Signs of a Bad Fuel Sending Unitby Chris Weis
Fuel gauges provide valuable information to a driver. Incorrect fuel level indications can be annoying, or downright dangerous. Unexpectedly running out of fuel at the wrong time could place a vehicle in peril. Inaccurate readings can stifle attempts to economize consumption. Compound or digital gauges, as well as mileage computers all rely on the performance of the fuel sending unit.
A fuel gauge that hangs up in one position before returning to normal operation, may be a direct representation of a float in the fuel tank. If the float sticks in one position due to mechanical deficiencies, a bump in the road may jar it loose, and normal function returns. A sending unit suffering from this condition can repeat the malfunction whenever the float reaches the sticking point due to fuel level changes.
Running on Empty
A fuel gauge needle that is resting on empty when the vehicle has fuel, may be due to total failure of the sending unit parts. This condition can also be caused by a defect in the wiring between the sending unit and gauge, or a fuse in the circuit. Some circuits employ constant voltage regulators or resistor coils to counter reactions to fuel movement in the tank. Proper operation of such devices should be confirmed before replacing a sending unit.
Steady false indications of a full tank may signify a shorted sending unit. It is as likely that shorted wiring or the gauge itself may be at fault. Full tank indications that seem to linger after fuel consumption occurs can be normal operation, inherent to the vehicle. A certain brand of vehicle is notorious for this harmless inaccuracy. Low fuel level warning lights that are illuminated prematurely are usually not related to the sending unit.
Complete testing of related components is the logical approach to avoid unnecessary and dangerous handling of the tank. Some vehicles have access panels to ease sending unit or fuel pump replacement, but most do not. Special testing equipment used by professionals eliminates wiring and gauges as possible faults before lowering the tank out of the vehicle. Fuel tank removal is a difficult task even with proper tools and equipment and should not be attempted by the uninitiated.
- "Electric and Electronic Systems for Automobiles and Trucks"; Robert N. Brady; 1983
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