MD5C cooling problem

Hello, I have a Volvo Penta MD5C with a cooling issue: when running the engine for a while at a moderate speed, the temperature buzzer goes off.

The amount of cooling water coming out of the engine seems to be normal. The water is warm, not hot. The impeller looks fine as well.
I removed the thermostat from its housing and checked it with hot water. It opens to about 1 cm (almost 1/2 inch) at 75 degrees celsius (167 F), so that seems to be OK.
Before the temperature buzzer goes off for real, it starts making a high squeaky sound and the warning light starts flashing, exactly at the rhythm of the engine.

Based on these three observations, I'm suspecting there is a problem with the temperature sensor. Maybe the sensor is malfunctioning, or it's something with the electrical connections to the sensor.

My main question is: how can I test the sensor? The sensor only has a single wire going to it, so I'm guessing it's probably an NTC or PTC. What should be the resistance when the engine is cold, and what is the minimum/maximum resistance that signals overheating?

Thanks for your help!

Hi Sander,
I have the MD5A (with 110s saildrive), and am having a similar issue with overheating.
Engine used to run 65-70 celcius last season (as it should on raw water cooling). I have the service manual and says thermostat starts opening at 65 and it fully open at 90. Mine is about 2mm open at around 65 degrees, and 6mm at I think about 80degrees.
A while back I fitted a temp sensor so I could use a proper temp gauge, rather than just the warning light.
I probably still have the ON/OFF temp probe somewhere, so will look at measuring if I find it and let you know (else see if the manual has the spec). I imagine that as they have one wire they go from very high resistance (kilo ohms +) cold, to very low (10s of ohms or less) too hot, over a very short temperature range (a couple of degrees). Naturally, grounded through the block. Have you checked with Volvo what temp the warning should come on? I'm guessing it's in the 80-90 degree range.

If the sensor proves OK, it might be worth running some descaler through the engine and letting it sit for a while. Worked wonders for my parents MD11 and a friend old Buhk. Something like CLR (Calcium, Limescale, Rust), or maybe NeutraSalt or SaltAway.

Let me know what you find, or if you have questions I might be able to help with..