Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsCharging System Diagnosis and RepairMedium
A technician is diagnosing a vehicle with an illuminated battery warning light. The battery voltage is 12.4V with the engine off. With the engine running at 2000 RPM, the voltage at the battery terminals measures 12.2V. A voltage drop test between the alternator's B+ terminal and the battery's positive post shows 0.1V. What is the MOST likely cause of this condition?
- AExcessive parasitic draw
- BCorroded battery terminals
- CFaulty voltage regulator
- DLoose alternator drive belt
Show answer & explanationAnswer & explanation
Correct answer: C. Faulty voltage regulator
The voltage drop test between the alternator and battery is within specifications, indicating good cabling. The battery voltage is not rising with the engine running, which points to a charging system failure. Since the battery is not being overcharged (voltage is low), and the alternator is not outputting sufficient voltage, a faulty voltage regulator is the most probable cause.
Why the other options are wrong
- A. Excessive parasitic draw would drain the battery when the engine is off, but the charging system should still attempt to charge it when running.
- B. Corroded battery terminals would cause a significant voltage drop at the battery posts, which is not indicated by the 0.1V drop between the alternator B+ and battery positive post.
- D. A loose alternator drive belt would typically cause a squealing noise and a much lower or no voltage output, often accompanied by a higher voltage drop across the belt.
Voltage Regulator Failure
A malfunction in the voltage regulator prevents the alternator from maintaining the correct charging voltage, leading to undercharging or overcharging.
- Regulates alternator output voltage.
- Internal or external component.
- Failure can cause battery undercharge/overcharge.
Memory trick: Regulate and Charge, or the Battery's in the Dark.