Rotor Test

Test rotor for grounded, open or shorted field coils, using an ohmmeter. See Fig 1 and Fig 2. Test for grounds between each slip ring and rotor shaft. No continuity should exist.
Check for open field by connecting ohmmeter leads across slip rings. Normal resistance reading with rotor at room temperature is 1.7-2.1 ohms for 117 amp. alternator; 1.5-2.0 ohms for all other models.
Readings between 2.5 and 3.0 ohms would result from rotors operating at higher engine compartment temperatures. Readings above 3.5 ohms indicate high resistance, possibly requiring rotor replacement. If reading is below 1.7 ohms (117 amp. alternator) or 1.5 ohms (all other models), the field coil is shorted.

Fig 1: Ohmmeter Probe Connections for Rotor Ground Test

Courtesy of CHRYSLER GROUP, LLC

Fig 2: Ohmmeter Probe Connections for Open or Short Tests

Courtesy of CHRYSLER GROUP, LLC