Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGauges, Warning Devices, and Driver Information Systems Diagnosis and RepairMedium
A technician is diagnosing an inoperative fuel gauge. The fuel tank is known to be half full. With the ignition on, the technician measures the resistance of the fuel level sender unit directly at its terminals and obtains a reading of 110 ohms. The vehicle manufacturer's specifications state that a half-full tank should correspond to a sender resistance of 50 ohms. Which of the following is the MOST likely cause?
- AA short to ground in the fuel gauge wiring
- BA faulty fuel level sender unit
- CAn open circuit in the fuel gauge wiring
- DA faulty fuel gauge in the instrument cluster
Show answer & explanationAnswer & explanation
Correct answer: B. A faulty fuel level sender unit
The technician measured the resistance directly at the sender unit and found it to be 110 ohms, which deviates significantly from the specified 50 ohms for a half-full tank. This direct measurement isolates the sender unit itself as the source of the incorrect resistance, indicating it is faulty.
Why the other options are wrong
- A. A short to ground in the wiring would typically cause the gauge to read full (low resistance) or blow a fuse, and would not affect the direct sender resistance measurement.
- C. An open circuit in the wiring would typically cause the gauge to read empty (high resistance or no reading) and would not affect the direct sender resistance measurement.
- D. The direct measurement at the sender bypasses the gauge and wiring, thus isolating the sender as the fault, not the gauge itself.
Fuel Level Sender Operation
A variable resistor located in the fuel tank, whose resistance changes with the fuel level. This resistance signal is sent to the fuel gauge or PCM to indicate fuel quantity.
- Resistance typically decreases as fuel level increases (or vice versa, depending on design).
- Direct measurement at the sender is crucial for isolating sender faults.
- Sender units can fail due to wear, corrosion, or contamination.
Memory trick: Sender's resistance, if wrong it's the source, not the wire or meter's force.