Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsGeneral Electrical/Electronic System DiagnosisMedium
A technician is performing a parasitic drain test on a vehicle that has a dead battery overnight. After connecting an ammeter in series with the battery negative terminal, the initial draw is 3.5 amps. After waiting for 15 minutes, the draw drops to 0.8 amps, which is still too high. What is the most appropriate next step to locate the source of the excessive drain?
- AImmediately begin pulling fuses one by one and observing the ammeter.
- BStart the engine and check the charging system output.
- CDisconnect the alternator and re-check the parasitic draw.
- DConsult the wiring diagram to identify circuits that are always powered.
Show answer & explanationAnswer & explanation
Correct answer: D. Consult the wiring diagram to identify circuits that are always powered.
Before indiscriminately pulling fuses, consulting the wiring diagram allows the technician to identify circuits that are always powered (e.g., memory, security, convenience modules). This systematic approach helps narrow down potential culprits and avoid disturbing critical modules unnecessarily. Pulling fuses directly can sometimes wake up modules or clear learned values, making diagnosis harder.
Why the other options are wrong
- A. While a common technique, it's more efficient and less disruptive to first identify likely circuits from a diagram.
- B. The charging system output is relevant for battery charging, not for identifying a parasitic drain when the vehicle is off.
- C. The alternator is a potential source, but it's one of many. Consulting the diagram helps prioritize and cover other possibilities.
Parasitic Drain Diagnostic Strategy
A systematic approach to identifying the source of an excessive electrical current draw when a vehicle is shut off, which can lead to battery discharge.
- Allow modules to 'go to sleep' before taking final readings.
- Draws typically should be < 50mA (0.05A).
- Use wiring diagrams to identify constant power circuits.
- Isolate circuits by pulling fuses or disconnecting components.
Memory trick: WAIT, then DIAGRAM, then PULL.