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Old 05-07-2019, 11:26 PM   #1
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Oil Pressure Alarm does not work

I have Ford Lehman 120’s in my 42’ G.B.. When I turn the engine switch to the “on” position prior to start my starboard engine alarm fails to sound. The alarm on the port engine works fine. The problem just recently occurred. Any suggestions on diagnosing the system?

It appears that there are two sensors or switches attached to a tee which is fed from engine galley oil. Both units have zero resistance when the switch is off. When the switch is on the larger “sensor” reads 9 volts while the smaller “sensor” reads 1.5 volts. Thoughts?


Jim Nance
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Old 05-07-2019, 11:36 PM   #2
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Turn on the ignition. Go to the horn with your multimeter and start testing for voltage. Work your way backwards until you find the problem.

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Old 05-08-2019, 07:40 AM   #3
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On that "T" one of the senders is for the gauge. The gauge sender is a variable resistor that varies depending on the oil pressure. Depending on the exact gauge the sender could go from say 10 ohms to as much as 280 ohms. The other will be a switch that is turned "on" by low or no oil pressure. With the engine not running, the alarm switch (which should be the smaller one, but I can't be positive) should read very low volts across it (no more than one or 2 tenths of a volt). If it is the alarm switch that is reading 1.5 volts, it has a problem. If you disconnect at least one of the two wires you can use the ohms function of your meter and it should read no more than 1-2 ohms. Be careful using the "Ohms" function on items which are connected elsewhere. Other items connected can cause false readings.

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Old 05-08-2019, 09:39 AM   #4
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Here is the schematic of how it is supposed to be wired.
Not pretty, but it should help you.
Attached Files
File Type: pdf FordLehmanEngineElectricSchematic.pdf (49.9 KB, 27 views)
Jay Leonard
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