On Tue, 26 Nov 2013 01:33:21 -0500, tm wrote:

If someone mixed up that wiring, it could present a short circuit
to the power supply and battery.

The alarm has been working for years, so, I'm pretty sure the wires
didn't all of a sudden get changed.

The good news is that the new transformer arrived today.

However, the voltage output wasn't at all what I had expected from a
16.5VAC transformer secondary.
http://farm6.staticflickr.com/5532/1...7612e531_o.gif

I measured:
a) 20 VAC across the secondary
b) 3.54 VAC across on side of the secondary to ground
c) 0.98VAC across the other side of the secondary to ground

Huh?