Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsCharging System Diagnosis and RepairHard
A technician is performing a voltage drop test on the charging system's positive battery cable. With the engine running at 2000 RPM and the headlights on high beam, a voltmeter is connected between the battery positive post and the alternator B+ terminal. The reading is 0.8 volts. What does this indicate?
- ANormal resistance in the positive battery cable.
- BA faulty alternator producing insufficient voltage.
- CExcessive resistance in the positive battery cable circuit.
- DA discharged battery drawing excessive current.
Show answer & explanationAnswer & explanation
Correct answer: C. Excessive resistance in the positive battery cable circuit.
A voltage drop of 0.8 volts across the positive battery cable during a charging system test with a load is excessive. Acceptable voltage drop across a main charging cable should typically be 0.2 volts or less. This indicates high resistance in the cable or its connections, impeding the flow of charging current.
Why the other options are wrong
- A. 0.8 volts is well above the acceptable range (typically <0.2V) for a main power cable, indicating abnormal resistance.
- B. A faulty alternator would be indicated by low voltage at its B+ terminal relative to ground, not necessarily a high voltage drop across the cable itself if the alternator is producing output.
- D. A discharged battery would draw more current, which can exacerbate voltage drop, but 0.8V across the cable still points to an issue with the cable's resistance, not just the battery condition.
Voltage Drop Test (Charging Circuit)
A voltage drop test measures the resistance within a specific circuit segment by comparing voltage readings at different points, indicating how much voltage is 'lost' across that segment under load.
- Performed with a voltmeter across the component or wire.
- Lower voltage drop indicates lower resistance.
- Typically, <0.2V drop is acceptable for main power/ground cables.
Memory trick: If the voltage drops high on the cable, its resistance is a stable fable.