XJ8 whining sound - voltage regulator?

Last week-end, I took my '99 XJ8 out and from the moment I started the engine, I noticed that it sounded a bit different. As I turned the radio on, I also heard a clear whinning in the speaker which revs up as the engine revs up.

This makes me think that the voltage regulator is not properly straightening the current to DC and that there are some residual harmonics which I can hear through the radio.

I will borrow an oscilloscope at work to check the shape of signal arriving to the battery when the engine is running.

Any idea?

Also does anyone know where the external regulator is located

Thank you in advance,


There is no external regulator. Your photo is the internal voltage regulator and in my experience looks to be a Nippon Denso and pretty robust.

I’m very interested to know how this turns out because I have the exact same issue.

I’m waiting for a slot with my friendly Jag Man to fit an ignition suppressor which, he told me, he’s never seen fitted to any XJ8 but it’s listed as a part of Jaguar Classic Parts so I figure it’s worth a go.

Dear All

Thank you for your messages. I borrowed an oscilloscope at work today and here is what the signal looks like on the battery posts with the engine running idle:

One can clearly see the fluctuations at a 100 ms period which matches exactly the 600 rpm at idle. Voltage is fluctuating around 12 V +/- 100mv. This being said, 100 mV is not a lot…

When the engine is running at +/- 1500 rpm the signal looks like this:

Again, the signal fluctuations match the engine rpms and the voltage is up to 12,8 V which seems to indicate that the alternator is delivering some power to the battery. The fluctuations are a bit higher around +/- 150 mV.

Are such fluctuations a normal value for a good working alternator?


I have never measured this on a car before, so I am not 100% sure, but it looks like you might have a dead diode in the rectifier. The alternator is a three phase generator and that means you should get a rectified wave firm like this (bottom wave form):

Your wave form looks like you are missing one phase.

Plus 12.8V at 1200 rpm indicates no charge worth having. I’d expect 13.5-14V maxing out around 14.8V at higher revs.

Bravo Peter, Experience talks!

You are absolutely right, this morning I took the same measurements on my daily driver (Peugeot 5008) and the signal looks pretty much the same (see below). But when I disconnect the battery, the engine keeps running (I didn’t do that more than 5 s not to damage anything).

On the Jag, as soon as I disconnect the battery the engine stalls indicating that the alternator is not delivering any power to keep the engine ignition running.

So I guess I am on for a new alternator…

Any advice on OEM vs original?



There you have a proper rectified 3 phase waveform. I would check the rectifier diodes on the alternator.

For those of us without oscilloscopes :), the output voltage is often enough to diagnose a charging system failure.

From a cold start, the voltage should be in the 13-14v range. Classic Jags like E-types might drop back down into the 12-13 range after running for a while because they have few electrical accessories compared to a modern car.

But something like a X300/X308 should always be in the 13-14v range at all times, since they consume enough electrical power running to keep the alternator busy.


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