Sierra 1500 2WD V8-5.3L Hybrid (2004)
Flexible Fuel Sensor: Description and Operation
FUEL COMPOSITION SENSOR DESCRIPTION
The fuel composition sensor (FCS), or flex fuel sensor (service parts term), measures the ethanol-gasoline ratio of the fuel being used in a flexible fuel
vehicle. Flexible fuel vehicles can be operated with a blend of ethanol and gasoline, up to 85 percent ethanol. In order to adjust the ignition timing
and the fuel quantity to be injected, the engine management system requires information about the percentage of ethanol in the fuel.
The FCS uses quick-connect style fuel connections, an incoming fuel connection, and an outgoing fuel connection. The two connections have different
diameters, to prevent incorrect attachment of the fuel lines. All fuel passes through the fuel composition sensor before continuing on to the fuel rail.
The fuel composition sensor measures two different fuel related parameters, and sends an electrical signal to the powertrain control module (PCM) to
indicate ethanol percentage, and fuel temperature.
The fuel composition sensor has a three-wire electrical harness connector. The three wires provide a ground circuit, a power source, and a signal
output to the PCM. The power source is vehicle system voltage, +12 volts), and the ground circuit connects to chassis ground. The signal circuit
carries both the ethanol percentage and fuel temperature within the same signal, on the same wire.
The FCS uses a microprocessor inside the sensor to measure the ethanol percentage and fuel temp, and change an output signal accordingly. The
electrical characteristic of the FCS signal is a square-wave digital signal. The signal is both variable frequency and variable pulse width. The
frequency of the signal indicates the ethanol percentage, and the pulse width indicates the fuel temperature. The PCM provides an internal pull-up to
five volts on the signal circuit, and the FCS pulls the 5 volts to ground in pulses. The output frequency is linear to the percentage of ethanol content in
the fuel. The normal range of operating frequency is between 50 and 150 Hertz, with 50 Hertz representing 0 percent ethanol, and 150 Hertz
representing 100 percent ethanol. The normal pulse width range of the digital pulses is between 1 and 5 milliseconds, with 1 millisecond representing
-40°C (-40°F), and 5 milliseconds representing 125°C (257°F).
The microprocessor inside the sensor is capable of a certain amount of self-diagnosis. An output frequency of 170 Hertz indicates either that the fuel
is contaminated or contains methanol (it should not), 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. Examples of
these substances include water, sodium chloride (salt), and methanol.
It should be noted that it is likely that the FCS will indicate a slightly lower ethanol percentage than what is advertised at the fueling station. This is not
a fault of the sensor. The reason has to do with government requirements for alcohol-based motor fuels. Government regulations require that alcohol
intended for use as motor fuel be DENATURED. This means that 100 percent pure ethanol is first denatured with approximately 41/2 percent
gasoline, before being mixed with anything else. When an ethanol gasoline mixture is advertised as E85, the 85 percent ethanol was denatured before
being blended with gasoline, meaning an advertised E85 fuel contains only about 81 percent ethanol. The FCS measures the actual percentage of
ethanol in the fuel.
