A technician is diagnosing a vehicle with a parasitic draw. After connecting an ammeter in series with the negative battery terminal and waiting for modules to 'sleep,' the ammeter reads 500mA (0.5A). The manufacturer's specification for parasitic draw is 50mA. What is the MOST appropriate next step?
- AStart pulling fuses one by one while monitoring the ammeter.
- BCheck the starter motor for a short to ground.
- CDisconnect the alternator and re-test the draw.
- DReplace the battery, as it likely has an internal short.
Show answer & explanationAnswer & explanation
Correct answer: A. Start pulling fuses one by one while monitoring the ammeter.
A parasitic draw of 500mA (0.5A) is significantly higher than the typical specification of 50mA, indicating a component or circuit is not properly shutting down. The standard diagnostic procedure for isolating a high parasitic draw is to systematically remove fuses (or disconnect circuits) one by one while monitoring the ammeter. The circuit that causes the amperage to drop to within specifications is the one containing the faulty component.
Why the other options are wrong
- B. A short to ground in the starter motor would likely cause a much larger current draw, potentially blowing fuses or causing the battery to rapidly drain even while cranking, but it's not the most common cause of a moderate parasitic draw like 500mA when the vehicle is off.
- C. While an alternator can cause a draw (e.g., faulty diode), pulling fuses is a more systematic approach to isolate the specific circuit, and then the alternator circuit would be part of that investigation if the draw persists.
- D. The battery is likely being drained by the parasitic draw, but it's not the cause of the draw itself. Replacing it would only temporarily solve the symptom, not the root cause.
Parasitic Draw Isolation
The process of identifying which circuit or component is causing an excessive electrical drain on the battery when the vehicle is off.
- Involves connecting an ammeter in series with the battery.
- Waiting for modules to 'sleep' is crucial.
- Systematically removing fuses helps pinpoint the faulty circuit.
Memory trick: To find the 'DRAIN', connect the 'METER', then 'PULL FUSES' to make it sweeter.