Yesterday I was using my defrost on high, worked fine. When I switched the fan to Medium it shut off, would not work on Low either.
When switched back to High it works fine. Nothing on Low or Medium. This occurs with heat, air, and defrost. 2003 Dodge Ram 5.7 L four door SLT. Any help would be greatly appreciated. Thank You, Bruce

 

Sounds to me like there is something wrong with your fan motor...

 

Try this, take off the negative battery cable from battery, wait about 5 minutes for everything to lose power. Pull wiring harness off from HVAC assembly. Turn the **** to the lowest speed, check continuity between ground and low speed. Do the same for the medium 2 speeds and high speed. If you have continuity at each location, your control is bad.

 

This is the classic symptom of the blower motor resistor having a blown thermal fuse. Replacing a blower motor resistor is easy and cheap ($18-25$). The resistor is accessed from inside the cab in the HVAC unit just above the passenger foot area. As a test, I pulled a BMR, cut out the thermal fuse, and crimped the fuse wires together. Installed it and all speeds worked. Since I didn't want a fire to start, I replaced it with a new BMR. It blew in 1 day. Turns out the blower motor was worn to the point that it didn't move enough air on the lowest setting and cause the thermal fuse to blow. Replaced the blower motor with a new one, plus a new BMR, and everything is okay. I've seen elsewhere where sometimes the blower motor relay is the problem. But, if the blower motor works on the high setting only, check the BMR first.

 

Replace the blower resistor.

 

it very well may be as simple as replacing the resistor, and even though its a cheap fix i would definately check before just going out and replacing it