Most would say it is the resistor causing the fan speed issue. So not sure on that one. I see other swap out the control unit without much success. Maybe check the wire harness to make sure all the pins contact well.

For the issue with the air only blowing out of the defrost vents, that is an issue with the mode actuator which is located just above the accelerator pedal to the right. It controls where the air is directed. Very common for them to fail.

 

The connectors as known to burn out because they are just barely good enough for the job. Examine them, and perhaps test them for voltage (different pins will get voltage depending on hvac blower setting).

 

It has become apparent that the highest fan speed will bypass the resister and that if that occurs and the fan is fine at the highest setting, the resister assembly is shot and it needs to be replaced. Have you solved your other issues?

 

My 1996 Sierra 2500 has the same issue; the only setting (on dashboard rotary-switch) that activates the blower-fan is 'high' setting. Assuming this (older) model ALSO has a 'resistor-assembly', would you happen to have a picture of it that you can send (in reply to this posting) so that I have a better-idea of what I'm looking for ? ? Much obliged, if you (or anybody-else) can help me with this, because I really rather-enjoyed the fan when I could run it on 'low' or 'medium' speeds ! ! !

 

@Lewtentent FYI, that member has not been on this site for two years. You're unlikely to get a response.