Help figuring out this engine buzzer circuit - SailNet Community
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post #1 of 12 Old 3 Weeks Ago Thread Starter
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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.
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Re: Help figuring out this engine buzzer circuit

I didnt digest everything above, but the buzzer alarm is probably oil pressure
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Re: Help figuring out this engine buzzer circuit

Yep, I'd agree, oil pressure. Turn the engine over and the oil pressure gets higher, turning off the alarm.
Switches come in normally open and normally closed. You just pick the right one for the job.

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Re: Help figuring out this engine buzzer circuit

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Yep, I'd agree, oil pressure. Turn the engine over and the oil pressure gets higher, turning off the alarm.
That would explain it if the buzzer is wired through the o.p. switch. But it's wired directly to ground. So how would it turn off when the engine turns over? That's the mystery.

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Re: Help figuring out this engine buzzer circuit

As the engine turns over the oil pump begins to pressurize the system. At some point the pressure will activate buzzer switch and go off. It may need replacement if it goes off too early (oil pressure too low). You would rather a warning that oil pressure is low, but not yet low enough to cause damage.

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Re: Help figuring out this engine buzzer circuit

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As the engine turns over the oil pump begins to pressurize the system. At some point the pressure will activate buzzer switch and go off. It may need replacement if it goes off too early (oil pressure too low). You would rather a warning that oil pressure is low, but not yet low enough to cause damage.
I'm missing something. The o.p. switch is not connected to any circuit. How would this happen?
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Re: Help figuring out this engine buzzer circuit

That's why I call it electrickery on a boat.
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Re: Help figuring out this engine buzzer circuit

My guess is that this is the answer:
You said the buzzer is grounded through a wire connected to the engine block. If you look closer at the connection point on the engine block, you should find it to be an oil-pressure switch connecting the ground side of the buzzer only when there is little or no oil pressure. When there is sufficient oil pressure, the connection at the oil-pressure switch will be disconnected from ground and the buzzer will stop.

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Re: Help figuring out this engine buzzer circuit

I think your wiring diagram is wrong. You can find the wiring diagram for the Westerbeke admiral and captain panel here:

https://www.google.com/url?sa=t&sour...CNdfZTzX7StOyK

On roughly page 40 or so (though the page label says 32)

If I remember right the audible alarm comes in with either oil pressure (steady) or water temp switch (pulsing).

As a previous poster said, it does that because the switch (es) close to ground when in fault and thus complete the circuit.


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Re: Help figuring out this engine buzzer circuit

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Originally Posted by BobT View Post
My guess is that this is the answer:
You said the buzzer is grounded through a wire connected to the engine block. If you look closer at the connection point on the engine block, you should find it to be an oil-pressure switch connecting the ground side of the buzzer only when there is little or no oil pressure. When there is sufficient oil pressure, the connection at the oil-pressure switch will be disconnected from ground and the buzzer will stop.
That sounds very promising. Thanks, I'll check that out when I'm next out there.
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