Blower Motor Stays on High
If your Blower Motor Stays on High you should suspect a burned out blower motor resistor—unless your vehicle has auto temp control (climate control). Those systems do not use a resistor. They vary the fan speed by sending a varying voltage from the temp control to the blower motor power module.
To test the system, connect a digital volt meter to the dark blue/light blue wire at the blower motor power module and see if the voltage varies as you change the fan speed with the control knob. If the voltage varies, then the control head is sending the proper signal and the blower motor power module is bad. Replace the module and you should get your blower working properly again.
© 2012 Rick Muscoplat