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Go Back   SailNet Community > On Board > Gear & Maintenance > Low Oil Pressure buzzer/alarm
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Thread: Low Oil Pressure buzzer/alarm Reply to Thread
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Topic Review (Newest First)
02-15-2014 02:56 AM
hjkane
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
04-21-2008 09:57 AM
sapadeni 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!!!!
04-16-2008 01:30 PM
Valiente Before we go cruising, loud low-oil pressure and coolant temp alarms will be in full effect. It's cheap insurance.
04-16-2008 10:30 AM
Quickstep192 In my case, there's no light installed now. I do have a temp gauge, but I don't stare at it all the time. I am also trying to avoid the circumstance where the belt breaks and I'm overheating without realizing it.
04-15-2008 09:58 PM
Valiente I had a alternator belt break last year running between the dock and the pump out. The same belt runs the FWC pump in the block, so I basically had cold lake water going through the heat exchanger, but very little heat exchanging going on. This was basically "at idle" for 20 minutes or so, as I didn't switch off while decrapping the holding tank.

The coolant got to 240 F before I noticed it. No harm done, apparently (except to my nerves), but for the funny smell and baking heat when I opened the deck hatch! The oil pressure stayed at its customary 55 PSI the whole time.

Now I carry two spare belts, including a Gates Green Line, supposedly the best in show.
04-15-2008 03:49 PM
Rockter I have not tried that one. I just look at the ammeter every now and again.

How often does a belt break? It is not a terminal failure, unlike oil pressure failure.
04-15-2008 03:49 PM
US27inKS
Quote:
Originally Posted by Quickstep192 View Post
I have a related question. I'm looking to add an audible alarm for my alternator so I'm alerted if the belt breaks. There is a lug on the alternator for an idiot light. The lug puts out +12v during normal operation and goes to ground if the alternator quits. Can I simply put a buzzer between that lug and a +12v source to create an alarm?
Yes you can. In fact that's exactly how the warning light works. You have to be a bit careful though, as many alternators need that 12 volts fed through the bulb to turn the alternator on. You may need to leave the bulb in place and add an alarm. Then make sure the alternator is still working properly with a meter.

edit: I just read hello's post and he's got a great point. Make sure you use a 12 volt source that is only on with the key on and you should be in good shape.

edit again: You really should use a relay if you use a buzzer that pulls any kind of amps. That's why I like those pezio alarms that only pull 0.015 amps.
04-15-2008 03:44 PM
hellosailor Quickstep-
In theory you could. In practice, you would be creating a path between two different "12" volt sources, one being perhaps a real 12V at the idiot connection, the other being 12-14.4V at the battery positive. that can set up a voltage loop which wastes power and screws up other things, like the brains in the regulator. Whether it works well or not will depend on the details of your setup.
So, you could try setting it up that way, and using a voltmeter and ammeter to check and see if it is behaving nicely. Or, use the idiot connection to power a relay. Idiot on, relay on, and if you use a "double throw" relay there will be a set of contacts that are OFF when the relay is on. When the relay turns off (because the alternator is off) another set of contacts comes ON, so the buzzer only comes ON when the alternator and relay are OFF.
(And of course that relay takes no power unless the alternator is ON.)

Your choice, spend $5 on a relay that isolates things and be sure are isolated, or take the multimeter to see the details. Or, just take a chance that the simple thing works. (I'm often not that lucky.[g])
04-15-2008 02:47 PM
Quickstep192 I have a related question. I'm looking to add an audible alarm for my alternator so I'm alerted if the belt breaks. There is a lug on the alternator for an idiot light. The lug puts out +12v during normal operation and goes to ground if the alternator quits. Can I simply put a buzzer between that lug and a +12v source to create an alarm?
04-15-2008 01:41 PM
sapadeni I'll try the "pos to ignition power" technique first since it appears to be the easier route. I'll try the relay if it still doesn't work. I don't have the buzzer with me so I'm not sure if it's a piezio. I do know that it's a pulsating buzzer (similar to a truck backup warning), and it does have a red and black lead.
Good thing I'm a much better mechanic than electrician.
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