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Old 04-14-2008
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Low Oil Pressure buzzer/alarm

Hello,

I have a 1985 O'Day 28 with a Universal model 5411 diesel. The low oil pressure indication on this boat is only a small red light on the ingnition panel, which is almost impossible to see on a sunny day. In attempt to be proactive, I wire a radio-shack buzzer in parallel with the inidicator light, and voila! nothing. The light is definitely lit when I turn on the ignition key so I know there's power in the circuit coming from the low oil pressure switch on the block. Could it be that the voltage to the indicator light is much less than 12 volts and that's why the buzzer won't work? The buzzer that I bought is rated at 9-16 volts DC. Admitedly I'm no electrictian, but I checked the wiring schematic and it seems like all the wiring to the panel would be 12 volts. I tried the buzzer to an adjacent gauge and it worked fine. Any thoughts???
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Old 04-14-2008
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Get a voltmeter on the indicator light circuit or read the voltage off the light base itself (if it's incandescent as I suspect a 23 year old lamp would be).

Find out the output of the low oil pressure sensor, or (my choice) replace the sensor with a 12 VDC one, and then you can run the audible alarm in series with the lamp, or just get rid of the lamp, assuming you don't have deaf helmspeople.

Another possibility is that you've either wired the buzzer incorrectly or that the amperage is insufficient to activate the buzzer. Try it with the lamp OUT of the circuit.

Which makes me think...how do you create the conditions for oil pressure low enough to light the lamp in the first place?
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The light is activated (circuit closed) by a pressure switch on the block. I just replaced the switch last weekend. Prior to that the light didn't work either. I would just rather have an audible alarm. Thanks for the advice.
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Old 04-14-2008
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Quote:
Originally Posted by Valiente View Post
Which makes me think...how do you create the conditions for oil pressure low enough to light the lamp in the first place?
Val, I'm assuming you were just having a brain fart because car, boat, whatever - Key ON engine not running equals OIL light on. If not fix it because it's broke.
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Old 04-14-2008
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I don't have an oil light, just a pressure gauge. And I don't own a car...just a couple of boats...heh.

So the light goes OFF when the pressure's low? Or are you saying that the light blinks on at "ignition" and then goes off in a couple of seconds? Anyway, I don't have it.

Basically, when I switch the key to ON, only the panel indicator lights light. I then hit the glow plug button and then the starter button. I have no "kill cable", either, which I've seen. I simply turn the key, meaning it's just a solenoid like my propane system. If it were to break, I would stuff a rag in the air intake, I suppose, to kill the engine. Then I guess I could hotwire the starter if I couldn't get it fixed quickly.

I've seen the low pressure sensors on Atomic 4s, but a lot of A4 owners buy a four-buck "squealing pig alarm" from electronics supply stores. That's basically what I want to install on my diesel.
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I'd be happy to use a "squealing pig alarm" if I can get it to work. Do you know how they wired it?
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[quote=Valiente;299908]So the light goes OFF when the pressure's low? Or are you saying that the light blinks on at "ignition" and then goes off in a couple of seconds? Anyway, I don't have it.
Basically, when I switch the key to ON, only the panel indicator lights light. I then hit the glow plug button and then the starter button. I have no "kill cable", either, which I've seen. I simply turn the key, meaning it's just a solenoid like my propane system. End of quote.
No.... the light goes ON when the pressure is low - It's 0 psi with the key on and the engine not running. Yes... I have a 2 solenoid activated 'kill switches' on my NC also instead of a manual 'pull cable' as I had on other diesels.
I have the gauge, light, and buzzer that came with my boat in the pilothouse and I added a second Gauge (edit) and buzzer for the cockpit. To further answer your question the oil pressure drops considerably when the engine heats up as I'm sure you notice on your guage. The buzzer I installed for the cockpit is adjustable so I try to adjust it so it doesn't buzz at normal operating temps but when it drops below that certain psi it goes off - trouble is that sweet spot is hard to adjust with the little mini-pot on the guage so many times with a hot engine when I drop down to Idle ..... buzzzzzzzz. I could lower the setting waaaaay down but I want to know about the slightest drop in oil pressure..... so I put up with some false alarms.
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Last edited by christyleigh; 04-14-2008 at 02:43 PM.
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Sapadini.

Did you perhaps wire the alarm in series with the light instead of in parallel with it? If it's in series, the light will work but be a little dim and the alarm will not work. It does this because the current flow is limited by the item that uses the least current. That doesn't leave enough current to operate the buzzer. Wire them in parallel and the circuit will flow enough current for both devices, assuming that you have enough current available from the fuse and the switch will flow enough current for the ground.
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Sapadeni-
I would guess that with the buzzer and bulb in parallel, what happens is that the bulb acts like a very low resistance (near zero ohms) so effectively it absorbs all the power and the buzzer gets none. If the bulb has a resistance of 1/2 ohm, and the buzzer is a maybe 600 ohms...the voltage divides proportionately and the buzzer won't sound.
The simplest most reliable solution will cost you about five bucks more. You disconnect the bulb, and replace it with an inexpensive 12-volt "dual pole" (DPDT or DPST, either will do) relay Wire up the relay coil to where the bulb was. The connect a 12-volt power lead to one side of both the relay's "NO" (normally open) contact pairs. Connect the blub to one of the paris, the buzzer to the other, and then connect them to ground. (If you buy a relay designed for car fog lights, there's usually a diagram included with it.)

The relay "goes on" instead of the originallight,and it isolates the light and buzzer so each one acts without interference from the other. That also will allow you to use a much more powerful buzzer.
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Quote:
Originally Posted by hellosailor View Post
Sapadeni-
I would guess that with the buzzer and bulb in parallel, what happens is that the bulb acts like a very low resistance (near zero ohms) so effectively it absorbs all the power and the buzzer gets none. If the bulb has a resistance of 1/2 ohm, and the buzzer is a maybe 600 ohms...the voltage divides proportionately and the buzzer won't sound.
The simplest most reliable solution will cost you about five bucks more. You disconnect the bulb, and replace it with an inexpensive 12-volt "dual pole" (DPDT or DPST, either will do) relay Wire up the relay coil to where the bulb was. The connect a 12-volt power lead to one side of both the relay's "NO" (normally open) contact pairs. Connect the blub to one of the paris, the buzzer to the other, and then connect them to ground. (If you buy a relay designed for car fog lights, there's usually a diagram included with it.)

The relay "goes on" instead of the originallight,and it isolates the light and buzzer so each one acts without interference from the other. That also will allow you to use a much more powerful buzzer.
While this explantion sounds good, it works a little differently. The current divides proportionally, not the voltage. With a parallel circuit, each leg of the circuit gets a full 12 volts and a full ground. Provided of course that the power supply and the ground are capable. The current on the circuit is increased by whatever the new item draws. In a series circuit, the total current flow is limited to whatever the lowest current flow item is, in this case the light bulb. While the light bulb's resistance is very low when read with an ohm meter, because of the inductive resistance of the filament when operating, the actual current flow is much lower than if it were just a resistor. You can prove this to yourself merely by applying ohms law to the 1/2 ohm light bulb. 12/.5 = 24amps in other words we would have a 288 watt warning light. In reality he likely has about a 3 watt bulb drawing .25 amps. A series circuit will only flow as much current as the LOWEST current user, in this case .25 amps. A quarter of an amp is not going to operate both the bulb and the buzzer, so the light comes on and the .25 amps passes through the buzzer to ground without sounding the buzzer.

I think that a relay would add unneeded complexity to it, unless the buzzer draws more amps than the switch will allow. If it does, he needs a new buzzer.
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