Oil Pressure Alarm

[rirvine]

The oil pressure alarm does not sound when the key it turned on.  My boat is a MKII #1383 with the Universal 35HP motor.  This problem has been intermittent for some time but now has gone hard not working.

A. The red oil pressure alarm light comes on and then goes off when the engine is started.  The means to me the alarm sensing unit is AOK.

B. I replaced the alarm unit with the loudest one Radio Shack had â€" know no oil pressure is bad news.

C. I measured the voltage across the alarm when the motor was not running with the key turned on and found it was less than 4Volts.  The label on the alarm takes more than 4.2 Volts to work.  I would have assumed this would be closer to 12V.

D. Unlike to water temp alarm, which just grounds the negative side of the alarm to activate it, the oil pressure alarm is some how involved with the lift pump and the pre-heat circuit.  Does anybody know:

1. How the circuit works?   I gather it is to shut off the pre-heat and the lift pump once the engine has oil pressure.

2. Where the 10A circuit breaker should in the circuit is located. I can find the main 20A breaker but not the 10A breaker in this circuit.

E. The Oil pressure is AOK when the engine is running â€" yes, I do have an oil pressure gauge.

Questions:

A. Do I need a special alarm unit that work with a lower voltages
B. Could something else in the circuit be broken?

Thank you for your help.
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[rirvine]

After numerous e-mails with Joe Joyce, I understand this problem a lot more but have no solution as yet. The problem revolves around how the "ground" circuit on the alarm buzzer ("P" terminal) works when there is no oil pressure (alarm switch is open).  When the alarm switch is closed, there are 12VDC on both the "+" and "P" terminals of the buzzer and it does not sound.  When the switch is open, there is still 12VDC at the "+" terminal (come from the key switch on my boat) but the "P" terminal attempts to find ground through the lift pump/preheat solenoid circuits.  The problem is there is too much resistance (945 ohms) in this ground circuit (outside of the alarm buzzer), and the voltage across the alarm buzzer never get above 4 volts on my boat â€" hence no alarm.  This all appears to be a little complex and the oil pressure alarm is not fail safe.

I believe that the circuit is this way so that the lift pump stops when the oil alarm switch opens.   The alarm should not sound when the engine has oil pressure or the key switch is in the pre-heat position.  

Has anybody had a similar problem?   If so how did you fixed it?

Thank you.
?

[jentine]

Jentine experienced a similar but opposite problem in 1992.  We had just departed New Bedford when the alarm sounded.  I checked the oil pressure and it was normal.  I returned to the dock and removed the terminal board from behind the ignition panel and threw it overboard.  I have experienced no problems with it since.
A vessel is required to maintain a lookout when underway.  During the performance of his duties is the time to check the engine (if runnning) and its systems from time to time.  An engine running out of oil is noticable to all but a deaf person.  There is a tonal change in the sound of the engine as well as the resultant clatter.
I would rather rely on me than an electrical alarm.

Jim Kane

[rirvine]

Ron, thank you.  GMTA as I arrived at the same conclusion earlier this week â€" replace this mess with something simple and failsafe.  I have the as designed circuit diagrams, have a few design ideas for the wiring, and have all the necessary parts.  The actually implementation will depend on how the circuits are implemented which wires are easiest to get to.

[rirvine]

This puzzle is finally solved.  The Catalina engine control panel has "red" lights across both the water alarm and the oil alarm buzzers.  With both of these connected, the lights will come on but the alarm will not sound.  With the one light disconnected (does not matter which one) the alarm would sound.   The solution is Crews Nest now has two separate buzzers â€" one for low oil pressure and one for high water temperature.  I come away from the probelm with a great deal of knowing about this all works.