Fuel gauge wrong

Complete fuel pump module. Contains fuel level sending unit, fuel pump, pulse modulator, bracket, connectors, and pre-filter

My fuel gauge is wrong

The fuel level sending unit is located inside the fuel tank. It has a float attached to an arm that rides up and down with your fuel level. The arm moves in an arc along a variable resistor that changes the voltage. In the old days, the dashboard fuel gauge responded directly to the amount of voltage it received from the fuel level sending unit. However, in late model vehicles, the variable voltage gets sent to a body control module (BCM) or generic electronic module (GEM), where the voltage is converted into digital data and then forwarded on a data bus to the powertrain control module and the instrument cluster.

Gasoline with too much sulphur can corrode the variable resistor in the fuel level sending unit and result in erroneous readings. Fuel level sending units are sold separately from the fuel pump. But you won’t find many shops that’ll install just a new sending unit. Sure, you may save a few bucks on the parts by buying just the sending unit, but then the mechanic will have to disassemble the old fuel pump module. Any screw up and you’ll be back at his door with a pump that doesn’t work. Few shops want to take on that liability so they almost always replace the entire fuel pump/sending unit module.

However, before you run it into the shop thinking that you’ve got a bum sending unit, take a while to check for corroded electrical connections because they’re pretty common. So if you’ve got fuel gauge problems, always start by checking the electrical connector right at the top of the fuel tank. Open the connector and check for corrosion. If you see any, clean them by following the instructions in this post. If the connector is clean, move on to the ground point for the fuel level sending unit. Remove the ground, sand the metal, and coat the ground screw and ground terminal with dielectric grease. Then reassemble and check the fuel gauge. If it’s still wrong, you may have a problem with the EVAP system or a bad module. Read this to find out how an EVAP problem can give you an incorrect fuel level.

© 2012 Rick Muscoplat

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