Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsStarting System Diagnosis and RepairMedium
A vehicle exhibits slow cranking, especially when the engine is cold. A battery load test indicates the battery is good. A technician measures 0.6 volts across the positive battery cable (from battery post to starter B+ terminal) during cranking. What is the most appropriate next step in diagnosing this issue?
- AReplace the starter motor.
- BPerform a parasitic draw test.
- CInspect and clean the positive battery cable connections.
- DCheck the battery's state of charge with a hydrometer.
Show answer & explanationAnswer & explanation
Correct answer: C. Inspect and clean the positive battery cable connections.
A voltage drop of 0.6 volts across the positive battery cable is excessive (typically should be 0.2V or less). This indicates high resistance in the cable or its connections, which will cause slow cranking despite a good battery. The next logical step is to address this resistance.
Why the other options are wrong
- A. While a bad starter can cause slow cranking, the excessive voltage drop on the cable points to a problem external to the starter motor itself as the primary cause.
- B. A parasitic draw would discharge the battery over time, but the battery load test already confirmed the battery is 'good' at the time of testing.
- D. The battery load test already indicated the battery is good, so further testing of its state of charge is redundant at this point.
Excessive Voltage Drop Diagnosis
Identifying the source of high resistance in a circuit based on voltage drop readings to resolve performance issues like slow cranking.
- Voltage drop exceeding 0.2V on main battery cables indicates a problem.
- High resistance can be due to corrosion, loose connections, or damaged cables.
- Addressing the source of resistance (e.g., cleaning connections) is the corrective action.
Memory trick: Slow crank, good battery, high drop? Clean the connections, don't stop!