Re: 1989 125 force water tempature
The switch on the surface of the head near the spark plug is an overheat indicator switch. It is normally open until the engine overheats at which time its bimetallic contact closes, causing the buzzer to sound. It has a 5/8 head and the bolt portion is 1/4-20.
On top of the head there can be either a switch or a temperature sensor. The hole is threaded 1/8 NPT. You should have received a sensor with your gauge. However, if you hook the send wire to the existing sensor and the gauge reads minimum or 0 with the engine fully warmed-up, then it is either broken or an overheat switch. If the gauge reads around 160 then you know it is a functioning sensor.
You can attach the orange wire to either but not both, but of course, the wiring will be slightly different for each. If you want to retain the overheat buzzer, use the orange wire for that and run another wire for temperature gauge.
If your gauge came with a sensor with 1/4 or 3/8 NPT threads, there is a boss on the block just behind the sensor on the head. This has plenty of meat to be drilled and tapped for a 3/8 NPT thread. Just don't tap too deep. National Pipe Threads are tapered and if you tap too deep, the sensor will not seal in them.
