Fan blower only works on high speed....

Incredibly enough, and this may sound counter-intuitive, but I had a bad ground where it connects into the blower motor behave in the same manner. I trouble shot the system all the way from the resistor, to the relay, to the switch with no definitive diagnosis.

The next step was to backprobe from the motor. I removed the panel/vent assembly from the passengers side to get to the motor, and since it happened to be after dark, I noticed arcing as the motor cut in and out, low to high... The motor ground was held on only with a small sheet metal screw mounted to the motor/blower chassis. When I tightened the screw everything worked properly...and at all speeds. After a few off/on cycles, the blower motor stopped working or would work only on high. The screw had so few threads in contact that it had backed itself out and would not tighten. I speculate that it would sometimes work on high only because the higher voltage bridged the gap at the bad ground contact. (thus the arcing)

I removed the blower/motor assembly, filed the paint from the area around where the ground screw mounted, and installed a bolt with a lock washer and nut into the hole. When I reassembled the blower/motor assembly I attached the ground to the bolt with a lock washer and nut. I have not had an issue since.

We went on a short trip this past week, and lo and behold the fan seemed to work just fine. All speeds worked as they are supposed to. Of course this was right after I had orderd a new dash fan switch!

So now I have an extra resistor assembly and fan switch for when the problems start again....the only thing I can think of that might have fixed the problem is that I pulled the wire harness off the switch while trouble shooting and maybe I improved the connection when putting things back to normal to go on our trip. It was difficult to get the harness back on so I used some electrical (light bulb) grease to help.

OK...I tried a new resistor assembly, and that did not fix the problem. As I note above, it seems that there was voltage to the resistor regardless of the switch position, but I've gone ahead and ordered a new switch....hopefully that will do it!

(Anyone know why the "Subscribe and send email..." option isn't working for me?)

FWIW...this is a part of the directions I used, from my Ford Expedition service manual, to to troubleshoot my RV blower problems.

I can't tell you, because I can't remember, which pin on the RV blower motor resistor block corresponds to which in the manual and I'm certainly not going to pull mine out to figure it out.

There's also a section on troubleshooting the relay and switch.

fourmat wrote:
with your fan switch in the lower settings do you have 12V to the resistor if you do then replace the resistor if you don't then it is the fan switch or wiring.

The fan switch seemed to provide 12v to the resistors no matter which setting it was on. I think I'll just replace the resistor assembly first as they are not that expensive ($12 vs $40 for the switch!). The black spot on the switch did NOT appear to be a burned area, but just grease or some fastening material.
Thanks to all for the help....I'll let you know the outcome!

Looking more closely at the switch. What is the small dark spot in the center? Is it burned right there or is it just grease or something?

From the picture it appears that the thermal fuse is the component that has over heated and failed open (like it was designed to do if the area gets too hot). The thermal fuse is that small cylinder shaped device. JMO

Your resistor doesn't "look" bad and the heat marks on the coil are normal as it's burning off energy instead of spinning the blower fast. You need to find the schematic or diagram for your chassis.

Do any of these look like your system
these?

Don't replace anything until you prove what is wrong. This may help to further troubleshoot. Notice in this video that there are 2 relays. Not that it's exactly your system. just one example.

with your fan switch in the lower settings do you have 12V to the resistor if you do then replace the resistor if you don't then it is the fan switch or wiring.