Low Oil Pressure buzzer/alarm - Page 4 - SailNet Community
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post #31 of 32 Old 04-21-2008 Thread Starter
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Well, your advice did the trick. I wired the pos from the buzzer to the ignition power out, and the buzzer neg to the oil switch lead, and Voila, I now have a low oil alarm. Thanks Guys!!!!
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post #32 of 32 Old 02-15-2014
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Re: Low Oil Pressure buzzer/alarm

The real problem here is the manufacturers went cheap and instead of using a relay for this they use the oil pressure switch (which applies 12V to the lift pump) to actually turn the alarms off when there is oil pressure. This oil pressure switch is truly a switch, On or Off and not a variable sensor. (For boats with an oil pressure gauge, there is a separate variable sender). The reason the alarms (light and/or buzzer) are supposed to turn on is they are wired across this switch. When the switch is open (no oil pressure) the alarms have 12V on one side and are tied to the top of the lift pump coil on their other side. The lift pump has low enough resistance to provide a current path to ground so these low current alarms go ON (or are supposed to), and when the oil pressure switch closes, it effectively shorts across the alarms turning them off, and applies 12V directly to activate the lift pump. The pump doesn't run without the oil pressure switch closed (aside from it's alternate path through the glow switch) because it takes more current than the alarms will pass through them. The problem is, since this is a pseudo ground, if there is the slightest increase in resistance in the line from the alarms to the lift pump and thus to ground due to corrosion, or loose connection, the alarms will not light or sound or will do so feebly. This is what we see most often. The best fix is to put a 12V relay with normally closed contacts powering the alarms and the relay activated by the oil pressure switch 12V. This will provide clear audible alarm and bright oil pressure lamp, will turn off when the oil pressure switch closes, and is actually fail safe since a relay failure will turn the alarms ON. Depending on the low current path through the lift pump is not reliable and will fail with even the slightest increase in resistance in the ground path. IMHO
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