3YM30 coolant buzzer going off, but engine not overheating

[Wotname]

Originally Posted by jbinbi

Thanks to all for their input. Not sure how the alarm is supposed to sound. It is not a continuous alarm, but rather an intermittent chirp. Every few seconds , but sometimes goes away for a few seconds (I think). I bring this up because I don't know if this is indicative of something wrong in the sensor (if it supposed to be continuous if the engine has overheated), or the way it supposed to work.

Also, the 2 sensor issue now has me more worried about the engine. Would it be true that running the engine for an hour at 120 C would seize the engine?

It is supposed to sound continuously (and loud).

The overheat alarm sensor is just a switch whose contacts are made when it overtemps. It uses the same "buzzer" as the low oil pressure alarm so it sounds exactly the same as the low oil alarm. Presumably your low oil pressure alarms every time as you start the engine.

As an aside, do you check the temperature alarm as a maintenance item. Many don't but one should as it never gets checked in normal use unlike the low oil pressure alarm. The wiring can be checked by simply pulling off the terminal at the sensor and shorting it to the engine block. The alarm should sound when terminal is shorted to the block. The only way to check the actual sensor is to remove it and heat it to its rated temperature and check the switch function with an ohmmeter.

[Wotname]

Originally Posted by Wotname

I reckon you would know if it had seized
As for damage, I can't say for sure as I have never tried it. 120C is awful hot for these engines but what does it normally run at. If it ran at say 100 to 105, then 120 isn't that much higher but if it normally runs at say 80 to 90, then 120 is high. Again I don't know what the pressure cap is rated at (in PSI etc) but that would also be a clue as to how much heat the engine can take. Higher PSI, higher temperature.

EDIT: If the coolant level in in header tank is OK (i.e. looking under the cap), then no boiling has occurred. This would then suggest no real damage has occurred as most damage occurs after coolant boils.

Further to this, I just checked the service manual which states the coolant alarm sensor is set at 93 to 97 C and engine thermostat is fully open at 85 C.

So 120 C is way high.

The boiling point of the Yanmar recommend coolants seems to be around 130 C with a pressure cap of 1 Bar so it is unlikely your coolant reached it's boiling point.

[boatbod]

Yanmar YM series engines can be used with conventional shaft or sail drive. In the case of a sail drive there is a water sensor to detect bellows failure. The same wiring harness is used on all YM engines; mostly this isn't a problem until moisture/corrosion in the connectors causes the water sensor alarm to activate intermittently. Symptoms would be exactly as you describe.

[Wotname]

Originally Posted by boatbod

Yanmar YM series engines can be used with conventional shaft or sail drive. In the case of a sail drive there is a water sensor to detect bellows failure. The same wiring harness is used on all YM engines; mostly this isn't a problem until moisture/corrosion in the connectors causes the water sensor alarm to activate intermittently. Symptoms would be exactly as you describe.

Good points but presumably issues with the sail drive wiring terminals would not cause water temperature gauge to read high.

I am speculating the water temperature gauge is an after market addition in the OP's situation (and supplied by Lagoon).

And of course the OP has the luxury of two engines so he/she can transpose parts between them for fault analysis

[boatbod]

Originally Posted by Wotname

Good points but presumably issues with the sail drive wiring terminals would not cause water temperature gauge to read high.

I am speculating the water temperature gauge is an after market addition in the OP's situation (and supplied by Lagoon).

Probably not, but the gauge is resistance based so a harness or ground problem could theoretically affect it. That said, if the gauge is high and the alarm is chirping, it just might be trying to tell you something...

[jbinbi]

Originally Posted by Wotname

It is supposed to sound continuously (and loud).

The overheat alarm sensor is just a switch whose contacts are made when it overtemps. It uses the same "buzzer" as the low oil pressure alarm so it sounds exactly the same as the low oil alarm. Presumably your low oil pressure alarms every time as you start the engine.

As an aside, do you check the temperature alarm as a maintenance item. Many don't but one should as it never gets checked in normal use unlike the low oil pressure alarm. The wiring can be checked by simply pulling off the terminal at the sensor and shorting it to the engine block. The alarm should sound when terminal is shorted to the block. The only way to check the actual sensor is to remove it and heat it to its rated temperature and check the switch function with an ohmmeter.

Here is an update. Checked the temp today 2x with a laser heat gun. This was after playing around in the engine room. I noticed that the wire on the temp switch can come loose easily, it is a 'sleeve' like connector going over essentially a terminal pin. I was able to slide it up the terminal easily, but not off. So I pushed it back down hard.

Started both engines. Cycled through temp on display. Both read 120C! So my guage does NOT send back temp to the display!

Then I ran both engines in neutral at 2500 rpm for 30 minutes. Checked the temp on them. On engine I thought was overheating, the temp at the thermo was 145F, on the top of the engine where the coolant goes in was 135, and coming out of the exchanger leading to the mixing elbow it was 82F. Both engines about the same.

Then went out for a sail and coming in motored heavily, ran the engine at at about 2900 rpm for 45 min. Temp was marginally higher in all spots, Say 160 at thermostat, and 90 coming out of exchanger. (see block diagram).

Wotname, you are correct, according the schematic, all alarms go to the same buzzer (see diagram), so not sure how it could 'chirp' unless it was a bad connection going in and out, there is a diode there and maybe connection was not continuous. Seems my engine is OK.

[boatbod]

The alarm will chirp if there is a partial short to ground. I.e. Corroded harness wires or moisture bridging a contract.