Automotive Alternator System Repair and Testing

Components of an Alternator

The alternator is formed by the stator, rotor, diode board, fan, pulley, brushes, and regulator.

Alternator Operation

When the motor turns the rotor within the stator, the rotor's magnetic poles (+ and -) produce a magnetic field, generating a current. Alternatively, this current is sent to a diode pack (+ and -), which converts it into continuous AC. The controller then regulates the electric current produced due to the applied electrical power.

Function of Regulators

The regulating circuit maintains current intensity such that all consumers receive approximately 14V.

• Excitation Terminal (Exc): Connects to the alternator output (+).
• Brushes (KE): The brushes feed power into the alternator.

Precautions When Servicing the Alternator

When working on the unit, do not start the engine, as this could damage the meter. Do not operate the vehicle without the battery and regulator connected, as this could break components of the vehicle's electrical system.

Verification of the Charging Circuit

The charge provided by the alternator to the battery should be 14V ± 0.5V. The ammeter should show a charge slightly superior to 4A at idle. The voltmeter should read 14.5V, indicating the regulator is functioning properly.

Checking Battery Charge

To check for discharge, disconnect one battery terminal cable. Use an ohmmeter between the disconnected cable and the terminal; if there is continuity, there is a discharge somewhere.

Rotor Testing

Using an ohmmeter across the slip rings, you should measure 2.0–3.0 Ohms. To check for a short to ground, touching the ring to the chassis ground should show no continuity.

Stator Testing

Test continuity between each stator terminal and ground; there should be none. The three stator points should have continuity between each other.

Diode Testing (Excitation)

Probe between the center of the star connection and the output.

Increasing Alternator Current and Tension

Increasing the current applied from the controller increases the magnetic field.

Stator Coil Connections

The stator coils can be connected in triangle (delta) or star (wye) configuration.

Where the cable from the center of the star connects:

• It is connected to the battery and used as a witness light (control light).

Diagnosing Failures

If the battery is discharged, the fault may be in the alternator or the regulator. If the regulator winding is suspected, check it with an ohmmeter. Ensure the plates are clean; dirt prevents good contact.

Terminal Locations

Where are the B+ and D+ terminals on the Alternator?

• B+ connects to the battery output.
• D+ connects from the diodes to the regulator excitation circuit.