Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisMedium
A technician is performing a parasitic drain test. With the ignition off and all doors closed, the ammeter connected in series with the battery's negative terminal shows a 500 mA draw. What is the next logical step in diagnosing this excessive drain?
- ABegin pulling fuses one at a time while monitoring the ammeter.
- BCheck the battery's state of charge with a load tester.
- CStart the engine and check charging voltage.
- DDisconnect the alternator to see if the drain stops.
Show answer & explanationAnswer & explanation
Correct answer: A. Begin pulling fuses one at a time while monitoring the ammeter.
A 500 mA (0.5 Ampere) draw is significantly higher than the typical acceptable parasitic drain (usually 20-50 mA). The most common method to isolate the source of an excessive parasitic drain is to systematically remove fuses, one by one, while observing the ammeter. When the current draw drops to an acceptable level, the last fuse pulled indicates the circuit containing the fault.
Why the other options are wrong
- B. While checking the battery's state of charge is important, it won't help identify the source of the drain itself.
- C. Starting the engine would be for charging system diagnosis, not parasitic drain.
- D. Disconnecting the alternator is a valid step if the drain persists after pulling fuses, as the alternator can sometimes cause a drain, but it's not the initial broad diagnostic step.
Parasitic Drain Test
A procedure to measure the amount of current drawn from the battery when the vehicle's ignition is off and all systems are in their 'sleep' state, used to identify excessive battery drain.
- Measured in series with the battery.
- Vehicle must be 'asleep'.
- Acceptable draw typically 20-50 mA.
Memory trick: To find the battery ghost, pull fuses one by one, watching the current meter.