It is an interesting anomaly. After running the car to being fully warmed up, I have experienced several incidences where, upon attempting to restart the engine, I have discovered that power to the car has somehow been, at least partially, removed. I noted with interest the a small red light in the oil pressure gauge is lit as well as a similar red light in the battery voltage gauge. However , no power to the ignition switch or starter or to most every thing else, including door windows, boot interior lights, etc. I have, so far, been able to recover by disconnecting the ground cable at the battery and tapping the relays in the boot fuse box and the bonnet fuse boxes. Upon re-connecting the battery ground, all is well, boot interior lights are lit, car restarts normally, and I am on my way. This has now occurred three or four times and I am reasonably convinced that some circuit relay is the culprit and I am looking for suggestions. Any and all ideas would be appreciated.

 

I had the exact same experience, right down to the two red idiot lights. Usually, if I did a hard reset, touching the negative cable to the battery's positive terminal, the car would restart, but it could take numerous attempts, sometime over the course of a few days. As a result, I never felt comfortable shutting the car off it it wasn't in my driveway. I got a new battery, checked every power connection I could, all the fuses and relays and cleaned up grounding points. Then, magically, the condition cleared up and i haven't had a problem in over a year. But I still hold my breath every time I start the car. Good luck. If you find the cause I'd love to know.

 

A good place to start with these cars is always the resting voltage at the battery. Anything under 12.6 volts is of concern.

 

I realize my post above could leave the impression the new battery helped. It didn't and was one of the first things I tried since I assumed it wouldn't hurt to replace a battery that was almost 19 years old.

 

^^ That battery may now hold the world record for length of service in a Jaguar....

 

I absolutely agree. The car had 55,000 miles on it when I bought it in 2012. So maybe the original owner changed it; I didn't check the date code. No matter, that battery didn't owe me a penny and it was still starting the car when I replaced it.