Alternator: Testing and Inspection

Fig. 1 Testing rotor coil resistance:

Alternator must be removed from vehicle for testing.
1. Check rotor coil resistance using an ohmmeter across terminals F and E, Fig. 1. Reading should be 4.2 ohms. If resistance is zero, a short is indicated. If resistance is infinite, poor brush contact, broken lead wire or an open rotor coil is indicated.

Fig. 2 Positive diode test connections:

2. Connect positive lead of ohmmeter to B terminal and negative lead to N terminal, Fig. 2. If continuity exists between terminals, one or more of the three positive diodes are shorted.

Fig. 3 Negative diode test connections:

3. Connect positive lead of ohmmeter to N terminal and negative E terminal, Fig. 3. If continuity exists between terminals, then one or more of the three negative diodes are shorted.