A vehicle's charging system is being diagnosed. The technician notes that the voltage at the battery is 12.3V with the engine off. With the engine running at 2000 RPM, the voltage at the battery increases to 14.5V. However, when the headlights, blower motor, and rear defroster are all turned on, the voltage drops to 13.0V and the battery light flickers. The alternator is rated for 150 Amps. A quick check of the alternator drive belt shows no signs of slippage or wear. What is the most likely cause?
- AA shorted field winding in the alternator.
- BAn undersized alternator for the vehicle's electrical demands.
- CA failing voltage regulator.
- DIntermittent open in the stator windings.
Show answer & explanationAnswer & explanation
Correct answer: B. An undersized alternator for the vehicle's electrical demands.
The symptoms indicate that the alternator can charge adequately under light load (14.5V) but struggles to maintain voltage under heavy load (drops to 13.0V and light flickers). This suggests the alternator, while functional, may not have enough current capacity to meet the vehicle's full electrical demands, indicating it might be undersized for the installed accessories or vehicle's design.
Why the other options are wrong
- A. A shorted field winding would cause a continuous no-charge condition, not a functional charge under light load.
- C. A failing voltage regulator would typically cause consistent overcharging, consistent undercharging, or erratic voltage, rather than just struggling under heavy load while performing well otherwise.
- D. An intermittent open in the stator would likely cause more severe and inconsistent charging issues, potentially with higher voltage fluctuations or a complete loss of charge, not just a struggle under heavy load.
Alternator Undersizing
An alternator may be considered undersized if it cannot meet the vehicle's total electrical current demands, especially under heavy load (all accessories on). Symptoms include voltage drop and battery light flickering under load, despite appearing to charge normally under light load.
- Alternator rated output must exceed maximum vehicle load.
- Symptoms appear only under heavy electrical load.
- Voltage drops significantly, battery light may flicker.
Memory trick: Big load, tiny gen, voltage low, start again.