Flex Fuel Sensor (NAS Vehicles Only)

The FFS (Flex Fuel Sensor), measures the ethanol to gasoline ratio of the fuel in the fuel line to the injectors. Flexible fuel vehicles can be operated with a blend of ethanol and gasoline, up to 100 percent ethanol. To optimize the ignition timing and the fuel injection quantity, the ECM requires information about the percentage of ethanol in the fuel.

The FFS uses a microprocessor to measure the ethanol percentage and fuel temperature, which it uses to produce and output a signal to the ECM. The FFS signal is a pseudo square-wave PWM (Pulse Width Modulation) voltage signal. The signal varies in both frequency and pulse width; The frequency of the signal indicates the ethanol percentage (-50 Hertz offset). The output frequency is linear with respect to the percentage of ethanol content in the fuel. The ECM provides an internal pull-up to five volts through a load resistor on the signal circuit, and the fuel sensor pulls the 5 volts to ground in pulses. The normal range of operating frequency is between 50 and 150 Hertz; 50 Hertz indicates 0% ethanol, and 150 Hertz indicates 100% ethanol.

The pulse width low state time indicates the fuel temperature. The normal pulse width is between 1 and 5 milliseconds: 1 millisecond indicates -40°C (-40°F), and 5 milliseconds indicates 125°C (257°F).

The processor inside the sensor is capable of some self-diagnosis. An output frequency of 170 Hertz or higher indicates either that the fuel is contaminated or that an internal sensor electrical fault has been detected. Certain substances dissolved in the fuel can cause the fuel to be contaminated, raising the output frequency to be higher than the actual ethanol percentage should indicate (for example; 180 Hertz or 190 Hertz). These contaminates may commonly be water, salt, methanol, or some other substances.