The blower motor is a variable speed motor. The higher the voltage applied to the motor, the faster the speed. Depending on the HVAC (Heater Ventilation Air Conditioning) option installed in the vehicle, blower speed control could be through a set of resistors or through a solid-state blower motor control module.
Battery voltage to the blower motor is supplied by the heater & A/C control by way of the blower resistor (or blower motor control module). At low and medium speeds, the voltage is stepped down by the blower motor resistors. At high speed, the blower motor relay is energized, removing the blower motor resistors from the circuit. Battery voltage is then applied directly to the blower motor through the relay. The motor will then run at maximum speed.
Several types of HVAC systems were available on these vehicles, from manual to fully automatic electronic control. The blower motor speed can be adjusted manually by pushing the fan switch up or down or automatically by placing the heater & A/C control in the AUTO mode.
In general, this system is reliable. Because so many of the control operations are electronic, special diagnostic equipment really should be used for system diagnostics and check-out. An authorized technician using a scan tool can most quickly locate HVAC problems.
An inoperative blower motor could be caused by the following: