The water temperature sensor is mostly used by the ECU for cold-start and run enrichment, engine over temperature limit and thermostatic radiator fan control. They come in different shapes and resistance values.

Operation
The resistance of the temperature sensor changes in relation to the temperature it senses. It has a negative temperature resistance co-efficiency (NTC). Each sensor has a preprogrammed curve to indicate its resistance to temperature. Note: The Spitronics ECU uses a 2K resistance NTC and this value is measured at 25°c. (See below a sample of such a curve.)

Electronics sensors
Spitronics build and supply small sensors for customers to build into their own units. There are 2 sizes but it does the same work. Note: These sensors require a pull-up resistor to let a constant current through them. (See below a picture of what these sensors look like in their raw form.)

How to measure a sensor
Use a multi-meter set to the 10,000 ohm (10 K) range. With the sender unit at approximately 25°c, measure the resistance. It should be between 1800ohm and 2200ohm at 25°c. Those two pins will be used for the ECU. The remaining two pins are normally used for the dash cluster and measure in the region of 3500ohm at 25°c. This same test can be done on engines with multiple sender units to determine which sender unit will be used for the ECU. Example: Toyota 1uz-fe, where there is three water temperature sender units.

Multiple Devices on One Sender Unit.
Do not share temperature sender units between two devices. Only one device may be coupled onto a sender unit at any one time. Both devices will excite the sensor with current which in turn will change the signal value and then both will display the wrong temperature. In such a case you may need to add an extra sensor for the Spitronics ECU. The only exception for this rule is when a data logging device is used that has no internal pull up resistor. Example: Race Technology DL1.

Sensor Location.
The ideal location for the temperature sender unit is on the engine side of the thermostat as this side will show the exact temperature at all times. The radiator side of the thermostat will show lower temperatures and take longer to react.

Air Cooled Engines
For air cooled engines it is better to measure cylinder head temperature for cold start compensation as the oil takes too long to pick up temperature. A simple cylinder head temperature sender unit can be made by clamping an NTC 2k ohm thermistor on a lug terminal and bolting it onto the cylinder head.