I have a 2001 impala and the blower motor only runs on high. I bought a new resistor and it had the same problem. Next i replaced the blower motor and it ran on low for a moment then quit, would only run on high again. I have voltage at the harness for each individual speed and at the connector to the motor itsself. Only had a test light because my meter is busted. The ground is ok also. Checked it with a continuity tester. I am stumped. Any help would be apreciated. Thanks.

 

The ground is what controls the speed, the resistor varies the resistance to ground thus speeding up or slowing down the motor. High speed is perfect continuity to ground. I had a 2010 Colorado last week that ran on high as soon as you turn on the key,even when the blower switch was in off. It would still run on high with the resistor un plugged. I found the blower wire harness had rubbed through on the metal lower glove box support thus giving it a perfect ground. Try wiggling the blower motor harness while the blower is on, see if it changes speed.

 

Looked for a bad wire and could not find anything. It does shut off with the switch. waiting for my meter to show up so i can compare voltage at the connector at the motor to see if this is any different between between speeds. Dont really know what else to look for.

 

Check the blower motor switch.

 

Had a graduation to get ready for so I just got back to it. Pulled the switch and it had continuity at it's respective spades. Checked the resistor and it increases with resistance in each spot. double checked voltage at the harness into the resistor and had between 9-10 volts at each speed except high which had almost 11. shouldn't it have 12 all the way threw and the resistor steps it down. Still only got high. The bcm doesnt have any thing to do with it does it? the hayes manual does not look like it does. Anybody anything? Thanks again!

 

The low voltage was cause by a burnt contact in the ignition switch. I took it apart cleaned it up and it worked. Also fixed my cruise problem. Hope this helps somebody.