Swapping ammeter movement w/ a voltmeter?

[jackw19]

More re Warning Lights

Crusher, that's the first explanation of the alternaor warning light operation I've been able to understand--and it's been ticking over in the back of the noggin for quite a while.

Is a somewhat subtle point that when the alternator starts running it ceases to function as a ground. And that the alternator no longer needs external external excitation once it's up and running is another complicating factor.

So whether or not the dash light is illuminated depends on the potential difference between its two terminals. This would explain why an incandescent dash light can have variable brightness. I guess an LED may tend to be full on or dark, although maybe they've got a limited range of brightness values.

Good to know your installation has survived since May. I was concerned about voltage excursions on the circuit up to 14-15v, but if it works--it works.

Thanks for your lucid exposition.

[75Crew]

Why not have both?

When I purchased my '76 W200 new, I ordered it with the tach and vacuum/voltmeter guage option. (A set of these just sold today on eBay along with an original tach for $140.) This truck also had the ammeter on it. I liked the setup really well back then, so when I started restoring my current '75 W200 crew cab, I decided to do it again. I bought a SunPro small tach that I mounted in the left side of the instrument panel. I had a Stewart Warner vacuum/voltmeter guage set that I had used on another W200 in the late 70's, so I brought them out of the attic and mounted them in the right side panel. Now I can see when the current is a little high (such as when I have all my auxilliary lights on at the same time) and I can see if the alternator is keeping up the battery voltage while supplying current to the loads. Plus, it makes for a nice look when I'm driving down the road.