2000 ford Taurus with about 300,000 miles has the AC fan stuck on high. I have replaced the blower motor resistor three times (which solved the problem) and they last about one month. Apparently, something is causing the resistor to go out. Otherwise the AC is working fine. Next diagnostic steps? Not the end of the world with a vehicle with 300,000 miles, but I am curious. If it is a simple fix, I will do it, otherwise I will live with it.
The blower is worn out and drawing too much current, burning out the resistors.
the high speed relay probably welded itself shut
New blower switch.
I believe the resistor that I replaced is in fact the switch. https://www.napaonline.com/en/p/ECHBR306?impressionRank=2
Are you referring to some device that the dial is directly connected to?