# Low Oil Pressure buzzer/alarm



## TSOJOURNER (Dec 16, 1999)

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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## Valiente (Jun 16, 2006)

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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## TSOJOURNER (Dec 16, 1999)

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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## christyleigh (Dec 17, 2001)

Valiente said:


> 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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## Valiente (Jun 16, 2006)

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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## TSOJOURNER (Dec 16, 1999)

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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## christyleigh (Dec 17, 2001)

Valiente said:


> 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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## US27inKS (Feb 6, 2005)

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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## hellosailor (Apr 11, 2006)

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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## US27inKS (Feb 6, 2005)

hellosailor said:


> 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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## hellosailor (Apr 11, 2006)

Dunno, John. If it is just a "simple" circuit, there's an oil pressure switch. But if it is hooked up with a pressure gauge and a sender, that's a variable resistor. Given that and the two unknowns (the bulb and buzzer)...the relay allows it to be treated with blissful ignorance.

In either case--if the bulb has near-zero resistance, as most panel bulbs do, it will act exactly the same way as a plain wire, effectively shorting out the terminals of the buzzer that is in parallel with it. That dead short ensures that the buzzer will never work (until the bulb burns out) because there is near-zero current available to it, regardless of the voltage. No?


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## US27inKS (Feb 6, 2005)

hellosailor said:


> Dunno, John. If it is just a "simple" circuit, there's an oil pressure switch. But if it is hooked up with a pressure gauge and a sender, that's a variable resistor. Given that and the two unknowns (the bulb and buzzer)...the relay allows it to be treated with blissful ignorance.
> 
> In either case--if the bulb has near-zero resistance, as most panel bulbs do, it will act exactly the same way as a plain wire, effectively shorting out the terminals of the buzzer that is in parallel with it. That dead short ensures that the buzzer will never work (until the bulb burns out) because there is near-zero current available to it, regardless of the voltage. No?


He has a light, which means he doesn't have (better not have) a variable resistor type of sender. With oil pressure it should be open (infinite reistance) and without pressure it should be a dead short to ground.

Like I said before, if the light bulb remains at or near zero resistance and acted like a plain wire, the fuse would blow. At .5 ohms and 12 volts you'd have a 24 amp 288 watt warning light.

The bulb only has near zero resistance when unlit. The whole inductive resistance thing is difficult to explain, and even harder to grasp. The concept is the same as with an electromagnet. If you take plain wire with a fuse in line, and touch one end to power and the other to ground the fuse will blow. Take the same wire and wind it tightly into a coil, and you get an electromagnet and the fuse doesn't blow. That's because in a wound wire you get a certain amount of inductive resistance when current flows. An ohm meter doesn't supply enough current to cause this, so you read whatever resistance the wire has if it were straight.

When dealing with inductors and light bulbs, you have to look at the power (watts) output. In this case we would have about 3 watts. If the light bulb is wired in series, the total amount of power available for the whole circuit is 3 watts. Enough for the bulb, but nothing else. A friend of mine showed me a great example of this many years ago. He was a part time TV repair man. When working on a TV that pops the fuse in the power supply, he would plug the TV into a special outlet that he made with a light bulb socket in series. He could limit the total power consumption of the TV with the wattage of bulb he put in the socket. A 100 watt bulb would power a 25 inch TV enough to get a picture about half the normal size right in the middle of the screen. It didn't have enough power to stretch the picture all the way to the edge. Same thing in here, except the wattage available is so low, the buzzer does nothing.


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## Rockter (Sep 11, 2006)

At the risk of sounding like the Old Man of Loch Ness.... been there done that.
You need a relay there my friend.
The internal resistance of the switch and bulb is limiting the current of the buzzer so that it is choked off when the buzzer load is needed. You simply have too much in-line resistance and there is not enough current in series to sound the buzzer.

This circuit works for me...

Image of oil pressure switch alarm with relay - Photobucket - Video and Image Hosting

... and has done for 15 years.

The oil pressure switch is from an old Mazda, the relay is from an old BMW, and the horn from an old Austin.

When the oil pressure is low (or zero), the oil pressure switch completes the circuit to engine ground. There is an audible click from the relay (if you have yor ear to it), then you are deafened as the relay calls in the horn.

Crucially, the circuit from ignition switch to horn is designed to take the full load of the horn.... the other wee circuit cannot, so it is used as a trigger only.... it only has to carry a wee current to close the relay, and it is capable of that.

Total cost, about $5, in 1992.

Rockter.


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## Rockter (Sep 11, 2006)

Oh, and I added a wee switch between oil pressure switch and relay to kill the alarm if I have my ears in the engine room and I have to shut off the motor at the motor kill lever.
If I did not, I would get deafened every time I shut down the motor at the engine, or I would have to run up on deck quickly to shut off the ignition.
It is not absolutely necessary, but it is desirable.


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## Valiente (Jun 16, 2006)

sapadeni said:


> I'd be happy to use a "squealing pig alarm" if I can get it to work. Do you know how they wired it?


