Hi.

2004 XJR. Battery replaced 8 months ago with a Yuasa 019.

Today when driving along the battery/alternator lamp came on. On restarting it went away. Thought nothing more of it.

Tonight I got the familiar drama with loads of warning lamps and system failures that normally occur when the battery is faulty.

But then the lights on the car went out. This was very scary as it was pitch black outside on a country road with no streetlamps.

I was under the impression that when the engine is running, all power comes from the alternator. Is that wrong?

I am really really hoping it isn't an alternator fault.

What else could it be?

Halfords tested the battery this morning before the incident and said it was good as their tester measured it as 860Ah (it was a 900Ah battery originally).

Could the battery be toast already?

 

Probably the alternator unfortunately. Something similar happened to me. One day driving all the warning lights came on and I limped it home. Battery had almost no voltage left. Took the battery in and exchanged it under warranty for a new one (was only a couple years old). Drove the car another week without issue and then the battery light came on and stayed on. Tested the system and found the alternator was not charging. It was not a fun job, but it is doable. Car has been great since.

 

Check the battery voltage with a decent multi-meter, it needs to be 12.6 volts at rest in the car to keep everything happy.
If below that then you will have various sub-systems telling you the sky is falling!

If you have a charger, put it on the battery to see if it will pick it up again overnight.
Everything should return back to normal with a decent voltage present (you might have to reset park brake).
Start the car and see what the battery voltage is doing with your meter while engine is running.
It should be getting charged by the alternator and yo should see the voltage going above 13 volts (more like 13.5 volts) to push charge into the battery.
Immediately after a start the alternator is likely trying to replace the energy used quickly so you might see a higher voltage across the batter for a short time.

If it all comes back to working, but dies again after a few more days, then you likely have a legitimate charging fault and unless it is a cable type problem (more common on the X-Types), then the alternator is the most likely culprit.

 

Chances are pretty good, replacing the brushes & bearings, will restore things back to normal.
Take it to an Auto Zone, or O'Rileys to have it tested, before installing

 

Mesure battery next morning . Slmost new it should still have about 12.4 to12.6v. The start engine and check charge again should be like 14.4V from the alternator dropping to about 13.4 after a minute. If not charging the alternator is about done.
take it out from below loosening engine mounts a bit and lift engine a bit with a pieceof wood under the oil pan. Get a compete referb done. 250€
jaguar 1000€
Replacement 500

 

Got a new battery yesterday, drove it all day and night, thought it was foxed.

But tonight the dreaded Battery lamp lit up again, so went straight home.

This is depressing.

It's gotta be the alternator right? There's no fuses or relays inbetween?

 

When an alternator fails it's usually the diode which is replaceable

https://www.google.com/search?q=car+...tor+diode&gs_l

 

Yes I know that, but I really really dont want to start trying to remove the alternator unless EVERYTHING else is ruled out. Could there be anything else it could be?

 

Did you happen to also pick up a multi-meter so you could check battery voltages at rest, immediately after starting and several minutes later.
Those readings will do a lot to tell if the alternator is trying to work, or if it is just a counterweight currently mounted on the side of the motor.

 

I have a multimeter so will check when home. When the battery drained the first time and the lights and everything went out, the engine still ran fine.

It's lile the alternator is powering the engine but nothing else.

 

Do you have the part number for the alternator?

Who is the OEM?

 

On my SV8, built in September 2003, the alternator is from Denso, but I do not remember the part number.

Best regards,

Thomas