Auto Air Conditioning & Heater Repair: BMW A/C, 2006 bmw 325i, head pulses

Question
Hello, I have a 2006 BMW 325i. The first symptom was my A/C would not go to the highest setting (speed). Then a day later it worked fine and would go to the highest setting. Then the next day it would not go to the highest setting again plus it would stop blowing then start again. I drove on the freeway for 30 minutes and the whole time the blower would start and stop. It blows cold air fine but would start and stop the whole drive home. Any suggestions what the problem could be or troubleshooting I can do? Thanks in advance

Answer
Chris,

 Without a graphing meter or scope it is difficult to make a 100 percent positive diagnosis but what I can tell you is what 90 percent of BMW's need when this symptom arises.
 BMW uses a final stage transistor module to control fan speed.  The HVAC head pulses a low current signal to the transistor and it duty cycles a high amperage circuit for the blower.  The are prone to overheating and premature failure.  A failing blower motor can cause damage to the final stage module as well but this is very uncommon.
 The final stage module is located in the air steam (to help keep it cool) and has an electrical connector. It is located in the hvac box on the passengers side left footwell.  

Wayne