Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsCharging System Diagnosis and RepairHard
A technician is diagnosing a 'battery light on' condition. With the engine running, a voltmeter measures 12.2V at the battery and 12.4V at the alternator's B+ terminal. The battery passes a load test. A scope test on the alternator's B+ output shows excessive AC ripple voltage, exceeding 0.5V. What is the most likely cause?
- AA faulty voltage regulator.
- BOne or more shorted or open rectifier diodes.
- CAn open stator winding.
- DA shorted or open field winding.
Show answer & explanationAnswer & explanation
Correct answer: B. One or more shorted or open rectifier diodes.
Excessive AC ripple voltage (above 0.5V) on the alternator's DC output is a classic indicator of a problem within the rectifier assembly, specifically one or more shorted or open diodes. These faults cause incomplete rectification of the AC voltage, allowing AC components to pass through to the DC output.
Why the other options are wrong
- A. A faulty voltage regulator would primarily affect the DC voltage level (over/undercharge) but wouldn't directly cause excessive AC ripple unless it also impacts the rectifier's operation, which is less common for this specific symptom.
- C. An open stator winding would typically reduce or eliminate output altogether, not primarily cause excessive AC ripple on the remaining output.
- D. A shorted or open field winding would lead to no or very low output voltage, not primarily excessive AC ripple.
AC Ripple Voltage Diagnosis
AC ripple voltage on the DC output of an alternator indicates incomplete rectification of the alternating current, commonly caused by a fault in the diode rectifier assembly.
- Measured with an oscilloscope or DMM with AC function.
- Normal ripple should be less than 0.5V (typically under 0.2V).
- Excessive ripple can damage electronic components.
Memory trick: If the AC waves are still there, the diodes aren't doing their share.