Yamaha Thermo Switch help

[andymach]

My Yamaha 40hp 2 cyl 1990 motor is sounding its overheat alarm. I have researched extensively and tried pretty much everything that is suggested on this board. <br /><br />1. Changed thermo switch.<br />2. Changed impeller<br />3. Changed thermostat<br />4. Checked for salt build up.<br />5. Used Salt Away treatment.<br /><br />I can touch the power head for a couple of seconds so I am pretty sure that the problem isn't the motor is actually overheating. I have also strong tell-tale flow.<br /><br />Today I fitted another new thermo switch as I read on this board that even new ones can be faulty. I tested the motor and it now reaches higher revs before the alarm goes off. <br /><br />This seems very strange to me.<br /><br />I spoke to Yamaha UK's tech dept some time ago and they said that there could be a short circut in the control box making the buzzer sound.<br /><br />Am I wasting my time taking the control box apart? In my research into overheating problems I have never come across the control box being at fault.<br /><br />Thanks in advance.<br /><br />Andy Mac

 

[sibelius99]

Re: Yamaha Thermo Switch help

Andy Mac,<br />This happened to me once with a older merc 75 hp. The problem was a faulty control box. When the engine got warm after starting it up the temp made something inside the controlbox short circuit. The solution for me was to take the whole controlbox away and let the tempsensors connect directly. Then I lost the first beep at start up (to check that the warning buzzer workes). I put in a simple switch to manually test it instead of spending the money on a new control box.<br />Hope this helps<br />Sibelius

 

[andymach]

Re: Yamaha Thermo Switch help

Sibelius<br /><br />Thanks for the reply.<br /><br />When you say that the temp made something in the control box short circut, do you mean that the thermo switch was activating and setting off the buzzer.<br /><br />My understanding is that the thermo switch on the engine only gives continuity ie completes the electrical circut at the given overheat temp. No other signal should be getting to the buzzer as if my motor isnt overheating there will not be a complete circut.<br /><br />Prehaps you mean that another circut which has nothing to do with the thermo switch circut is making it activate.<br /><br />Once again, many thanks.<br /><br />Andy Mac

 

[sibelius99]

Re: Yamaha Thermo Switch help

Andy Mac,<br />Yes the problem was the control box itself. Motor never overheated but when it got to correct operating temperature something in the control box grounded the buzzer connection.<br />The thermo sensor creates a grounding when activated in overheat conditions and the buzzer goes off. In my case the thermo sensor never activated (easy to test with a simple volt meter). <br />The mercury control box was sealed and could not be repaired. Instead of getting a new one i connected the thermo sensor directly to the buzzer so that in case of overheat condition the thermo sensor would ground and the buzzer sound. In order to be able to check the buzzer I installed a manual switch to be able to ground the buzzer alone to test its function.

 

[sibelius99]

Re: Yamaha Thermo Switch help

Hey, I am not a native English speaker (like I need to tell you that.....)<br />When I say "ground" I mean closing the 12 V circuit<br />/Sibelius

 

[andymach]

Re: Yamaha Thermo Switch help

Sibelius<br /><br />This idea sounds really promising. I will take control box apart and clean all connections for the buzzer. There may be some corrosion in there as the box is now 12 years old. I also like the idea of connecting the buzzer directly to the sensor at the engine. I am thinking that there might be the same male & female connectors in the control box that I can simply attach to the thermo swith.<br /><br />I will be delighted if this sorts it out as this has been on going for a couple of years now.<br /><br />Cheers<br /><br />Andy Mac