Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsCharging System Diagnosis and RepairMedium
A customer complains of a dead battery every morning. The battery tests good and the charging system charges at 14.2 volts. A parasitic draw test is performed, and with the ignition off and doors closed, an ammeter shows a draw of 250 milliamps (mA) after 30 minutes. What is the next logical diagnostic step?
- ACheck the resistance of the main battery cables.
- BReplace the battery.
- CBegin pulling fuses one by one to isolate the circuit.
- DReplace the alternator.
Show answer & explanationAnswer & explanation
Correct answer: C. Begin pulling fuses one by one to isolate the circuit.
A parasitic draw of 250mA is excessive (typically should be below 50mA). Since the battery and charging system are confirmed good, the next step is to identify the source of the draw. This is done by systematically removing fuses until the draw drops to an acceptable level, thus isolating the faulty circuit.
Why the other options are wrong
- A. Checking battery cable resistance is relevant for charging or starting issues, but not for an excessive draw when the vehicle is off and the charging system is confirmed good.
- B. The battery tested good, so replacing it would not solve the underlying draw issue.
- D. The charging system is functioning correctly (14.2V), so the alternator is not the problem.
Parasitic Draw Test
A parasitic draw test measures the amount of current consumed by vehicle components when the ignition is off, identifying excessive draws that can drain the battery over time.
- Performed with an ammeter in series with the battery.
- Typical acceptable draw is 20-50 mA.
- Requires waiting for modules to 'sleep' (often 20-30 mins).
Memory trick: When the battery dies in the night, a hidden current is taking flight. Pull fuses to find the light.