A vehicle's charging system light is on. A technician tests the 'S' (sense) terminal circuit at the alternator and finds 0 volts with the ignition on, engine off. The battery voltage is 12.6V. What is the most likely cause?
- AA faulty voltage regulator within the alternator.
- BA discharged battery.
- CA short to ground in the 'S' terminal wire.
- DAn open circuit in the 'S' terminal wire or its connection.
Show answer & explanationAnswer & explanation
Correct answer: D. An open circuit in the 'S' terminal wire or its connection.
The 'S' (sense) terminal provides the voltage regulator with a direct reading of system voltage, usually from the battery or a main distribution point. If there is 0 volts at this terminal when the ignition is on (and battery voltage is present), it indicates an open circuit in the wiring or connection to that terminal. Without this sense voltage, the regulator cannot accurately monitor system voltage and may not activate the alternator, causing a no-charge condition and the battery light to stay on.
Why the other options are wrong
- A. A faulty voltage regulator might cause over/undercharging, but it relies on the 'S' terminal input to function. If the input itself is 0V, the problem is upstream of the regulator's internal sensing.
- B. A discharged battery would show lower voltage, but the 'S' terminal should still reflect whatever battery voltage exists, not 0V if the battery is at 12.6V.
- C. A short to ground in the 'S' terminal wire would likely cause a blown fuse or draw down the source voltage, possibly not 0V unless it's a complete short to ground that prevents voltage from reaching it.
Alternator Sense (S) Terminal
The 'S' (sense) terminal on an alternator provides the voltage regulator with a direct feedback voltage from the battery or main power distribution, allowing the regulator to accurately monitor and maintain the vehicle's system voltage.
- Crucial for accurate voltage regulation.
- Often connected to battery positive or a main fuse box.
- An open circuit here can cause undercharging or no charging.
Memory trick: If 'S' has no volts, the regulator's sense jolts.