Variable Resistors

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THERMISTOR OPERATION
In the example below, as the engine warms up, R2 (water temp sensor) increases in temperature; this lowers the resistance of R2. The drop in R2 resistance results in a decreased voltage drop across R2 (less voltage is used). This decreased voltage drop across R2 causes the voltage drop across R1 to increase. R1 now uses more voltage. This increased voltage drop of R1 results in a lowered measured voltage by the monitor circuit and is seen by the computer as an increase in engine temperature. The reverse is true as the water temp sensor cools.