I purchased my 2001 Jaguar S-type 3.0 V6 in November 2024 with about 125,000 miles on it. It appears to be in good condition, runs well. I am in Los Angeles CA. This is my daily driver, and I'm an apartment dweller, so I don't have a lot of facility for being a "backyard mechanic." Each time, I had not driven the car the prior day. When I drive, it shows average gas mileage of about 18 mpg in the city, and about 28 mpg on the highway.

Twice now, once a month ago and once today, I went out to the car and the battery is absolutely dead. No lights, no key fob, etc. The battery is correct and only a couple years old and connections look clean. When I charge it, the charger indicates 13.3 volts and it fully charges after a couple hours or so at about 6 amps.

These are odd things in the car that perhaps have an impact:
1. The headlight sensor that tells the autolight to turn on is not working properly; when in the auto position, the headlights are always on but when I turn off the car, the headlights go off after the set delay. After the last failure, I stopped using the autolight, but the other day, I had used it, letting it turn off the headlights after the delay.
2. Occasionally, maybe four times since I bought it, the error message "Right Brake Light Malfunction" comes on, but after a period of time, that resolves and the light works. (I do not know why this happens, so if anyone has a theory on THIS too, please let me know.)
3. Previous owner put in odd things; two cameras (front windshield and rear windshield), a video screen in place of the NAV box, and somehow destroyed the cassette player and removed the CD player from the trunk in so doing. He added a bluetooth device to feed the radio, powered by a USB wire. He had some other contraption I don't know about but it was run from a USB wire above the dashboard.
4. There is a "wild wire" coming out of the trunk fuse box, which today I disconnected in case that's the battery drain. The battery is not fully charged yet, so I don't know but assume that wild wire powers the cameras, which I do not need.

Any comments appreciated.

 

Look up threads about
quiescent current

It will take patience and care to find the power drain(s).

 

Totally dead, colder than Nancy Pelosi's heart? That makes me suspect a connection problem, and not necessarily just a discharged battery. Typically, when a battery runs down for whatever reason, there's usually still a little oomph (sorry for the technical term) remaining to power some lights. Sure, a massive short can totally discharge a battery, but that is fairly rare.

My first thought was a problem where the battery ground cable connects to the body. There's a short cable bolted to the body, close to the battery. This bolt extends into the wheelwell. Moisture can wick up through the threads and cause corrosion, interfering with the connection. For giggles, disconnect and thoroughly clean this connection. This is a known trouble spot with these cars.


13.3V is an odd number, and not in the range I'd expect to see either while charging or at rest.

If recorded while charging, that's too low. Typically, you want to see around 13.7V+. Any less while charging indicates either a defective charger or a battery on its last legs, unable to take a charge.

If recorded at rest, that's pretty high. Turn on the headlights for about 15 seconds to deplete the surface charge for a more meaningful reading.

Fully charged after two hours? That doesn't sound right either. If so, the battery wasn't nearly as depleted as you were thinking. For a completely dead battery, I think you'd need to charge for at least 12 hours, if not longer. Kinda fits my preconceived hunch about a connection problem, versus a discharged battery.

I would suggest taking the battery to any auto parts store that will test it for you. If the car will start, you can just drive there. If not, remove the battery and take it in. Any place that sells batteries will have a high tech charger/scanner machine that can determine the battery health, and not just the state of charge. If indeed it did fully discharge twice, the battery is probably a borderline paperweight by now. Test it to be sure.




Is it this guy, for the transit isolation relay?:

https://www.jaguarforums.com/forum/s...s-trunk-98093/

 

I agree with Karl. Sounds like a loose ground wire or something like it. Also, don't put the emphasis on the 13.3V, which the charger indicates, but use a multimeter.
What I wanted to add - but I might be wrong here...: In cases like those we always suspect a loose ground wire, but is there maybe a relay somewhere, where the internal contactor gives an intermittent contact only, causing your problems? Just an idea - I may be wrong, as mentioned...

PS:
Thinking about it more meanwhile, I may actually be on something there. I just remembered a short video more clearly, which I saw recently (which may have been in the back of my mind unconsciously, when I mentioned a relay a possible failure source)...: I saw a video about a Mercedes AMG, which had to be towed - and the problem was a relay. There were 2 identical relays next to each other - it wasn't mentioned, what the other relay was good for, but they simply took the good relay and put it, where the damaged one was, which prevented starting of the engine, and suddenly the AMG roared up again...

 

Thanks, will work on that connection.

The battery did charge and the car is, once again, running fine.

 

Will do. Thanks.

 

The wild wire was an added wire to power the cameras.

 

One more quick check for ya. There is megafuse in the trunk, hidden behind the liner outboard of the battery. This thread, from a later model, shows the general location:

https://www.jaguarforums.com/forum/s...-issue-230037/

Now that I think about it, I'm not positive about the location on an early model, but it's somewhere near the battery and rear power distribution box. I'm not saying you'll find the megafuse smoked like that example, but do make sure the connections are tight. Pretty much all current to the front of the car passes through that fuse. Look for discoloration from overheating, indicative of a marginal connection. Be careful with a wrench, as the fuse carries hot battery power. Disconnect the battery ground cable before checking the fastener torque.