Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsCharging System Diagnosis and RepairMedium
A vehicle's charging system is being diagnosed. With the engine running at 1500 RPM and all accessories off, the voltage at the battery is 13.8V. When the headlights, rear defroster, and blower motor on high are activated, the voltage drops to 12.8V. The battery is fully charged and tests good. The alternator's maximum rated output is 120 amps. What is the most likely issue?
- AA failing alternator unable to meet current demand.
- BA parasitic draw exceeding alternator capacity.
- CA faulty voltage regulator allowing excessive voltage drop under load.
- DAn open circuit in the battery cable.
Show answer & explanationAnswer & explanation
Correct answer: A. A failing alternator unable to meet current demand.
A significant drop in charging voltage (from 13.8V to 12.8V) when a substantial electrical load is applied, even with a good battery and a seemingly functional voltage regulator (as it was maintaining 13.8V initially), strongly indicates that the alternator is unable to produce the necessary current to meet the demand. This points to a failing alternator.
Why the other options are wrong
- B. A parasitic draw affects the battery when the engine is off; it doesn't manifest as a voltage drop under load when the engine is running, as the alternator should be supplying power.
- C. A faulty voltage regulator would typically cause consistent overcharging, undercharging, or erratic voltage, not just a significant drop under high load if it was maintaining good voltage initially.
- D. An open circuit in the battery cable would cause high resistance and voltage drop, but typically affects starting and overall charging, not specifically a drop under load from an otherwise normal 13.8V.
Alternator Current Output Test
An alternator current output test measures the maximum current an alternator can produce under various loads and RPMs, indicating its ability to meet the vehicle's electrical demands.
- Performed with an inductive amp clamp and voltmeter.
- Compares actual output to rated output.
- Helps identify alternators that cannot keep up with demand.
Memory trick: When demand is high, and voltage starts to die, the alternator can't supply.