Flex Fuel sensors measure the ethanol to fuel ratio, in cars that run with multiple fuel sources.
Flex-fuel cars can be run by a mixture of ethanol and petrol, and go as high as 85% ethanol. To adjust the ignition point, and the correct amount of fuel, the engine control needs information about the percentage of ethanol in the fuel.
The sensor measures the alcohol levels in the fuel, as it passes through, to determine the amount of ethanol and sends an electric signal to the engine control(ECM), to indicate the ethanol %.
Flex-fuel sensors have 3 wires so that there is a power source, signal exit to the ECM, and a ground connection. The power source is the batteries positive charge, and the ground connection wire is connected to an engine to ground connection. The signal circuit carries the ethanol % via a frequency signal.
The alcohol % signal is sent to the ECM from the sensor in Hz, and a normal drifting frequenzy would be around 50-150 Hz, and a reading of 180-190 Hz would indicate the fuel is polluted.