Hi, I have a 2001 Blazer with fan speeds 1,2 3 working but 4 is not.

I was told that it is highly likely that the relay for the Hi-Speed motor needs to be replaced. I am picking one up tonight but my questions where does get installed? Is it under the hood?

Thanks.

 

This applies to manual HVAC only, not automatic climate control.


There are only a few things that can cause only the high speed to be inoperative. Check fuse #9 in the instrument panel fuse block first. A faulty high blower relay would be the next place to look for a problem. The high blower relay has 5 wires and it's mounted to the center of the firewall, under the hood. The wire colors are: Black, purple, 2 orange and a dark blue.

 

I have a short in the connector that plugs into the relay. I sometimes have fan speeds 1 & 2 but usually not 3. If I wiggle the connector around I can get the 3 to work as long as I am applying pressure to it. I have the connector but have not had a chance to install it yet. To find it I turned the key to the on position but did not start the engine (so I could hear the blower). I placed the speed selector to 3 and wiggled the wires at the connector around. The fan will sometimes come on at 3, most of the time at 1 & 2. I always have fan speed 4 (default speed).

 

Possum, The high blower relay affects all 3 lower speeds equally. They either all work, or none of them work. When speeds 1, 2, or 3 are selected, resisted voltage from the blower motor resistor travels through the relay in the unlatched position. When you select high speed, the relay latches and simultaneously removes voltage from all 3 lower speeds, and applies battery voltage directly to the blower motor.


If you lose any one or more of the lower speeds, the blower resistor is more than likely the problem. The blower resistor wire harness is near the relay, and disturbing the wires on the relay might be wiggling the wires on the resistor causing speed 3 to fail. Could be a faulty connector on the blower resistor, a bad resistor, or a wire in the process of breaking.

 

It is the connector to the blower resistor that is bad not the relay. I got my terms mixed up. See attached picture.

 

Yup, that's the resistor. That's the piece that changes resistance to the blower motor. Each of those "spring" looking things has a different resistance. When/as they burn up, you lose a speed, (except high) no resistor is used for high speed. The resistor assembly is totally bypassed by the high blower relay when it latches.