I went to see my wife at work in my F-250, and when I left I turned the heater to full blast. About 30 seconds later, I smelled something electrical burning, and suddenly the cab filled with smoke. I pulled over and shut the truck off. I looked around and didn't see any flames in the cab or under the hood. The smoke cleared a minute later. I started the truck back up, and it quickly filled with smoke again. I shut the truck off again. I turned off the heater and the radio, and started the truck again. No smoke, and the truck still runs fine. I turned on the radio, works okay and no smoke. I turn on the heater, and smoke starts pouring from the floor vents. By this time, smoke is coming from the cowl panel as well. I checked a couple times, and the smoke was directly related to the heater being on. I left it off and went to work. When I got to work, I turned the heater on, but no smoke this time. I tried it again after work, and still no smoke. The cab stinks, so I hosed it with some air sanitizer. Everything still works though, the heater fan and controls, all the lights under the dash everything in and on the truck still works. What the heck could have caused this? I haven't had time to take anything apart yet, but for the last 2 days, the smell would reappear briefly when the heater came on, but otherwise everything seems normal.

 

your ballast resistor is what burned. its in the plenium

 

Its either what ford390gashog said or could be that there are leaves in the heater box. Even if it is not because of burning leaves, to avoud future hassle I would still clean all of whatever is in there out and then put a screen (household window screen is fine) under the cowel. The pre 87 F Series cowel doesn't do much to keep the leaves out. They still get in on my 95. We had a 85 that we had to clean out once a year after about 3 years we just cut a peice of window screen and put it in. Once it is done you can hardly notice it.

 

I'm not a Ford expert. But many vehicles blower speeds are controlled by switching blower power through resistors which give off a lot of heat when the blower is on any position other than high. These are in the airstream of the blower to dissapate the heat so it sound like some debris has gotten in there and the resistors are burning it. Put your blower on "HIGH" and start it up. If no smoke, there is your problem.

 

Not saying it could not be something else, but I have had and do have more than one ford truck and at various times the cab will fill with smoke with various smells. Well after much adieu and scary moments I find that leaves have gotten into my heater system. I just stop the truck and let them burn out and then go on my way. To this day nothing seems to have gotten damaged.

 

Yep, burning leaves, install a screen, remove blower motor and clean it out.

 

I cleaned the cowl up not long after winter started, and now there are no leaves where I live. I will pull the motor out though and see if anything got in there that I couldn't see from the cowl. So do you think one of the resistors burned up, or something that touched a resistor burned up?

 

If all your blower speeds work, the resistors survived.

 

Yeah, same thing happened here. Driving 60 Mph with the heat on full blast, then smoke started pouring out. When i turned the heater down to low it went away. So i'm guessing with all the air coming in and stuff it started a rats nest on fire or some leaves. i've got all that cleaned out of there now and have no problem.