Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsCharging System Diagnosis and RepairMedium
A technician is diagnosing a 'no-charge' condition on a vehicle where the battery light is illuminated. After verifying the battery is good and the drive belt is properly tensioned, the technician performs a voltage drop test on the charging circuit's positive side. With the engine running and all accessories on, the voltage drop between the alternator's B+ terminal and the battery's positive post is 0.6V. What does this indicate?
- AThe alternator is operating correctly.
- BExcessive resistance in the positive charging cable.
- CThe voltage regulator is faulty.
- DA short circuit in the vehicle's wiring harness.
Show answer & explanationAnswer & explanation
Correct answer: B. Excessive resistance in the positive charging cable.
A voltage drop of 0.6V between the alternator's B+ terminal and the battery's positive post indicates excessive resistance in the positive charging cable or its connections. A healthy positive circuit should have a voltage drop of no more than 0.2V (0.5V for heavy-duty systems) under load.
Why the other options are wrong
- A. A 0.6V drop indicates a problem in the circuit, preventing the alternator from fully charging the battery.
- C. While a faulty regulator could cause a no-charge, this voltage drop specifically points to cable resistance, not regulator failure.
- D. A short circuit would likely cause a blown fuse or melted wire, not just a voltage drop of this magnitude in the charging cable.
Charging Circuit Voltage Drop
Voltage drop tests measure resistance in a circuit. On the positive side of the charging circuit (alternator B+ to battery positive), a drop exceeding 0.2V (or 0.5V for heavy-duty) indicates excessive resistance, often due to corroded cables or loose connections.
- Positive side drop limit: ~0.2V (passenger car).
- Negative (ground) side drop limit: ~0.1V.
- High voltage drop reduces charging efficiency.
Memory trick: Voltage dropped, resistance hopped, charge stopped.