Rotor Field Winding

Check rotor for grounds or an open circuit, using either a 110 volt test lamp or an ohmmeter set at the x 1000 scale. See Fig 1 . To test for grounds, touch leads to rotor shaft and, in turn, to each slip ring. Reading should be very high (infinite). No continuity should exist with either slip ring (test lamp should not light). If lamp lights or if continuity is shown, check soldered connections at slip rings. Replace rotor, if connections are OK.
Test for an open field by connecting an ohmmeter set to the x1 scale, touching leads to each slip ring. Continuity should exist, with a resistance of 2.4-3.5 ohms. If infinite resistance is indicated, replace rotor.
To test for shorts, connect a 12 volt battery and ammeter in series with both slip rings (or an ohmmeter may be used). See Fig 1 . Current draw should be 4.0-4.5 amps. Ohmmeter resistance should be 2.4-3.5 ohms (2.75-3.0 ohms for heavy-duty alternator).

Fig 1: Bench Testing Rotor For Opens, Shorts And Grounds
Courtesy of GENERAL MOTORS COMPANY
Excessive amperage draw or low resistance readings indicate shorted windings. Replace rotor. If rotor tests okay, but alternator output is low, continue with other tests.