I have replaced the resistor under the passenger side dash 5 times and as soon as I do the air comes on and works fine for a day or two then stops working again. As soon as the resistor is replaced it immediately starts working properly. I replaced the blower motor about 9 months ago when the first resistor blew thinking that was the problem. Also this is probably a separate problem but air and heat both work fine but only come from the defrost. I have checked all vacuum lines and replaced the vacuum reservoir under the passenger side of the bumper. Jeep is a 99 XJ with 180k miles

 

The blower itself is responsible for cooling the resistor (which gets red hot like the heating elements of a hair dryer). Because of your door mode problem, it sounds like air is not going over the resistor, causing it to meltdown within a short time of being used. Focus on fixing the blend/door issue, and that will likely solve the resistor meltdown issue.

 

Everything I have found relating to the air only coming from the defrost points to a vacuum line issue. I will replace the connectors in the lines and go from there. Thanks for the quick reply

 

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