Automotive Service Excellence (ASE) A6 Electrical/Electronic SystemsCharging System Diagnosis and RepairMedium
A technician is diagnosing a vehicle with a continuously illuminated battery warning light. The battery voltage with the engine off is 12.4V. With the engine running at 2000 RPM, the voltage at the battery terminals remains at 12.4V. A check of the main alternator output cable to the battery shows 0.0V drop, and the alternator case to battery negative has 0.0V drop. The technician then checks for AC ripple at the alternator output and measures 0.8V AC. What is the most likely cause?
- AA shorted field coil in the alternator.
- BA loose alternator drive belt.
- CA faulty voltage regulator.
- DAn open diode in the rectifier bridge.
Show answer & explanationAnswer & explanation
Correct answer: D. An open diode in the rectifier bridge.
A high AC ripple voltage (typically above 0.5V AC) at the alternator output, especially when the DC voltage is not increasing, is a strong indicator of a faulty diode in the rectifier bridge. An open diode allows AC current to pass through without being properly rectified, reducing DC output and increasing ripple.
Why the other options are wrong
- A. A shorted field coil would result in no magnetic field, thus no output, but not necessarily high AC ripple.
- B. A loose alternator drive belt would cause slippage and undercharging, but typically wouldn't produce high AC ripple; it would simply reduce overall output.
- C. A faulty voltage regulator would lead to consistent over- or under-charging, but usually doesn't cause high AC ripple unless it's impacting the rectifier directly.
AC Ripple Voltage (Rectifier)
Excessive AC ripple voltage (typically >0.5V AC) in a charging system indicates a problem with the alternator's rectifier bridge, usually an open or shorted diode, leading to poor DC conversion.
- Normal AC ripple should be less than 0.5V AC.
- High ripple indicates faulty rectifier diodes.
- Can cause electrical interference and battery damage.
Memory trick: Ripple high, diode's cry, DC output starting to die.