Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsStarting System Diagnosis and RepairMedium
A technician is diagnosing a parasitic draw on the battery that is causing a no-start condition after the vehicle sits overnight. The draw is measured at 250 mA. Which of the following is the most appropriate initial diagnostic step after verifying the draw?
- ASystematically remove fuses one by one while monitoring the current draw.
- BReplace the battery and retest.
- CDisconnect the alternator and retest the draw.
- DPerform a continuity test on the starter motor windings.
Show answer & explanationAnswer & explanation
Correct answer: A. Systematically remove fuses one by one while monitoring the current draw.
A parasitic draw of 250 mA is higher than acceptable (typically 50 mA or less). To locate the source of the draw, the most effective method is to systematically remove fuses, one at a time, while monitoring the current with an ammeter. When the draw significantly drops, the circuit associated with that fuse is the culprit.
Why the other options are wrong
- B. Replacing the battery is a temporary fix; the underlying draw will quickly discharge the new battery again.
- C. While the alternator can cause a draw, it's only one potential source. Systematically removing fuses is a more comprehensive and efficient way to isolate the circuit.
- D. A continuity test on starter motor windings is for diagnosing specific starter issues, not a general parasitic draw which could be from any number of vehicle systems.
Parasitic Draw Diagnosis
The process of identifying components that continue to consume electrical current when the vehicle is off, leading to battery discharge.
- Measure current draw with an ammeter after vehicle 'sleeps'.
- Acceptable draw is typically 50mA or less (check OEM specs).
- Isolate the faulty circuit by systematically removing fuses.
Memory trick: Draw's a ghost, so pull a fuse, then watch the amps for what to lose.