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Help figuring out this engine buzzer circuit
I'm in the process of building a new instrument panel using near identical circuits but adding some extra components and cleaning up the wiring. I took pretty good pictures and diagrams before I started disconnecting everything. I have most of it figured out but there's a buzzer in the layout that I can't understand how it ever worked as it did. Hoping some more experienced folks here can help me understand.
Let me start by saying I know this is not the way an engine alarm is supposed to be wired. I'll fix that going forward. I want to fully first understand the existing (insufficient) scheme to verify the notes I took and make 100% sure I know how the old scheme worked before I implement a new one, and there aren't some variables out there I don't understand.
The existing scheme employs a 3-position ignition switch to start a Westerbeke 30C (diesel) with a simple 4099 piezo buzzer serving as an alarm. Here's what happens: the buzzer sounds when the key is in the on or start positions, stops sounding once the engine has started, and starts sounding again when I pull the engine-stop lever, stops again when I turn the ignition to off.
In the existing scheme there are 4 parallel circuits running from the "on/accessory" position of the ignition switch: one to the panel instruments (water, oil pressure, tach), a second to the buzzer, a third to an engine clock, and a fourth (according to the Westerbeke diagram) to the o.p switch and alternator. The instrument circuit is grounded through a wire from the engine harness. The buzzer is grounded through a wire connected to the engine block. The engine clock is grounded through a distribution panel (in turn connected to the engine block).
So far so good. What I don't understand is why the buzzer stops sounding when the engine turns over. While this is a preferred result, it runs counter to my understanding of electricity.
Possible explanations include: 1) I don't understand electricity; 2) there's some voodoo electrical magic in the wire harness ground I don't understand; 3) my diagrams are wrong and my photographs are misleading (i.e., the ignition switch wasn't actually wired as I described above); 4) the buzzer isn't actually turning off when the engine is running; it's just being drowned out (which I doubt), or possibly running relatively softly.
Can anyone help me untangle this mystery? I realize the supplied information is imperfect.
Alternatively and on the #3 theory above, I've been trying to imagine a scheme that would behave as I described but I'm coming up short there too.