When I first start the car I got really good voltage but as I drive it slowly goes down to the point where it’s almost triggering the battery light I think it’s a alternator but these old jaguars are weird so I thought I’d ask 1986 xj6

 

Yes, probably the alternator...but just as easily could be a loose belt.

Over the years we've seen a few cases where the alternator output wire is so corroded that it won't carry current. Possible, but less likely.

Cheers
DD

 

When I start my Series 3, there's a lag of maybe 10 seconds before the voltage slowly rises to good voltage. I always assumed that's normal - but when I read this -- made me wonder. Is there supposed to be a lag like that?

 

Every....and I mean every....Series III Jag I've owned or worked on has the same lazy voltmeter. I can't explain why.

It might be merely a quirk of the design.

If it's a fault, well, virtually all Series III cars seem to be afflicted.

Cheers
DD

 

I think that the voltmeter is heavily "damped". That is to stop it jumping around and twitching with every slight voltage change with variations in electrical load.
Just hark back to old ammeters and how they bounced around with changes in electrical load.

 

As Doug noted, it seems that all our cars suffer from the same ailment. I took my meter apart and cured the issue, only for it to return after a year of driving.
This is what I found. Of the other three meters, all have a metal needle, except the voltmeter, whose needle is plastic. The voltmeter is analog.

From Google "In analog voltmeters (often seen as a needle moving across a scale), a coil of wire is placed within a magnetic field. When voltage is applied, a current flows through the coil, creating a magnetic field that interacts with the permanent magnet. This interaction causes the coil (and attached needle) to deflect, indicating the voltage."
If the needle is metallic it will be affected by the meter's magnetic coil, resulting in false readings.

On my meter, the needle was "bowed" backwards, so it just touched the indicator plate, causing it to stick at the minimum position. A flick of a finger would release it and it would read normally. I guess that the heat from the meter's coil, eventually warps the plastic needle.

I used careful heat from a cigar lighter after positioning a toothpick to "rebend" it into shape. Worked great, until a year later it deformed once again.

Now I just live with it.
Ian

 

Just like the gauge in "China Syndrome." Who knew ? I remember that scene like it was yesterday. The anxiety Jag lovers feel when gauges are critical is probably close to that shown in the movie.