Re: how does over temp switch work?
The overtemp switch is a simple bimetallic design. heat bends the internal bimetal strip and at a certain temperature it goes to ground on the engine block where it is mounted. Since they are basically internal and sealed, there is no way to test the switch on the engine, other than heating the head with a propane torch. Grounding the wire will test the system from power source to engine but not the switch itself.
Power is USUALLY taken from the "I" or "A" terminal of the ignition switch. This will turn off the buzzer when the switch is off as this is an accessory terminal powered only when the switch is on. However, power can be taken from the battery terminal of the ignition switch or any positive wire under the dash although strictly speaking, this is not correct.
Power is routed through one wire or terminal of the buzzer or light and the wire (orange on older models) that goes to the engine is connected to the other. If the buzzer is functional and has power, when the bimetal closes, a circuit is completed and the buzzer sounds. Since I am half deaf, I prefer a light. You can buy 12 volt signal lights in a couple of colors (I prefer red) at Radio Shack for only a couple of bucks
If you want to install a gauge, you must either disconnect the bimetal switch or use a separate wire. The gauge sending unit can be mounted in the head where there is a stainless plug or you can drill and tap a boss in the block, location depending upon the size of the engine. Three and four cylinder engines have one right behind the head at the top of the block.
Strictly speaking, it is not absolutely necessary to drill and tap for a gauge sender: I have seen on older Evinrudes the sender held in contact with the outside of the head by a simple "L" bracket attached to one of the bolts. It works, but since only the end of the sensor is in contact and the body is exposed to air, it is slow to heat and slow to respond to changes.
