Bench Testing: Rotor

WARNING: This page is about a different car, the 2004 Subaru Legacy. However, it is still accessible from the selected car via links, so may be relevant.

Inspect the slip rings for contamination or any roughness of the sliding surface. Repair the slip ring surface using a lathe or sand paper.
Measure the slip ring outer diameter. See ROTOR ASSEMBLY SPECIFICATIONS table under GENERATOR SPECIFICATIONS. If the slip ring is worn replace rotor assembly.
Check the resistance between slip rings using circuit tester. See Fig 1.
Check the continuity between slip ring and rotor core or shaft. See Fig 2. If resistance is 1 ohm or less, the rotor coil is grounded, and so replace the rotor assembly.
Check the rear ball bearing. Replace if it is noisy or if the rotor does not turn smoothly.

Fig 1: Checking Resistance Between Slip Rings

Courtesy of SUBARU OF AMERICA, INC.

Fig 2: Checking Continuity Between Slip Ring & Rotor Core Or Shaft

Courtesy of SUBARU OF AMERICA, INC.