Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGauges, Warning Devices, and Driver Information Systems Diagnosis and RepairMedium
A technician is performing a diagnostic check on a vehicle with an inoperative fuel gauge. The fuel tank is known to be full. When measuring the resistance of the fuel level sending unit, the technician expects to see a reading of approximately 20 ohms. Instead, the multimeter shows an 'OL' (open loop) reading. What is the most likely cause?
- AA short to power in the sending unit circuit.
- BAn open circuit within the sending unit or its wiring.
- CA miscalibrated fuel gauge in the instrument cluster.
- DA short to ground in the sending unit circuit.
Show answer & explanationAnswer & explanation
Correct answer: B. An open circuit within the sending unit or its wiring.
An 'OL' (open loop or overload) reading on a multimeter when measuring resistance indicates an open circuit. This means there is a break in the electrical path, either within the sending unit's internal resistance element or in the wiring leading to it, preventing current flow.
Why the other options are wrong
- A. A short to power would not typically be detected as an 'OL' reading when measuring resistance, and would likely cause other electrical issues.
- C. A miscalibrated fuel gauge would still receive a resistance signal, but display it incorrectly; it would not cause an 'OL' reading at the sending unit itself.
- D. A short to ground would result in a very low resistance reading, close to 0 ohms, not 'OL'.
Multimeter 'OL' Reading (Resistance)
An 'OL' or 'Overload' reading on a multimeter when measuring resistance indicates an open circuit, meaning there is an infinite resistance or a break in the electrical path.
- Common for broken wires or internal component breaks.
- Prevents current flow in the circuit.
- Opposite of a short circuit (near 0 ohms).
Memory trick: OL: 'OPEN' circuit, 'LOST' connection, 'OH NO!'