Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsCharging System Diagnosis and RepairMedium
A vehicle arrives with a complaint of a dead battery after sitting overnight. The battery tests good, and the charging system performs within specifications (14.2V at 1500 RPM, 110 Amps output). The technician performs a parasitic draw test and measures 350 mA after 30 minutes of the vehicle being off and locked. What is the most appropriate next step in diagnosing this issue?
- AInspect the alternator for internal shorts.
- BReplace the battery, as it must be faulty despite testing good.
- CPerform a voltage drop test on the charging circuit.
- DBegin pulling fuses one at a time to isolate the circuit.
Show answer & explanationAnswer & explanation
Correct answer: D. Begin pulling fuses one at a time to isolate the circuit.
A parasitic draw of 350 mA is significantly higher than the typical acceptable limit (usually 20-50 mA). Since the battery and charging system are confirmed good, the next logical step is to isolate the circuit responsible for the excessive draw by systematically removing fuses and observing the current drop.
Why the other options are wrong
- A. The charging system is performing within specifications, indicating no internal shorts in the alternator itself.
- B. The battery tested good, so replacing it without addressing the draw will only lead to another dead battery.
- C. A voltage drop test on the charging circuit is for charging efficiency, not for identifying a parasitic draw when the engine is off.
Parasitic Draw Diagnosis
Diagnosing an excessive parasitic draw involves measuring the current draw with the vehicle off and then systematically isolating the faulty circuit by pulling fuses until the current draw returns to an acceptable level (typically 20-50 mA).
- Excessive draw drains battery overnight.
- Acceptable draw: 20-50 mA (varies by vehicle).
- Fuse pulling isolates the faulty circuit.
Memory trick: Draw is high, battery dies, pull the fuse, find the lies.