Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGauges, Warning Devices, and Driver Information Systems Diagnosis and RepairMedium
A technician is diagnosing a complaint of an inoperative fuel gauge. The fuel tank is known to be full. When checking the fuel level sender unit at the tank, the technician measures 20 ohms between the signal wire and ground. What should the technician conclude?
- AThe fuel level sender is faulty and indicates an empty tank.
- BThe fuel level sender is operating correctly for a full tank.
- CThe fuel level sender is shorted to ground.
- DThe fuel level sender has an open circuit.
Show answer & explanationAnswer & explanation
Correct answer: A. The fuel level sender is faulty and indicates an empty tank.
Most automotive fuel level senders use a variable resistor where resistance is lowest when the tank is empty (e.g., 20-70 ohms) and highest when the tank is full (e.g., 200-300 ohms). A reading of 20 ohms with a full tank indicates the sender is faulty and is sending a signal consistent with an empty tank.
Why the other options are wrong
- B. A correct reading for a full tank would typically be the highest resistance value, often 200-300 ohms, not 20 ohms.
- C. A short to ground would typically result in a 0 ohm reading, not 20 ohms.
- D. An open circuit would result in an 'OL' (over limit) or infinite resistance reading, not 20 ohms.
Fuel Level Sender Resistance
Automotive fuel level senders are variable resistors whose resistance changes based on fuel level. Typically, resistance is lowest when the tank is empty and highest when full.
- Resistance range varies by manufacturer (e.g., GM: 40-250 ohms; Ford: 16-158 ohms).
- Lower resistance usually means empty tank.
- Higher resistance usually means full tank.
- An in-spec full tank reading is crucial for accurate gauge operation.
Memory trick: Empty Equals Low Ohms, Full Equals High Ohms.