Riding down the road on one of our hot summer days. The a/c is working textbook then all of a sudden the HVAC blower fan jumps to high speed and doesn't respond to the fan speed control. I get home, turn off the ignition and the fan is still blowing wide open. I pulled the under hood fan fuse to kill it. All the expert advice I can find says that the blower control module (BCM) is bad so I changed out the BCM and now I can't get the fan to blow at all. I tried another BCM (different brand) and still no fan. I did all of the electrical testing for the BCM and all of that reads normal: 13.8V main battery feed from the 40A under hood fuse and the two fuses under the back seat that power the HVAC module are good at 13.8V. The varying ground pulse on the purple wire from the HVAX module (which is supposed to govern the fan speed) goes from 0V to 13.8V as it should. All indications are that the fan should be blowing but it's not. I also tried a new HVAC control module and still no fan. I also found that the rear HVAC control (rear of the console facing the back seat) is dead and I have the replacement for that on order but I can't see from the schematics how that would effect the blower fan. So I'm thoroughly stumped at this point. Anybody got any ideas here?

 

Oops. Forgot to add this is a 2004 Trailblazer, 4.2L, with the automatic climate control.

 

have you tried applying 12v directly to the motor?

Please post the schematics you have.

 

Yes. I tested the fan separately.

 

Check the ground for the fan.

 

Problem solved. It was an intermittent in the 5-pin connector. Once again proving that intermittents send more mechanics to Hell than any other cause.

 

Thanks for letting us know you found the problem and what the problem was!