A technician is diagnosing an inaccurate fuel gauge. With the ignition off, the technician measures the resistance of the fuel level sender unit at the instrument cluster connector, and then again directly at the sender unit in the fuel tank. Both readings are identical at 75 ohms. The fuel tank is known to be exactly half full, and manufacturer specifications indicate 50 ohms for a half-full tank. What is the MOST appropriate conclusion?
- AThere is a short to ground in the wiring between the sender and the cluster.
- BThere is an open circuit in the wiring between the sender and the cluster.
- CThe instrument cluster fuel gauge is faulty.
- DThe fuel level sender unit is faulty.
Show answer & explanationAnswer & explanation
Correct answer: D. The fuel level sender unit is faulty.
The fact that the resistance reading is identical (75 ohms) when measured both at the instrument cluster connector and directly at the sender unit indicates that the wiring between the sender and the cluster is intact and not introducing any additional resistance, shorts, or opens. Since the measured 75 ohms does not match the specified 50 ohms for a half-full tank, the sender unit itself is providing an incorrect resistance value.
Why the other options are wrong
- A. A short to ground would typically result in a very low resistance reading or a blown fuse, not an identical, but incorrect, 75-ohm reading.
- B. An open circuit would result in an infinite resistance reading (or a significantly higher reading) when measured, contradicting the identical 75-ohm readings.
- C. Since the resistance reading at the cluster connector matches the sender, the gauge is receiving the incorrect signal from the sender. The fault is upstream of the gauge.
Circuit Isolation by Resistance Measurement
Measuring resistance at different points in a circuit allows technicians to isolate faults to a specific component or section of wiring. Identical readings confirm wiring integrity up to that point.
- Measure at component terminals to test the component itself.
- Measure at harness connectors to test wiring integrity.
- Compare measured values to manufacturer specifications.
Memory trick: Same ohms, different spots, sender's the fault, no wiring plots!