Unknown source of alarm tone

[Bob Endeavor]

1996 Baja 240 with Mercruiser 7.4 carbureted. Last time we were out an alarm sounded and I can find no reason. Engine was cold, idled away from shore, accelerated to plane and alarm started. Gauges when this occurred, Oil pressure is in limit (approx 40), Temp showed high close to 200 but went down to 180 or so and alarm still sounding, Tach showing 3,000 or more RPM, Trim gauge does not work, Volt shows low at or below 11. Shut down for few minutes restarted and still loud alarm. Checked water flow as there was debris in water seemed OK. Brought boat home and started in driveway let idle for a while and no alarm. The volt meter does no longer works, have to knock on it to get it to move and then it onlyreads about 10V. I don't see how it has an out of limit sender. I previously had a problem the you helped with THANKS AGAIN! http://forums.iboats.com/forum/boat-...-safety-switch. Any ideas where to start looking? I will replace the impeller and thermostat as I'm concerned about the temp getting too close to 200 degrees.
Thanks in advance for your ideas.

 

[Silvertip]

Voltmeters do not have a sender - they measure battery/charging voltage and all it needs is a positive and negative connection (and an internal light connection). You need to either replace the voltmeter or use a hand-held voltmeter to measure battery voltage with the engine off. Then start the engine and rev to about 1200 rpm. Measure the voltage again. Test #1 should show 12.6 volts (12.4 - 12.6 is ok). Test #2 should show 13.5 - 14.5. If voltage is the same as test #1 the charging system is not working. I'm not familiar with the alarms on your specific boat but perhaps there is a low voltage alarm. Looks like you have some issues that need fixing.

 

[alldodge]

1996 Baja 240 with Mercruiser 7.4 carbureted. Last time we were out an alarm sounded and I can find no reason.

Three things can cause a solid alarm, low oil pressure, engine overheat and low drive lube. Have you checked your drive lube bottle for lube?

The 200 degrees is not good, check your raw water pump impeller.

 

[Bob Endeavor]

Thanks for your replies. I will check both and report back.

 

[HT32BSX115]

Temp showed high close to 200 but went down to 180 or so and alarm still sounding Any ideas where to start looking? I will replace the impeller and thermostat as I'm concerned about the temp getting too close to 200 degrees.
Thanks in advance for your ideas.

Howdy,


That alarm system on your 7.4 engine is a rather simple circuit. 2 wires go to your alarm buzzer. 12v DC power and a ground connection. The "Ground" connection is actually the "signal wire" connected to oil, temp, and drive lube sensors.

Any time the key is on, 12vDC is provided to the buzzer.......... Since the oil pressure is zero (before starting) the buzzer should sound. (testing the buzzer every time)

"Sensors" back in the engine compartment include an oil pressure, temp and drive lube monitor switches.

Those switches are connected to ground and the "sensor" wire such that when any ONE (or more) of them closes, it completes the circuit to ground to sound the alarm.


Once the engine produces sufficient oil pressure to open the switch, the buzzer will stop (since the temp is cool and presumably, the drive lube monitor is full)

Understand that ANY "completion" of the circuit to ground will activate the buzzer.

You can check this by getting a wire and holding one side to any ground point (block etc) and touching the other end to the temp, oil pressure or drive lube monitor switch. (the drive lube monitor switch connections are not usually exposed but the oil pressure and temp usually are)

Also if the sensor wire that goes from the engine to the instrument panel (buzzer) is frayed or exposed and touches any ground point, it will also activate the buzzer.

To isolate switches, with the alarm sounding, disconnect each one, one at a time. when you disconnect the one that is closed, the alarm will stop. If all three are disconnected and the alarm still sounds, the wire is frayed and is touching a ground some place between the engine and the buzzer.

If you're really reaching 200 degrees (open or closed cooling?) It's possible that the engine is REALLY getting too hot.

Change your raw water impeller and if it's disintegrated, you'll need to remove and clear all the debris out of your oil cooler because it's likely plugged.

AND I would check the oil cooler anyway for debris from previous failures. Mine was COMPLETELY plugged with impeller fragments from multiple raw water impeller failures from a previous owner.

Regards,


Rick

 

[Bob Endeavor]

THANK YOU, THANK YOU , THANK YOU I believe the problem was low drive lube. It looked to me to be full to the line but I did add more lube to fill the reservoir. I also changed the engine mounted impeller (what a chore!) which was in near new condition and a new thermostat. No alarm tone and the boat ran nice and cool, never over 160. I'll change the other impeller before we go out again.
Really appreciate all the responses and the time and energy you put into helping on this forum. THANKS AGAIN!