A vehicle's battery light remains on after starting. The technician measures 12.8V at the battery with the engine running at 1500 RPM. A scope connected to the alternator's 'L' terminal (lamp control) shows a steady low signal (close to 0V). What does this indicate?
- AThe alternator is undercharging or not charging.
- BThe battery is fully charged and the alternator is healthy.
- CThe voltage regulator is sending an intermittent signal.
- DThe alternator is overcharging.
Show answer & explanationAnswer & explanation
Correct answer: A. The alternator is undercharging or not charging.
The 'L' terminal on many alternators provides a ground signal to illuminate the battery warning light. A steady low signal (close to 0V) on the 'L' terminal, combined with a low charging voltage (12.8V is below target 13.8-14.8V), indicates that the alternator is not producing sufficient output, and the warning light circuit is correctly signaling this condition.
Why the other options are wrong
- B. If the alternator were healthy and the battery fully charged, the charging voltage would be higher (e.g., 13.8V-14.8V), and the 'L' terminal signal would be off (high or open, depending on design).
- C. An intermittent signal would cause the light to flicker, not remain steadily on. A 'steady low signal' points to a consistent problem.
- D. Overcharging would result in high voltage and the 'L' terminal signal would likely be off or high (depending on design), not low.
Alternator L-Terminal Signal
The 'L' (Lamp) terminal on an alternator typically provides a ground signal to illuminate the battery warning light when the alternator is not charging or is undercharging. When charging correctly, this terminal is usually pulled high or open, turning the light off.
- L-terminal: Lamp control.
- Low signal (ground) = light on (no charge).
- High/open signal = light off (charging).
Memory trick: L is low, light will glow, charge is slow, or no show.