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SX200 Water Temp Horn Going Off

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  • SX200 Water Temp Horn Going Off

    Horn goes off after going 4000 RPMs for a few minutes.
    Engine de-scaled, new tstats, new impeller, poppet. Took readings of heads with IR Heat gun and only showed temperatures from 120-140 F.

    New thermo sensors and switch?

  • #2
    Remove heads, clean deposits from water passages, drill out broken bolts, go boating.


    • #3
      Do you have a Yamaha multi-function tachometer? If so, what are you seeing on it?


      • #4
        have you pulled and tested the temp switch and sensors to see if they function in spec?


        • #5
          What you will "probably" find is the area where the sensor fits into the motor is all blocked up, no amount of saltaway, vinegar, fresh water or other famous method will fix it, you need to remove the heads and clean it properly, or just continue to flush it, replace parts and hope for the best, which usually never works.


          • #6

            1. It does go into limp mode and the water icon blinks on Yamaha multi-Function.

            2. Will pull the sensors and switch and test.

            3. Please confirm the method to test sensors as attached.

            4. What is the difference between the switch and sensor?

            5. Looks like sensor should not have continuity until temps reach 183F. IR Temperature Gun highest reading was 144F.

            Attached Files


            • #7
              switches are open or closed

              sensors are variable resistance.

              were you shooting the area at the switch or sensor when taking the temp?

              pull the switch and test
              look to see if that motor also has sensors and find,pull them and test.

              shooting temp in the area or even in the hole you pull them out of should give a better idea of what they are seeing while motor is running in alarm situation( if that is a blind hole and water does not dump out when you pull them)
              Last edited by 99yam40; 09-05-2019, 08:05 AM.


              • #8

                Thank you for responses.

                I shot the temp at head near spark plugs but agree I should have shot temp in the area of the hole you pull the sensors out of would give a better idea of what they are seeing while in alarm

                Will pull 2 sensors and 1 switch and test them.




                • #9
                  Have not had a chance to test sensors and switch yet.

                  If the switches are simply on and off and the sensors have variable resistance does it make sense that it is the sensors going off at a lower temperature?

                  Are the sensors connected to the CDI or to the switch?

                  Why are their 2 switches and only 1 sensor?

                  Not able to get to boat right now as it is remote.


                  • #10
                    are you sure there are 2 sensors and one switch?
                    my C40 has 2 switches and only one head being a 3 cylinder.
                    one switch tells the cdi the motor is warmed up and the other tells it the motor is overheating.

                    I am sure the sensor would give an accurate temp reading of the block, water inside the block , or the head.
                    a switch would just close at a temp close o its set point
                    need a service manual to tell you what you have and set points


                    • #11

                      you are correct, there are two switches and one sensor. Apologies for my confusion.