Fuel Tank Pressure Sensor: Description and Operation

Description
The Fuel Tank Pressure sensor is a 3 wire strain gauge sensor much like that of the common MAP sensor. However, this sensor has very different electrical characteristics due to its pressure differential design. The sensor measures the difference between the air pressure (or vacuum) in the fuel tank and the outside air pressure.

The sensor mounts at the top of the fuel tank sending unit. A three wire electrical harness connects it to the Control Module. The Control Module supplies a 5 volt reference voltage and ground to the sensor. The sensor sends a voltage between 0.1 and 4.9 volts. When the air pressure in the fuel tank is equal to the outside air pressure, such as when the fuel fill cap is removed, the output voltage of the sensor will measures 1.3 to 1.7 volts.


Operation
When the air pressure in the tank is 4.5 inches H2O (1.25 kPa), the sensor output voltage should measure 0.5 plus/minus 0.2 volts. When there is neither a vacuum or a pressure in the tank, the sensor voltage should be at 1.5 volts at 14 inches H2O (-3.75 kPa) the sensor output voltage should be 4.5 volts plus/minus 0.2 volts.

When energized, the evaporative Canister Purge Valve allows the fuel vapor to flow from the EVAP canister to the engine. The normally closed valve is pulse width modulated by the Control Module in order to precisely control the vapor flow. The valve opens during the Enhanced Evaporative Diagnostic Test in order to create a vacuum in the fuel tank and then closed in order to seal the system.