Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsCharging System Diagnosis and RepairHard
A vehicle's battery light comes on while driving and stays on. The technician performs a battery test, which passes. The charging voltage at the battery is 12.6V with the engine running. An alternator output test shows the alternator is producing 80 amps under load, but its rated output is 140 amps. Which of the following is the most likely cause?
- AA seized alternator pulley.
- BA faulty voltage regulator causing undercharging.
- CAn open circuit in the alternator's ground wire.
- DA shorted stator winding within the alternator.
Show answer & explanationAnswer & explanation
Correct answer: D. A shorted stator winding within the alternator.
If the alternator is only producing 80 amps when rated for 140 amps, it's significantly underperforming. A common cause for reduced output, especially when the voltage regulator might still be attempting to regulate, is an internal alternator fault like a shorted stator winding. This reduces the effective winding area, leading to lower current production.
Why the other options are wrong
- A. A seized alternator pulley would likely shred the belt and cause a complete no-charge condition, not a reduced output of 80 amps.
- B. A faulty voltage regulator causing undercharging would typically result in low voltage (e.g., 12.6V), but a specific current output reduction to 80A from 140A under load points more directly to the alternator's internal generation components.
- C. An open ground wire would cause a complete loss of alternator function, resulting in no output, not a reduced output of 80 amps.
Alternator Stator Winding Faults
Faults in the alternator's stator windings, such as opens or shorts, directly reduce the alternator's ability to generate its full rated alternating current, leading to undercharging and reduced maximum output.
- Stator windings generate AC before rectification.
- Shorts reduce effective turns, lowering output.
- Opens can cause complete loss of a phase, severely reducing output.
Memory trick: If the amps are low, the stator's flow might be slow.