Wiper Operates Only At One Speed (Same In Lo & Hi)
WARNING: This page does not describe the selected car, but rather 10 other vehicles, including the 1990 GMC Vandura Special, 1990 GMC Vandura, 1990 GMC Rally Camper Special, 1990 GMC Rally, and 1990 GMC Magnavan. However, it is still accessible from the selected car via links, so may be relevant.
Remove wiring from wiper motor terminals No. 17, 18 and 19. Operate wiper with switch in both LO and HI positions. See Figure . If wiper operates correctly, problem is in wiring between wiper switch and motor. If problem is still present, repair wiper motor, checking for low and high speed brush leads shorting together internally.