Jump starting a V12 HE

Probably a dumb question… My car’s battery is flat.
Can i simply use another car to jump start the XJS?
Is there anything particular that I should look put for?

Hello Han - first step is to check the bad battery fluid level - if you can remove the caps on top - might have paper label covering the edge of the cap covers - feel around to see if you can tear the paper label so that you can use a flat blade screwdriver to remove the cover - probably have two covers - look at the fluid level - add distilled water to bring the level up to at least cover the metal plates inside, preferrably fluid level up to bottom of fill holes - connect the cables from the helper car (engine off on both cars) with matching polarity (positive to positive and negative to negative) start the helper car and let it run for approximately twenty minutes - watch the dead battery and see if any bubbles are appearing in the fluid - might only be a few bubbles - if every cell hole is showing a bubble once in a while, then the jump procedure may help - if no bubbles, then battery is too far gone - if getting bubbles, then turn off the helper car, leaving the cables connected - attempt to start the dead battery car - if the dead car starts, remove the cables, while it is running, first from the dead car, the negative first, then the positive - make sure that the cable ends do not touch each other - then remove the cables from the helper car - it is not good to have the helper car running, if the dead car starts, with the cables connected, as this will cause damage to the alternator of either or both cars - if you can not start the dead car, but did see bubbles, then you may have to let the helper car restart and run longer, maybe about a half an hour, to get the dead car to a point that it will be able to start while using the assist of the helper car - please be careful to not let the cable ends touch at any time - if you can, use a volt meter to measure the dead battery before starting these steps, and then measure the battery, before starting the dead car (with the helper car negative cable off) to see if the dead battery is trying to rejuvenate - if you can get to at least 10 volts on the dead car, then, with the cables connected, you should be able to get the dead car to start - Tex - sent 10/11/2021 1158hrs. EST USA.

Whew, the most complex set of instructions as to cranking another I ever encountered.

No more jump cables around here. A smart charger and a jump box. the latter a new toy and as yet to be used… May do it in a day or two l Jeep battery is too low to crank.

Unless Han lives in Bolivia - he should’t follow this guide. Sice last decade - we have batteries that are unservicable. The chance of hurting yourself while topping up - is 100k times greater than the risk of damaging the battery itself.
Also - if the “metal plates” inside the battery are exposed - it will serve you well as a corrosive paperweight…
Opening - not mentioning topping it up with DI water may only damage some of these days designs…

You don’t need to wait any 20 minutes, it will be ready to crank after 5 (unless - you want to use another car as battery charger, then go for 8 hours min.)

The only thing that you wil have to do (especially after long nap in the barn) - is to clean battery terminals and clamps. Later on, gving a smear of copper grease on these - will preserve them forever.

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Thank you Terry(??)
My battery is the maintenance free type, so there is no cap to open.
Taken note of your advise.

I have the jump starter, but was just worried as some manufacturers advise against jump starting a car using another car’s battery… but I think it was for newer cars. But I guess the XJS is safe to do that.
Thank you.

Managed to jump start the car with my portable jump starter.


i usually put vaseline or light grease over cable ends… after cleaning both the posts and inside cable end clamps

I’ve never done that before, applying vaseline or grease. Is that to prevent corrosion?

yes keeps air and corrosive fumes away

Yes you can jump start your XJS with another car and jumper cables. I would have thought someone would have said “yes” by now. We can do better :slight_smile:


funny after reading this yesterday… left lights on while having coffee… friend jumped me… battery to battery… no prob.

My 89 does not have a “key is off lights are on” beep. At some point I will wire one up.

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let us know how etc… parts wiring

Should be pretty straight forward. Off the top of my head you need a small 12v relay and a buzzer of choice. You need the 12v wire that is hot when the headlights are on, and another 12v that is hot when the ign is on. You need a ground.

My quick and dirty logic:

One side of the buzzer goes to the wire that is hot when the headlights are on.
The other side of the buzzer (ground) goes to the switched side of the relay, normally closed. The other side of the normally closed contact goes to ground. So when the relay is not energized the wire is grounded.
The coil side of the relay has one side to a switched ignition source, the other side to ground.
When the lights are on and the ignition is on, the relay is open, the buzzer is powered from lights but no ground thru the relay for the buzzer
When the lights are on and the ignition is off, the relay is closed, buzzer is powered from lights and grounded thru relay and buzzes
When the lights are off and the ignition is on, there is neither power nor ground for the buzzer
When the lights are off and the ignition is off, there is no power for the buzzer.

I wrote this pretty quickly so feel free to point out logic flaws.

possibly there is a unit from another car that would wire in easily

Its for the saloon, but imagine also applies for the XJS.