Re: new Merc 50 ELPTO overtemp horn test
Well, Merc uses tan or blue tan wiring for he OT sensor which should be embedded in the water jacket somewhere around the spark plugs location.
You could locate the OT sensor and follow the wire to a junction block or to a bayonet connection if your engine doesn't have a junction block. Either place, turn the ignition key to RUN and short the wire (going to the wiring harness, obviously) or from the terminal block, to the engine block, and you should hear a solid alarm.
Mark