Strange Voltage problem XJ12

Hi guys,

yesterday while driving, my battery light came on.
When I measure the voltage on the battery, with a running engine, the voltage is 13.8V.
The battery is brand new.
IMHO that’s right on the mark where it should be.

Any suggestions as to why the battery warning light comes on then?

Your alternator may be ‘on its way out.’ Alternatively, the problem may be just a loose or corroded battery terminal cable or ground strap.

Strangely, the charging voltage is within specs. When revving the engine, the battery light grows stronger…

Would be a shame to swap alternators when it’s not broken.

Verzonden vanaf mijn Galaxy

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Time to get out the VOM. Probe volts at battery + & - posts. Record. About 13 or so?

Start the engine. read volts again. At idle any better? Should be a bit more 13 + ?

Rev it up a bit. More volts ?


Hi Carl, done that allready.
Idle (750 rpm) Voltage 12,5 V
3000 rpm 13.8 V

And still the light is on…

Verzonden vanaf mijn Galaxy

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sounds a bit on the low side. With an alternator you should see close to 13 at idle and close to 14 short above that, say at 1000 to 1500 (not at 3000 rpm). That being said: there must be a fault in the circuit as at 3000 rpm and 13.8 V the idiot light should be off for sure - not brighter!

How about the history: when did the malfunction occur first time? What did you do prior to that? Any other incidents?

Good luck


75 XJ6L 4.2 auto (UK spec)

It came on for the first time just yesterday.
Nothing before that, the car has proven to be extremely reliable until now.

Gues I’ll just have to risk it and buy a new alternator. They are not expensive, around €120,- excl.VAT.

Verzonden vanaf mijn Galaxy

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At 13.8 V the idiot light should not be on.
Check your connections, maybe loose cable or short circuit somewhere?

If you go for a new alt check this out, same price but modern design and you get 120A.

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Before you do that, Patrick - check the brown/black wire connection to the alternator, as also suggested by Aristides…

If the wire is disconnected and shorts to ground, you will have the symptoms described.

Your voltage readings indicates that the alternator works as it should - and the fault relates to the warning light only…

xj6 85 Sov Europe (UK/NZ)

Oh boy, I love this forum already…

Will check the wire this afternoon…thanks!

Strangely, the warning light grows stronger when revving the engine higher.
On idle, it is barely visible at all…to me that indicates a working alternator, but broken wire / short etc.
Had the alternator died, it would either deliver no current or a current that is way to high imho…

Perhaps the alt has lost one phase? I’m not sure what the symptoms are for that, but I know an o-scope shows too much “ripple” in the voltage.

As the alternator of the Jaguar should provide direct current, the scope should always show a straight line, i think…

Bad diode,. Back feed to the light? Purely a guess !


I agree with Frank. your alternator is not at fault. your issue lies in the wiring somewhere. your light is being supplied with power from somewhere to illuminate it.

an alternator, regardless of the car, supplies alternating current, not direct current, hence the name ‘alternator’

direct current is supplied by the battery, the alternating current charges the battery, through relays and diodes.

The alternator does, but the current is transformed to direct current in order for the battery to be able to store the energy. So, when connectong a scope to a live feed, it measures dc. When connecting the scope over the seperate phases of the alternator, then it’s a different thing…

Verzonden vanaf mijn Galaxy

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The ac the alternator generates is in most cases already coverted to dc before it leaves the alternator to the wiring loom.

Verzonden vanaf mijn Galaxy

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The AC is rectified to pulsing DC by 6 diodes in the back of the alternator.

More specifically, the light is being grounded from somewhere.

One side of the bulb always gets 12v+ when the key is on. The other side is grounded in the alternator (thus it illuminates) until the alternator begins charging. At that point 12v+ is provided to both sides of the bulb and it extinguishes.