How does my temp switch work? Evinrude 50hp 72

[kippstakes]

Putting on a replacement head and am not sure if my temp switch is trustworthy- looks pretty old. I would like to test it, but would like to know how they work first. My understanding is that there is material inside the switch that changes/lowers resistance as it heats, and when it gets hot enough it basically becomes conductive. Does this sound right? If so, is that material the black plastic looking stuff in the switch or the metallic stuff beneath it?

Thank you

 

[racerone]

Re: How does my temp switch work? Evinrude 50hp 72

It is not the black plastic stuff.----The sensor wire has 12 volts on it.------When the sensor ( metallic stuff ) on this wire gets hot enough then it goes to ground.--Horn sounds.

 

[kippstakes]

Re: How does my temp switch work? Evinrude 50hp 72

Ah so it's the metal on the tip of the wire itself? Got it. I have tested the wire (which is out of the casing) and I'm getting continuity from the tip up through the wire. Would this be normal or is the tip shot?
Thanks for the quick reply.

 

[F_R]

Re: How does my temp switch work? Evinrude 50hp 72

I respectfully sort of disagree. I believe there is a thermal switch inside that thing. But I've never torn one apart, so might be mistaken.

 

[F_R]

Re: How does my temp switch work? Evinrude 50hp 72

OK, from the book: Should an irregularity develop in the cooling system and cause abnormal water jacket temperatures, the switch closes the horn circuit. The switch may be tested by submerging it in hot oil with a thermometer to check oil temperature. The contacts should be found to close at 211 degrees F, +/- 6 degrees, and open at 175 degrees F +/- 7 degrees

 

[kippstakes]

Re: How does my temp switch work? Evinrude 50hp 72

That sounds about right, but I'm not sure HOW it closes the circuit. It grounds the alarm horn out which engages the horn when the head gets hot enough. The thing is the switch in my head seems very simple, basically a nickel-sized dish filled with hard black plastic in which there is a pencil tip sized node (best word I can think of) crimped onto the end of the wire lead. There is a spring that holds the disk in place in the head, and a rubber plug. It looks like this:

http://thumbs1.ebaystatic.com/d/l225/m/mPVGKXAXzKFqpU-Hb5jtxrA.jpg

Now the wire seems to conduct throughout, from the node tip all the way to the end. When inserted into the plastic, the node touches inside the metal disk and conducts through the disk, which would conduct to the head. This means that this switch would continuously conduct to the head and ground/close the circuit. I don't get it.
Can anyone weigh in?