I believe it was in line with the ignition key, so it would go "eeeeEEEKKK" at start for a second or two and then stop. If it went on again, you were below 10 PSI or something along those lines.

Check out entry #4 in this thread. The poster is a friend of mine and one clever fellow:

Checklist for a troublefree spring startup - Moyer Marine Atomic 4 Community


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## US27inKS (Feb 6, 2005)

Wow. I would not have thought of using a car horn for a buzzer. In that case, by all means use a relay.


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## US27inKS (Feb 6, 2005)

Why would you use a car horn, when you can get one of these instead? McMaster-Carr Put part no. 56965T13 in the search box on the left. I couldn't get the link to work.

At only $7.06 and it draws 0.015 amps, any oil switch in the world will operate it with no relay required. A 90db tone should be able to be heard easily even with the engine running. If that's not loud enough you can move up to a backup horn and get to 107db, but the price is high.

Unlike a car horn, you can easily mount one of these to the back of a panel, or could easily be hidden anywhere you like. At such low cost, low amperage and small size, you could mount a couple of them, one inside and one outside.


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## Rockter (Sep 11, 2006)

US..
Having suffered the chagrin of twice pumping the sump dry, and that wee light, unseen glowing in the daylight, I was trying to make sure that I would hear the next one.
It was raining, I was in a wee villiage called Ardrishaig in West Scotland, and there was a scrapyard there.
If you are content with the 0.015 amp buzzer, then go for it.
The good point about the relay is that it does not have a current limitation on it, this side of sanity, anyway..... you can call in horn and light if you wish. I added a pressure guage also, below decks.
There is no way that you cannot hear that horn.


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## TSOJOURNER (Dec 16, 1999)

Wow, I'll work on digesting the information you guys provided. As I see it, I'm looking at several options:

1. Try reversing the polarity of the buzzer in parallel with the panel light. (I originally wired it with the buzzer positive to the low oil switch, neg to the black leading to the bulb).

2. Add a relay in place of the present light and tap the light and buzzer off the relay.

3. Run the pos of the buzzer to the ignition "on" wire, then the neg of the buzzer to the lead running from the switch.

Did I miss anything? I'll try these options out this weekend. Thanks a million for taking the time to help me out. It's definitely been informative. I'll let you know how I make out.

Scott
S/V Inspiration


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## US27inKS (Feb 6, 2005)

Rockter said:


> US..
> Having suffered the chagrin of twice pumping the sump dry, and that wee light, unseen glowing in the daylight, I was trying to make sure that I would hear the next one.
> It was raining, I was in a wee villiage called Ardrishaig in West Scotland, and there was a scrapyard there.
> If you are content with the 0.015 amp buzzer, then go for it.
> ...


I actually have one in my auto shop attached to my alarm system. It works as a door buzzer so that I know when someone comes in the front door. I can hear it 80 feet across my shop with the air compressor and air tools running. I would trust it.

BTW this buzzer puts out 90db, the same as a car horn.


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## US27inKS (Feb 6, 2005)

sapadeni said:


> 1. Try reversing the polarity of the buzzer in parallel with the panel light. (I originally wired it with the buzzer positive to the low oil switch, neg to the black leading to the bulb).


This is the problem right here. When you attached the neg lead for the buzzer to the black (I'm assuming neg lead) of the light, you did the same thing as hooking it directly to the oil sending switch. OK so far. When you hooked the positive up to the oil sending switch, it has the same effect as if you hooked both the positive and negative wires to the neg post on the light. When the switch grounds, the entire wire from the switch to the bulb becomes ground. You had both wires attached to ground. Put the pos lead of the buzzer to ignition power, and you should be good.


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## hellosailor (Apr 11, 2006)

Scott, if your buzzer is a real buzzer (basically a relay) they are not polarized and will run regardless of which wire goes to which. If it is a piezo "buzzer", etc. it will be polarized.

If you do use something like a car horn, they can draw a lot of power and normally are driven by horn relays in cars. You would need to check the power draw and perhaps use a common Bosch 30A-rated relay to make sure the relay points don't get burned from the load, a typical cheap relay from Radio Shack might only be rated for 2 amps. Enough for a piezo alarm but not a car horn.


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## TSOJOURNER (Dec 16, 1999)

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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## Quickstep192 (Jan 6, 2001)

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?


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## hellosailor (Apr 11, 2006)

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])


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## US27inKS (Feb 6, 2005)

Quickstep192 said:


> 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.


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## Rockter (Sep 11, 2006)

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.


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## Valiente (Jun 16, 2006)

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.


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## Quickstep192 (Jan 6, 2001)

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.


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## Valiente (Jun 16, 2006)

Before we go cruising, loud low-oil pressure and coolant temp alarms will be in full effect. It's cheap insurance.


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## TSOJOURNER (Dec 16, 1999)

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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## hjkane (Sep 26, 2011)

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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