What's up with my oil pressure alarm??

[aliddell]

I have a 93 Pacific Seacraft with a 3JH2E Yanmar. The oil pressure alarm goes off and light is on up to about 2000 or 2200 rpm's. There appears to be one wire from the sending unit to an oil pressure gauge which reads normal: 30 at idle and 60 at 1500 and above. The wire to the light and alarm on the panel must come off the pressure gauge. Are the alarm and light both bad? Thanks in advance for any insight.

[nigelmercier]

Sender unit may have two outputs, plus (perhaps) a ground. One output gives varying signal for gauge, the other is on/off for lamp.

[aliddell]

Only one wire coming off the end of the sending unit.

[nigelmercier]

Look for another sender.

[PuttingDoctor]

Take a look at the sender itself. When I had an alarm on my 4 cly. Yanmar we found it mounted on an extension that was beating some air. Senders are cheap enough to replace and carry a spare.

[barnakiel]

If gauge present there will be another wire somewhere. Find it 'backwards' from the gauge (block/instr. manual) to the block.

Check the lamp/buzzer wire if it is sitting tight on the sender and in the block.

Remove the potentially faulty sender, clean it - there may be a small particle in the hole or other dirt present. Retest after cleaning and making sure the connections are tight and clean.

If the problem persists, while the gauge shows good reading, replace the lamp/buzzer sender.

Over last 10 years or so I have seen oil senders go kaput on ours and other boats,

b.

[aliddell]

I did find what I think is a switch mounted on the stem of the sending unit, between it and the block. This is probably the problem but it looks like it would be almost impossible to get to it.

Thanks all, Andy

[a64pilot]

Originally Posted by aliddell

I did find what I think is a switch mounted on the stem of the sending unit, between it and the block. This is probably the problem but it looks like it would be almost impossible to get to it.

Thanks all, Andy

99% of the time, switches operate lights and buzzers, sending units operate gauges.
You can confirm that is the LOP (low oil pressure) switch by disconnecting the wire, majority of the time its the ground that is switched and with it disconnected when you turn the key on, you will get no light or buzzer.

[BentP]

Did you check if the oil pressure actually is to low?? Check oil level, filter etc.? Sometimes, the alarm is correct....

[Sinbad51]

Not sure of your setup, but I had an older yanmar that had a sending unit for pressure gauge and a separate sending unit fpr the alarm and idiot light.

[Captain Bill]

The oil pressure sending unit is separate from the idiot light sender. The standard placement for the idiot light sensor is towards the rear of the engine on the left side when facing the front of the engine at about the same level as the oil filter. The standard location for the pressure sensor is on the oil filter mount. I have seen after market additions of a pressure sensor mounted on a tee with the idiot light sensor.

[agb512]

Anybody know how to tell the difference between a switch and sending unit with a multimeter? I assume the sending unit would read 0 with engine off and the switch would have some resistance?

[barnakiel]

On off sensors (alarms) are contact. Impedance zero or infiniti

Gauge sensors are ohms in any position. Look up the manufacturer to tell how many ohms in which position. I think most read hundreds.

b.

[exMaggieDrum]

Oil pressure (OP) switches come in different PSI ratings. I.e. they are rated to switch at lower or higher pressure. If the last person to replace the switch had put in one too high for your engine it might take higher RPMs to switch.

Oil pressure alarm switches usually are the normally closed (NC) type so there is a closed circuit that turns the alarm light/buzzer on when you turn the ignition switch on. The NC switch opens and breaks the circuit when the oil pressure gets high enough so the buzzer/light goes off. That is why it takes some engines a second or two to stop the buzzer. (Water temp and oil temp alarm switches in contrast are usually NO - they complete a circuit when the temperature gets too high.) As noted above, oil pressure gauges and water temp gauges are not switches but variable resistors that change with the rise and fall of the temps.

The circuit for OP switches is usually to ground. I.e. the wire coming from the panel and ignition is the positive side to the light/alarm and the other wire from that light/alarm is the negative side and goes to the OP switch on the engine. The OP switch is usually grounded to the engine via the pipe and the engine is grounded/negative back to the battery.

However, some switches may not ground to the engine because they are at the end of a flexible hose instead of a pipe or whatever. The switch would have to have two contacts then. One wire on the switch goes to the light/alarm and one goes to a good ground/negative so that there is a full circuit. You can buy both. You can even have a dual contact OP switch on there when you don't have to if it is already grounded to the motor. You have to run a separate ground then with the other contact.

Switches can also be on the positive side but OP switches are not usually.

Testing an OP switch is simple with a ohmmeter or multimeter as noted by another poster. You set the meter to ohms in the lowest range. Put one lead on to a good ground or an unpainted part of the engine. Put the other lead on the switch connector. With the motor not running you should get a buzz and either zero or some very low ohms reading. Anything but 0 or very low means the switch is closed and not working as a NO switch. Start the motor and the switch should be open when there is oil pressure and you should get no reading/buzz which is proper for an open circuit. If the switch is rated too high because the last replacement was wrong, it would be closed until the pressure got to its rated PSI and then it would open.

Oil pressure switches for this type of application are usually pretty low, e.g. 5 PSI or so. You can't tell by looking at it which it is although it might be marked on it. It is common for the oil pressure gauge sender to be teed with the oil pressure switch. Mine is. The oil gauge will tell you the exact pressure but the oil switch is to alert you to a catastrophic low oil pressure as a last resort.

If your switch is too hard to get to, you can put in a pipe or pipe/elbow to get it out to make it easier to get to in the future. You would still have to get in there to do that though in the first place. Mine was a son-of-a-gun to work on so I did that.

Hope this helps. BTW - engine circuits like this often have color-coded wires that are the same for many engines. Ironically, there isn't a specific color for the oil pressure switch but there is for the oil pressure sender to the gauge (light blue). But note that not sometimes the colors get changed or not followed. Here is one chart that you might save and have on your boat to use for troubleshooting other things, provided your boat uses this scheme:

http://www.defender.com/pdf/abyc-wire-color-chart.pdf

[agb512]

Great answers, thank you for the detailed explanation. I had the connection from my brain to my hand wardbacks.