A technician is diagnosing an inoperative fuel gauge. The fuel tank is known to be full. When the technician disconnects the fuel level sender unit's electrical connector, the fuel gauge immediately drops to 'Empty'. When the technician jumps the two terminals of the disconnected connector together with a wire, the fuel gauge immediately reads 'Full'. Which of the following is the MOST likely cause?
- AAn open circuit in the wiring between the sender and the cluster
- BA faulty fuel level sender unit
- CA faulty fuel gauge in the instrument cluster
- DA short to ground in the wiring between the sender and the cluster
Show answer & explanationAnswer & explanation
Correct answer: B. A faulty fuel level sender unit
This test procedure effectively isolates the gauge and wiring as functional. When the connector is disconnected (simulating an open circuit), the gauge reads 'Empty'. When the terminals are jumped (simulating a short/zero resistance), the gauge reads 'Full'. This confirms the gauge and the wiring are correctly responding to resistance changes. Since the fuel tank is full (which should cause a 'Full' reading from the sender) but the gauge was inoperative, the sender itself is failing to provide the correct resistance value, indicating it is faulty.
Why the other options are wrong
- A. If there was an open circuit in the wiring, jumping the terminals at the sender wouldn't make the gauge read 'Full' as the open would still exist between the jump and the gauge.
- C. The tests confirm the gauge responds correctly to open and short circuits, indicating the gauge itself is functional.
- D. A short to ground in the wiring would typically keep the gauge reading 'Full' constantly, and the tests performed would likely not yield the described results.
Fuel Gauge Circuit Test
By manipulating the resistance input to the fuel gauge circuit (open circuit, short circuit), a technician can determine if the gauge and wiring are functional, thereby isolating a fault to the sender unit.
- Open circuit at sender connector typically causes gauge to read 'Empty'.
- Short circuit at sender connector typically causes gauge to read 'Full'.
- If gauge responds correctly to these inputs, the gauge and wiring are good.
Memory trick: Gauge responds to tests, wiring's fine; sender's the fella, out of line!