This sound like a bad temperature sending unit mainly located on the intake manifold sometimes by the thermostat housing ,it has one wire plug.
If you mean where is the temperature sending sensor it is in the water outlet housing next to the thermostat. There are 2 sensors for the engine temperature. One for the computer which controls the radiator fan and one for the gauge on the dash.
MY 1991 VW DID THIS & THE RADIATOR RES. TANK HAS A SENSOR IN IT TO LET YOU KNOW TO ADD WATER TO THE SAFE LEVEL ON THE TANK.
water temperature. 180-220 degrees
Celsius
Where the top hose of the radiator goes is called the water outlet pipe. The sensor for the coolant temperature is in this pipe. There are 2 sensors. One goes to the gauge on the dash and the other goes to the computer. Not sure which is which, but if you unplug the one that goes to your dash the temperature gauge will peg.
no water
The temperature sender for the coolant is located in the water outlet. The upper radiator hose goes to the water outlet. The sensor with 2 wires goes to the gauge and the sensor with 3 wires goes to the computer.
To answer you question more thoroughly I will need more detail because there are several different types of water qauges. You have a water temperature gauge, water pressure gauge and water level gauges.
It is in the water outlet. It is a casting with 4 holes in it. One for the top of the radiator cooling hose. One for the gauge sensor. One for the computer computer, and another one for the heater hose. There are 2 temperature sensors. The one with 2 wires instead of 1 is for the computer. The sensor with 1 wire is for the gauge.
Take wire off temperature sender look a guage. If guage at cool, temp sender needs replacement. If guage still at hot, temp sender wire shorted to engine or vehicle body.
The most effective location for a water temperature gauge on a vehicle is on the head or the side of the block. Generally, water temperature gauges are more accurate and fit better in these locations.
The temperature sensor is on the water outlet housing. This is where the big hose from the top of the radiator meets at the engine.