Hi there,

I will agree with Art on the sudden loading theory.

I purchased the "Automotive Electric/ Electronic Systems" book that Bosch prints. Art recommended to me a few weeks back. I got it off Amazon for about $8 shipped! Looked really new and its well illustrated too!
It's a great little book, Art, thanks!

You did not mention if the instrument panel goes "nuts" if the headlights are on or not?

The I don't know much about HID headlights. I think they use a transformer don't they?
Any people that I have known, that used aftermarket head lights, have had issues with the transformers, sooner or later!

All of this might be a coincidence that started from worn brushes or stator rings in the 100 Amp alternator. 10 years is quite a bit of time if you have not serviced them.

I'm curious to know where the voltmeter is connected. Is it into the cars circuitry or is hooked directly to the battery's posts?

The instrument panel is having an issue to be acting that way.
It seems to me I would look art the fuse panel out feeds to the instrument panel or the headlights switch itself. Most of the power should goes through the "infamous" headlight relay connector, on a regular 240.

For some reason, the battery light is seeing a ground circuit through the alternator or a part of the diode circuit for several lights mounted on the instrument panel.
The tachometer bouncing sounds like a on-off grounding or instrument powered issue to be along with the gauges.

If you remove the cluster you will access to almost all of those goodies!

I hope you are able to keep it down to a low tech hunting endeavor!

That's about all I could handle if it were my car.
(:)

Phil