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Alternator: Basic Functions and Common Problems

Role of the Alternator

The alternator is a device that generates electricity by using the engine’s rotation. It supplies power to electrical components in the car such as headlights, the navigation system, and the air conditioning. It also charges the battery. While the engine is running, the alternator provides most of the electricity used in the vehicle.

Main Components

  • Pulley
  • Rotor (a rotating electromagnet)
  • Slip rings and brushes
  • Stator coil
  • Diode (rectifier)
  • Voltage regulator
  • Cooling fan
  • Housing (case)

Structure and Operation of Alternator

  1. When the engine starts, the rotation of the crankshaft is transferred via a belt to the alternator’s pulley.
  2. This rotation turns the rotor inside the alternator. The rotor functions as an electromagnet by receiving excitation current through the slip rings and brushes.
  3. As the rotor spins rapidly, it creates a changing magnetic field around the stator coil.
    This changing magnetic field induces alternating current (AC) in the stator coil.
    Since the car’s battery and electrical components operate on direct current (DC), the diode (rectifier) converts the AC into DC.
  4. Next, the voltage regulator keeps the voltage stable, ensuring a steady power supply to the battery and electrical systems.
  5. Because the alternator spins at high speeds, it generates heat inside. To cool it down, a built-in cooling fan draws in air to dissipate the heat.
    All these parts are enclosed within the alternator’s housing.

Common Symptoms and Causes of Alternator Problems

Common Symptoms and Possible Causes of Failure

  • Battery Warning Light Turns On
    • Low alternator output (due to a faulty regulator or worn-out brushes)
    • A loose or broken belt
    • Poor electrical connection (such as damaged or corroded wires)
    • The regulator sending out abnormal voltage
  • Battery Frequently Dies (Even After Replacing with a New One)
    • The alternator is not charging properly (low voltage or current output)
    • A faulty stator coil or diode in the alternator
    • Parasitic drain (something is using power even when the ignition is off)
    • Corroded or loose battery terminals
  • Engine is Hard to Start or Won’t Start
    • The battery is not charged due to a faulty alternator
    • A weak or dead battery
    • Damaged or corroded battery cables
  • Headlights or Electrical Components Work Unreliably
    • Unstable output voltage (faulty voltage regulator)
    • Worn slip rings or brushes
    • Low alternator output
    • Faulty diodes
  • Unusual Noise from the Engine Compartment
    • Worn or damaged bearings inside the alternator
    • Squealing drive belt (too tight, too loose, or worn)
    • Broken or rubbing cooling fan
  • A Strong, Sharp Smell
    • The alternator is malfunctioning and supplying too much power to the battery
  • Steering Feels Heavy While Driving
    • The alternator is faulty, causing the electric power steering (EPS) assist to weaken or stop working
  • Engine Stalls While Driving
    • The alternator is not producing enough power, so the electrical system doesn’t get enough electricity
    • The ECU shuts down to protect itself from overvoltage or overcurrent
    • The battery is completely discharged

Alternator Disassembly and Inspection

  1. Rotor Inspection
    After correcting any contamination or roughness on the slip ring surface with sandpaper, check continuity between the slip rings using a circuit tester set to the resistance range. Continuity must be present.
    Next, check insulation between the slip rings and the rotor core using a megger. Contamination such as brush wear powder adhering to the slip ring surface may cause insulation failure; clean and correct as required.
  2. Stator Inspection
    Using a circuit tester set to the resistance range, check continuity between each phase lead wire from the stator and the neutral point (N terminal). Continuity must be present.
    Subsequently, check insulation between the stator windings and the stator core using a megger.
  3. Diode Inspection
    Using a circuit tester set to the resistance range, measure resistance between the B terminal or E terminal and each diode terminal. Perform measurements with the tester probes in both polarities:
    • Positive (+) probe on the B or E terminal and negative (–) probe on the diode terminal.
    • Probes reversed.
    • A diode is considered serviceable if the difference between forward and reverse resistance values is sufficiently large.
  4. Brush Inspection
    Verify that the brushes move smoothly within the brush holders by lightly pressing them with a fingertip. If movement is impeded by dust or brush wear powder, clean and correct as required.
    Measure the brush length from the end of the holder using a vernier caliper. Replace brushes if wear exceeds the specified limit.
  5. Bearing Inspection
    Rotate the bearings lightly with the fingers. The bearings must rotate smoothly without roughness, abnormal noise, or excessive play. Replace bearings if any defect is found.
  • Performance Test (Using Alternator Tester)
    Conduct the following tests with an alternator tester:
    • No-load rotational speed test
    • Output rotational speed test
    • Output voltage test
  • Oscilloscope Test
    Set the oscilloscope to the specified range. Connect the probe to the alternator B terminal and the ground lead to the E terminal.
    Start the engine and observe the waveform at idle speed. If the stator windings and diodes are normal, a proper voltage waveform will be obtained.
    Under load conditions, the waveform may fluctuate due to voltage regulator control. Wait until the waveform stabilizes, then reconfirm.
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