How do the voltmeters work in these trucks?

[kipps]

I'm holding two instrument clusters.

An '87 cluster has three pins on the back of the voltmeter.

A '91 cluster has an ignition hot and a ground going into the voltmeter, and a separate wire going to the battery light. It looks like the battery light needs to be grounded through that wire in order to work. I may have misread the printed circuit, though.

How does that three-pin voltmeter work? How does the battery light work?

I'm working on a LS swap, and am trying to literally merge the old and new harnesses, including the new underhood fusebox.