Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsStarting System Diagnosis and RepairMedium
A technician is performing a voltage drop test across the main battery cables during engine cranking. The negative battery cable (from battery post to engine block) shows a reading of 0.8 volts. What does this reading indicate?
- AThe negative battery cable has excessive resistance, impeding current flow.
- BThe battery is fully charged and providing sufficient voltage.
- CThe starter motor is drawing excessive current.
- DThe positive battery cable is faulty and requires replacement.
Show answer & explanationAnswer & explanation
Correct answer: A. The negative battery cable has excessive resistance, impeding current flow.
A voltage drop of 0.8 volts across the negative battery cable is higher than the typical specification (usually 0.2V or less), indicating excessive resistance in the cable or its connections. This impedance will restrict current flow to the starter motor.
Why the other options are wrong
- B. Voltage drop testing measures resistance in a circuit, not the battery's state of charge, although a healthy battery is needed for accurate testing.
- C. Excessive voltage drop is caused by resistance in the cable itself, not by the starter motor drawing too much current, although high current draw can exacerbate the drop if the cable is already compromised.
- D. The test result specifically refers to the negative cable; it does not directly indicate an issue with the positive cable.
Voltage Drop Test (Negative Cable)
Measures the voltage lost across the negative battery cable (from battery post to engine block) during cranking to identify excessive resistance.
- Performed with a voltmeter while the engine is cranking.
- Acceptable voltage drop is typically 0.2V or less.
- Higher readings indicate excessive resistance in the cable or its connections.
Memory trick: High drop in voltage means high resistance, a bad connection or cable's existence.