Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsCharging System Diagnosis and RepairMedium
A vehicle's charging system warning light is illuminated, and the voltmeter shows 12.0V with the engine running. A visual inspection reveals a severely corroded battery positive terminal. After cleaning the terminal, the warning light remains on, and voltage is still 12.0V. What is the most likely issue now?
- AThe voltage regulator has failed.
- BThe alternator drive belt is slipping.
- CThe battery is internally shorted.
- DThe alternator's main output wire (B+) has high resistance or an open circuit.
Show answer & explanationAnswer & explanation
Correct answer: D. The alternator's main output wire (B+) has high resistance or an open circuit.
Even after cleaning the corroded battery terminal, the charging light remains on and the voltage is still at battery voltage (12.0V), indicating no charging. This suggests a complete break or extremely high resistance in the main charging circuit from the alternator to the battery, often due to a faulty B+ wire or connection.
Why the other options are wrong
- A. While a failed voltage regulator can cause no charge, the complete lack of charging (12.0V) after addressing a known resistance point (terminal) makes a B+ wiring issue a more direct explanation for no output reaching the battery.
- B. A slipping drive belt would typically cause some, albeit low, charging output and often an intermittent light, not a complete lack of charging (12.0V) and a continuously on light.
- C. An internally shorted battery would typically show much lower voltage and would not likely allow the engine to run without immediate stall.
Charging System Wiring Integrity
The integrity of all charging system wiring, especially the main alternator output (B+) and ground cables, is critical for efficient power delivery and proper system function.
- High resistance or open circuits prevent proper charging.
- Can manifest as voltage drops or no charge.
- Corrosion is a common cause of resistance.
Memory trick: Cleaned the battery, but the charge is still unseen; look to the main wire, it's the next scene.