The shop that restored the car caution me about static battery drain, and installed a cut--off switch to deal with it.

I measured the drain -- 0.3A -- and suspect that it is created by the clock, which happens to be inoperative, as the wiring diagram shows no other likely suspect. Unfortunately, the screw attaching the electrical feed to the clock is covered by a tight wire, which would seem to preclude disconnecting the clock without disconnecting the panel and disturbing all the wiring and sensor feeds behind the panel. I'm inclined to leave the situation as-is, and rely on the battery maintainer and cut-off switch to keep the battery topped up.

Comments? Suggestions?

 

I have battery cut-off switches on all of my current Jaguars (as per my signature)
As some only get driven once a month it is essential that those electrical systems like clocks, and anti-theft in the later cars are not sucking the battery down.
A couple of moments to reset the clock and in the later cars a minute or two to reprogramme the radio are small penalties to pay, and a fully charged battery lasts a lot longer

 

The clock should never pull more than 0.03 Amps, even if it is stopped. When it runs it pulls maybe a tenth of that. If you have a 0.3A drain then there is something wrong, either in the clock or elsewhere. The clock can be electrically disconnected from underneath with a short or angled screwdriver, just be sure to disconnect the battery first since that wire is not fused. The clock can also be removed while you're there, and you can send it to www.jaguarclock.com or any other instrument restorer so you will have a functioning clock.

 

Agree totally with Mike's advice for your XK120 situation.

In each of my early models (Mk1, Mk2, S Type, S1 E-Type) I used the services of a local jeweler who rose to the challenge and rewarded me with fully working clocks . . . in one case, making new hands to replace those that had fallen off!

A word of warning re newer models . . . a big tick to battery tender / maintainer / trickle charger but only if of the really 'smart' version that not only powers down once the battery is fully charged, but powers up automatically with any charge depletion . . . however, a big 'no-no' to battery isolators in these later models. Those models are designed to enter sleep mode, drawing a maximum of just 30ma which is next to nothing in a fully charged battery.

Importantly, each time you isolate the battery, you will delete all the stored ECU and TCU data, which means the car must re-learn every time you drive it. Surely the solution is to identify and fix any excessive battery drain and let the car operate as intended.