I don't know if I have an answer for you, but I can share my experience.

I bought my 78 Bronco last September. The heater in it didn't work, so I decided to fix it. I checked the fuse for the heater and it looked ok, but I replaced it anyway. The fan switch was burnt from what I assume was a short, so I replaced it. Nothing changed, the fan never came on. I checked the resistor with a test light and it worked on all settings as I moved the fan switch. I went all the way down to the plug at the bottom of the heater assembly where it plugs into the fan motor by the floorboards. I was getting power there, so I figured the fan motor was bad. I pulled everything out and got the fan motor out. I clipped the wiring, since I was going to replace the motor anyway, and I hooked it straight to my Bronco's battery and suddenly it started working. It ran, but it made god awful noise. I'm assuming that straight power from the battery was enough to finally get it moving, but being hooked up inside the Bronco it just didn't get enough juice to get it to move with the condition it was in. The new motor I put in works great now, but I found that the fan wheel is... out of round I guess. It makes crazy vibration when its at full speed, but it certainly spins up like nobody's business.

Even without the resistor plugged in, your fan should spin up at the highest setting since it bypasses the resistor for full speed or so I'm led to believe since it'll do that on mine if I take the resistor out and turn on the fan. I'm not sure what else would cause it to not come on if you're getting power at the fuse block, fan switch, the resistor, and at that plug that I described down on the left side of the heater assembly by the floorboard. I'll think about it some more, maybe something will come to me. Oh and make sure you have good grounds!

 

the relay that provides power to the fan switch is activated by power coming through the AC mode switch, so you should check to make sure that's working

 

I'd bet it's a bad motor, even though it works off the battery - it's usually worn/dirty brushes. Have one right now that will only start about 50%, will work the rest of the time if you reach in the floor vent with a pencil and "bump" it (not recommended). Sounds fine when it's running, maybe a little slow, but not enough you'd notice if you weren't listening for it. It'll do the same thing, works perfectly fine if you hook straight to the battery - assuming it's an amperage thing, the rest of the wiring seems to be fine, full voltage.

 

 

 

Going to hop onto this thread because I'm having a similar issue. Just got home on leave the other day, took the truck out this morning and fan quit working on me. Switch in the cab normally stays on vent, but since it's a bit warmer today I tried the AC. Now, when I have the ignition on, but the motor (engine) off, I can hear and more importantly feel the fan running. But when the motor (engine) is running I'm getting squat out the fan and when I listen real close I can't hear it running.
When I first got the truck, I had problems with the blower motor and got someone in to help me as I know nothing about these trucks. Feel certain he bypassed the resistor for the speed setting because it used to run like two speeds; low and full whack.
Thinking I might have to replace the blower motor but I've heard it's a real b!*ch of a job to do? Especially since I've pretty much rebuilt the dash and don't really want to go into tearing it apart, although will if I have to.
Any ideas?