Two things commonly cause repeat failures, especially when complicated, unreliable computers are added to do things computers were never needed for before. In this case, suspect the fan motor is tight and drawing high current. That will overheat the high-power switching transistor in the module. Heat is the deadly enemy of transistors. In this circuit they are turning on and off hundreds of times per second. The ratio of "on-time" to "off-time" is varied to vary fan speed. When the transistor overheats it will short and remain turned on all the time. That's why the fan would run at high speed.
A second common problem is something in the wiring or a corroded connector pin. Unplugging a connector will often make it scratch a clean spot that will let it work when it's reconnected, ... Until it gets corroded again. That corrosion is usually just a very light coating and can be hard to see.
The place to start is by measuring the current flow to the fan motor. I don't know what "normal" is but I'd expect to see around six to eight amps on the highest speed. An experienced AC mechanic would know for sure what's normal.
Tuesday, December 6th, 2011 AT 10:35 PM