Overheating Yanmar?

[Zephyrus]

I have a 1984 Hunter 31 with the original Yanmar 2GMF fresh-water cooled 13 HP motor. When I motor at 2500 rpm or lower the temperature is fine. When I approach 3000, the audible alarm at the cockpip panel begins to emit a faint alarm. As I continue to motor (in gear, under way) at 3000, the alarm gradually gets louder. When I back down to 2500 rpm or lower, the audiable alarm gadually fades away. Is fading in and out the normal function of this alarm? I assume it is, so I've taken some steps to stop the overheating. I've checked the raw water strainer and pump and they are ok. I've replaced the thermostat and cleaned the raw water side of the heat exchanger. Still no improvement. I'm next going to perform a flush on the fresh water system. If this does not do it then I'm fresh out of ideas. Has anyone had this experience that can steer me in the right direction? Any suggestions would be greatly appreciated. I'm about to make a 25 nm trip to her winter home with most of the trip being under power.
Thanks, Ted in NJ

[cal40john]

The alarm is connected to both the coolant temperature and oil pressure senders. These are both just switches, so the alarm should either be off or on, no in between.

Have you looked to see which warning light is on while the alarm is sounding?

Does the alarm sound loud and strong when you turn the key to on with the engine off?
Do the charge and oil pressure lights come on with the key on and the engine off?

If you have the tachometer version of the panel, the switch that turns on the instruments light when pushed to one side, tests the coolant indicator light when pushed in the other direction.

Could be a defective oil or temp sender. They should read either as a short or open. Oil is closed (short) no pressure, coolant is open cold.

Could also be a poor connection to or from the alarm, such that you do have an engine problem, but poor alarming.



John

[noelex 77]

Dear Ted
I am no expert, but it sounds to me more like an electrical (wiring or sender) issue than a true overheating problem.
Try turning off the ignition disconnect the wire from the sender (It will be screwed into the block often near the thermostat about 2cm diameter and copper coloured with one wire coming out of it) and measure with a multimeter on the Ohms scale from the sender terminal to ground. It should read infinite Ohms when the engine is cold and close to 0 ohms when the engine is overheated (about 95 deg c).
The oil warning light will read the opposite 0 ohms when shut down and infinite when the engine is running.
Some multimeter's have a temp measuring thermocouple (or use an infrared thermometer) try this on the sender (ideally measure the temp before you have a problem ) with the engine running it should be about 85 deg C.
Hope this helps John

[never monday]

whats the max RPM you can get in gear underway?

[Zephyrus]

Thanks, guys. It never goes into full-volume alarm and the light never lights. I'm going to check the things that you recommended. I do have an analog thermometer strapped on the closed loop hose that's between the thermostat and the heat exchanger and It's never read more than 160F (71C). A faulty sender would be a heck of a lot better than a cronic overheating problem! I'll keep you posted.
Ted in NJ