I just finished responding to a TCA group question about wiring of old F-3 horns. I've written a full description of the wiring and thought I'd share it with this group.
BTW, note that this is often a futile exercise. An original horn that still works is rare, and the replacements don't sound the same.
But here's my explanation
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Below is a wiring diagram for F-3 horns, showing only wires related to the horn. I'm trying to illustrate more clearly what is on page 153 of the service manual.
One important thing to know is that the battery/relay bracket and the relay frame are all at 1.5 volts from the locomotive frame. The top of the battery is touching the bracket, and the relay frame is screwed to the bracket.
There are three contact strips on the relay. I'll call them 1,2 and 3, from the outside inward.
The top of strip 1 (black) is one side of the relay coil. The bottom is attached to the locomotive frame by a screw holding the relay bracket. That screw is insulate from the bracket.
The top of strip 2 (green) is attached to track power. Exactly where doesn't matter. It's the same connection as the headlight and the power contact on the e-unit.. The bottom connects to the other side of the relay coil.
Strip 3 is the battery power (red). The top is connected to the horn. The bottom makes contact with the relay armature, which is hinged on the relay frame, which is connected by the frame to the + side of the battery.. Thus when the relay is energized, the battery is connected to the horn.
I've tried to illustrate this by using separate colors for ground, track power and battery power.
