Dear Tom and Ray:
Driving down the road in my '89 Nissan pickup, I smelled smoke. I assumed it was outside and rolled up the driver's-side window and turned off the fan. Immediately, smoke came out of the heater vents. It was so thick that I could not see out of the windshield. I pulled over, stopped and put on my emergency blinkers. I grabbed my handy fire extinguisher and jumped out of the truck. I stood there, afraid to open the hood, thinking air would make the fire worse. When a friend showed up, we opened the hood -- and could see nothing! I drove it carefully to my friend's house -- now no smoke. I took it to the mechanic, and he could find nothing. What could be the cause? Many thanks!
TOM: It was a sign from above, Sara: Time for a new truck!
RAY: No, I'm guessing you burned up your blower motor, Sara.
TOM: If the blower motor started to fail, it would draw too much current and overheat. That would cause the insulation around its wires to heat up and burn, creating what? Smoke!
RAY: And that smoke would come right through the vents and into the passenger compartment.
TOM: So take the truck back to your mechanic and ask him to remove the blower motor and give it a good sniff.
RAY: The wiring itself is inside the motor, so it's hard to inspect visually. But if that motor was the source of the smoke, you'll be able to smell it immediately.
TOM: If it smells like smoke, then replacing the blower motor ought to solve your problem.
RAY: If the blower motor isn't to blame, then it's possible that some piece of debris -- a stick, some leaves or an abandoned public-radio tote bag -- got lodged against the exhaust system, perhaps between the engine block and the manifold, and that could have caught fire. Maybe it burned up entirely, or fell out when you moved the truck, in which case there might not be any evidence for your mechanic to find.
TOM: But have him check again. Start with the blower motor. It'd be good to know what happened. I know you have your fire extinguisher handy, but fire can be unpredictable. And if whatever is in there ignites again, the fire could spread and take your whole truck with it. What a lucky break that would be!