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I just finished rebuilding the #4 cylinder on my '01 HPDI 200 Yamaha with 244 hrs on it.  After rebuilding it I replaced all of the temp sensors for the water temp and port/starboard head temp sensors.  While replacing the sensors I wanted to be sure my overheat alarm horn was working correctly b/c it did NOT go off when my #4 cylinder failed.  They say if it is a detonation failure, as mine was, it may not go off.  Nonetheless I wanted to be sure the overheat alarm horn was working correctly.  

My overheat alarm horn was located under the drivers console and about 1.5" in diameter.  I disconnected the two connectors for the horn and connected it directly to my 12V battery charger.  The horn sounded correctly (if it doesn't go off at first, reverse the polarity of the connections).  Next I reconnected the horn and turned the key to the on position.  I then located the two cylinder head temp sensors in the top of the port and starboard cylinder heads.  These sensors are about 3/4" in diameter and have two wires going to them.  You can disconnect the wires at the sensor and touch the wires together.  With the key in the on position, the overheat alarm should sound when the wires are grounded together.  Repeat this process for both temp sensors.  If you want to go one step further, you can heat a pot of water to about 210* F on the stove and then place the removed sensors in the water for about 5 secs and the alarm horn will sound.  You can also do the same with an ohm meter with the sensor in the pot of water and the sensors should read less than 5k ohms (i.e. continuity) when the water temp is above 195* F.  

Knowing all of your temp sensors are working correctly as well as your overheat alarm is great piece of mind.  Always remember to check your oil and temp gauges/lights regularly on your boat motor as your driving it just in case the alarm horn doesn't sound.  You don't want to go through rebuilding your motor or buying a new powerhead, trust me!

-Rich
 
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