Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsCharging System Diagnosis and RepairMedium
A technician is diagnosing a no-charge condition on a vehicle. With the engine running, a voltmeter connected from the alternator's B+ terminal to ground reads 14.2V. However, the battery voltage simultaneously reads 12.5V. What is the MOST probable cause?
- AFaulty voltage regulator
- BExcessive parasitic draw
- CInternally shorted battery
- DOpen circuit in the B+ cable between the alternator and battery
Show answer & explanationAnswer & explanation
Correct answer: D. Open circuit in the B+ cable between the alternator and battery
The alternator is producing 14.2V, indicating it's functioning correctly and the voltage regulator is doing its job. However, the battery is only receiving 12.5V, meaning there's a significant voltage drop or an open circuit between the alternator's output and the battery. An open circuit in the B+ cable (main power cable) would prevent the alternator's output from reaching the battery, leading to this discrepancy.
Why the other options are wrong
- A. A faulty voltage regulator would typically result in incorrect voltage output FROM the alternator, not a difference between alternator output and battery input.
- B. Excessive parasitic draw would drain the battery when the engine is off, but the alternator would still deliver its full output to the circuit when running, even if the battery charges slowly.
- C. An internally shorted battery would quickly drain, potentially boil, and not hold any charge, but the alternator would still attempt to charge it, likely showing a lower voltage at the alternator as well due to the short.
Charging Circuit Open
An interruption in the electrical path between the alternator's output and the battery, preventing charging current from reaching the battery.
- Prevents current flow.
- Alternator may show good output.
- Battery remains uncharged.
Memory trick: Good Alternator, Bad Connection: Trace the Wire, Find the Problem.