Field Coil Open Test
WARNING: This page does not describe the selected car, but rather 10 other vehicles, including the 1998 GMC Yukon, 1998 GMC Suburban, 1998 GMC Pickup, 1998 GMC Cab & Chassis, and 1998 GMC C3500 HD. However, it is still accessible from the selected car via links, so may be relevant.
Remove armature from frame and field assembly. Using self-powered test light, place one lead on field lead connection and other lead on one insulated brush. See Fig 1. Test light should illuminate. If test light does not illuminate, field coil is open. Replace field and frame assembly. Repeat test for each insulated brush.