High Engine Temp Buzzer

[wcconway]

We got caught in a severe unexpected squall, in a very narrow channel that is heavy with barge traffic. Out of nowhere the high engine temp buzzer starting screaming, but the temperature gauge never showed an increase in temperature. We got out of the channel, dropped the anchor and turned off the engine. We waited about 15 minutes, cranked the engine and the alarm was no longer screaming and the temp was fine. Motored about 2 miles, got caught in another squall and the buzzer started screaming again, but with no increase in temp we finally came to the conclusion it must have been a short from the blowing rain. We motored about 8 miles with no temp increase and duct tape over the buzzer.

Anybody else had this issue?

[R.B.]

I don't think the buzzer is just for high temp warnings. We just finished a 2 week cruise and the buzzing could be stopped by changing the RPM. I did have the buzzer start up on me and it made noise for awhile and then my fuel pump died. The next day the fuel pump was working again but I did replace the pump. Now when the buzzer starts up, I just change the RPM and it seems happy again.

[djones]

I've been all over this circuit while doing the restoration of our project boat, hull #13.

Basically, the key to the alarm circuit is the oil pressure switch. Instead of the usual "one terminal to ground" style of pressure switch, this one has two terminals that short together when there is sufficient oil pressure.

Before the engine starts, the power to the fuel pump is temporarily provided by holding the ignition key switch in the "start" position. This does two things, it turns on the glow plugs, and it also powers the electric fuel pump. While holding the key switch in the "start" position, you then push the real "start" button to engage get the engine moving.

When the engine starts, you let go of the ignition key switch (and also the button), and the temporary power to the fuel pump is removed. If the engine has really started, an alternate source of power to the fuel pump is proved THROUGH the 2 contacts of the oil pressure switch (since there is now sufficient oil pressure with the engine running that will close that pressure switch). This is a protection "feature". If you run out of oil, the oil pressure switch should open, and the power to the fuel pump will be removed, starving the engine of fuel and stopping it before things sieze up and get expensive to repair.

There is also a small resistor in the circuit for the audible alarm that has a nasty habit of breaking in two. This can cause some false sounds from the alarm buzzer. It's hard to find because it looks just like a wire with heat shrink on it near the glow plug solenoid.

Just giving additional info!

Thanks
Don Jones "Onatop" C350 #13
Oak Island / Southport, NC