A technician is performing a parasitic drain test on a vehicle that has a history of a dead battery after sitting for a few days. After installing an ammeter in series with the negative battery cable and waiting for the modules to 'go to sleep', the technician measures a current draw of 150 mA. What is the MOST appropriate next step for the technician?
- ADischarge the battery completely and then recharge it to recondition it.
- BBegin systematically pulling fuses one by one while monitoring the ammeter.
- CStart the engine to ensure the charging system is functioning correctly.
- DImmediately replace the battery as it is likely weak.
Show answer & explanationAnswer & explanation
Correct answer: B. Begin systematically pulling fuses one by one while monitoring the ammeter.
A parasitic drain of 150 mA (milliamperes) is excessive; most vehicles should have a drain of less than 50 mA (and often much lower, like 20-30 mA) after all modules have gone to sleep. To pinpoint the source of this excessive draw, the next step is to systematically pull fuses one by one from the fuse boxes, monitoring the ammeter after each fuse is removed. When the current draw significantly drops, the last fuse pulled identifies the circuit containing the parasitic draw.
Why the other options are wrong
- A. Discharging and recharging a battery will not fix a parasitic drain; it's a diagnostic step, not a solution for the underlying problem.
- C. Checking the charging system is for when the battery dies while driving, not when it dies after sitting, which points to a parasitic drain.
- D. While a weak battery might exacerbate the problem, the 150 mA draw is the root cause of the battery dying, not necessarily a symptom of a weak battery itself. Replacing the battery without fixing the drain will only lead to another dead battery.
Parasitic Drain Isolation
To isolate an excessive parasitic drain, systematically remove fuses one by one from the vehicle's fuse boxes while monitoring the ammeter. A significant drop in current draw indicates the circuit responsible for the drain.
- Excessive drain: >50 mA (general guideline).
- Ammeter must be connected in series with battery cable.
- Allow modules to 'sleep' before testing (usually 20-60 mins).
Memory trick: High drain, fuse pull, find the culprit, rule the school.