I've been told that this is possibly your problem.
Customer Concern: HVAC. Has the auto a/c system. The fan is running on high speed at all times. You can turn the control panel completely off and the blower fan is still running on high speed.
Tests/Procedures: 1. Check for a voltage signal on the Blue/Yellow wire at the fan control amp . If it has a no voltage, then the fan amp would be bad. If it has a continuous "high" voltage signal, then the control panel may be at fault. For low to medium speed, it should have 2 to 4 volt signal, for high speed, it should be about 9 to 10 volts.
Potential Causes: Blower Fan Amplifier