A vehicle's charging system light remains on after starting. A technician measures 12.6 volts at the battery with the engine off, and 12.4 volts with the engine running at 2000 RPM. A check of the alternator's field circuit reveals no voltage present at the 'L' (lamp/indicator) terminal when the ignition is on, engine off. What is the most likely cause?
- AA faulty battery temperature sensor.
- BAn open circuit in the field control circuit to the alternator.
- CA shorted alternator output wire.
- DExcessive parasitic draw.
Show answer & explanationAnswer & explanation
Correct answer: B. An open circuit in the field control circuit to the alternator.
The 'L' terminal (or similar indicator terminal) provides initial excitation voltage to the alternator's field winding and controls the battery light. If there's no voltage at this terminal with the ignition on, engine off, it indicates an open circuit in the field control wiring or a fault in the PCM/regulator control, preventing the alternator from being excited and thus preventing charging.
Why the other options are wrong
- A. A faulty battery temperature sensor affects charging voltage regulation based on temperature, not the initial excitation or activation of the alternator.
- C. A shorted alternator output wire would likely blow a fuse or cause severe voltage drop, not specifically prevent field excitation.
- D. Excessive parasitic draw would discharge the battery when the engine is off but would not directly prevent the alternator from charging once the engine is running, unless the battery is too low to excite the field.
Alternator Field Excitation
Alternators require an initial small current, known as excitation current, to flow through the rotor's field winding to create a magnetic field, which is necessary for the alternator to begin producing output voltage.
- Usually provided via the 'L' terminal or a dedicated excitation wire.
- Often routed through the battery light circuit.
- Without excitation, the alternator will not charge.
Memory trick: If the 'L' isn't lit, the field won't get its bit.