checked Pin 18 at the ECU connector in the trunk, and still had ground continuity (even with the white wire not connected to anything up front and showed no ground continuity at the front of the car).
After this, I checked the PLUG on the white wire from the Ignition Amplifier that is only about 2" long (it plugs into the white shielded wire), and had ground continuity at the Amplifier Plug.
So … I have ground continuity at Pin 18 (with the wire disconnected up front), ground continuity at the short white wire plug (at the Amplifier itself), but no ground continuity on the shielded wire over by the right fender. All the other wires (and plugs / ground) going to the Ignition Amplifier have been cleaned and check.
Is it possible the Ignition Amplifier has an internal short? Why would I get ground continuity at back in the trunk at Pin 18 (with the white wire disconnected), but not have ground continuity on the cut white wire up at the right front fender (which is what I would expect if there was no short in the shielded wire)? Is there a junction box (or connectors) up front on the right side (back towards the fire wall) the white wire runs through that may have an issue?
Any thoughts or ideas would be greatly appreciated. Thanks for all the help and suggestions.