Heater/SMOKE/(almost)FIRE problem - Any predictions before I tear into it?!

[69GMCLonghorn]

So a couple weeks ago, in my '70 non-A/C. I got about a mile from the house, I start smelling something burning of a non-electrical nature. Within 20 seconds, the cab was FULL of smoke and I had to pull over and shut it down! No burned wires, no HOT wires, nothing smoldering, literally NO sign of catastrophe! I limped it home and parked it. Same thing happened a week later. So now I've discovered that the smoke is coming out of the lower heater vent as the truck warms up. The blower motor is not on. I've actually put my hand on it and it's not even warm. The wires aren't hot either.

Now, I'm sure there are 300 leaves, etc in the heater box, but in cold weather when the truck isn't even up to temp, there's no WAY that stuff could start cooking... Every truck of this era would have burned up by now if that was the case.

So who'd like to make a prediction of what I find when I tear this thing apart??

[Mister-B]

Originally Posted by 69GMCLonghorn

So a couple weeks ago, in my '70 non-A/C. I got about a mile from the house, I start smelling something burning of a non-electrical nature. Within 20 seconds, the cab was FULL of smoke and I had to pull over and shut it down! No burned wires, no HOT wires, nothing smoldering, literally NO sign of catastrophe! I limped it home and parked it. Same thing happened a week later. So now I've discovered that the smoke is coming out of the lower heater vent as the truck warms up. The blower motor is not on. I've actually put my hand on it and it's not even warm. The wires aren't hot either.

Now, I'm sure there are 300 leaves, etc in the heater box, but in cold weather when the truck isn't even up to temp, there's no WAY that stuff could start cooking... Every truck of this era would have burned up by now if that was the case.

So who'd like to make a prediction of what I find when I tear this thing apart??

Could be something like a dead mouse or leaves that did burn at one point when it got up to temperature, and now any air (even cold) blowing past it sends the smell in to the cab

[Molberg]

Could be motor burnin up too if someone jammed a larger than required fuse in. Or wrapped the blown one in tinfoil. Lol.

[69GMCLonghorn]

I hadn't thought about a once-living-now-dead item, like a mouse... As for the smell, it was actual smoke, not just eau de smoke... This weekend, I should have the time to rip it apart and see what is going on. More to come.

[Longhorn 70]

Was the fan on low? Something could have been on the ballast resistor.

[marvgarr68]

Resistor bad

[davepl]

Fuse? Fusible links? Can't imagine why it would smoke if everything was properly protected.

I fixed one once that only did that at startup. Turned out it was the reverse light circuit, so it only happened when he backed up out of the garage.

[luvbowties]

Originally Posted by 69GMCLonghorn

So a couple weeks ago, in my '70 non-A/C. I got about a mile from the house, I start smelling something burning of a non-electrical nature. Within 20 seconds, the cab was FULL of smoke and I had to pull over and shut it down! No burned wires, no HOT wires, nothing smoldering, literally NO sign of catastrophe! I limped it home and parked it. Same thing happened a week later. So now I've discovered that the smoke is coming out of the lower heater vent as the truck warms up. The blower motor is not on. I've actually put my hand on it and it's not even warm. The wires aren't hot either.

Now, I'm sure there are 300 leaves, etc in the heater box, but in cold weather when the truck isn't even up to temp, there's no WAY that stuff could start cooking... Every truck of this era would have burned up by now if that was the case.

So who'd like to make a prediction of what I find when I tear this thing apart??

You stated blower motor not on...you sure? If by chance it WAS on, even LOW, the squirrel cage fluttering across a leaf(or several leaves)will cause "frictional smoke/fire" while motor stays cool.

Had it happen on a truck whose blower motor ran on LOW all the time--even with heater turned completely off. It was designed as a safety measure, I guess, maybe somehow to keep the cab ventilated? Possibly a p/o thought it was a good idea & modded it?
Sam

[Bgrdmoose]

Had the same thing happen 3 weeks ago in my '72 suburban w a/c. It was the resistor inside the heater box. The coils turned bright orange and smoked. Replaced it and no more problems.

[Hawghauler]

It seems to me you said a "Non-electrical" smell. It sounds like you may have something on your header/exhaust manifold burning. Nothing would get hot enough to burn non-electrical components except the exhaust. As to how it got into the heater vent, It may be coming from holes in the firewall. If it's electrical, the comments stated above would certainly apply. There are also wires you may not be able to see without deep rooting that are shorting. But again not in the heater box. The most likely heat source in the heater box that could reach those temps would likely be the resistors for the motor. They may not smell electrical if they are in contact with combustible materials and are heating up. If you say the motor wasn't on, I'd consider checking the motor wiring to see if your getting power unintentionally, i.e. faulty switch, shorted wires, faulty wiring...
Good luck

[Longhorn 70]

You stated that the blower wasn't on. In my 70 the blower runs very slowly when in the off position. I don't know if they all do. There is one or 2 ¼" bolts under the heater housing in the cab that hold in the dropping resistor assembly. Remove those, pull the dropping assembly. Then get the vacuum in there. Clean out the crud in there. Turn the blower on high and get the vacuum in there again. Let us know how many mice and leaves you find.

[69GMCLonghorn]

Thanks, gang. I will be pulling the entire outer heater box off since I need to replace the heater core anyways (starting to see signs of leakage). So whenever I can find more than 5 minutes to work on the truck, I should be able to get the big picture of what's going on. I'll post back when I open it up.

[69GMCLonghorn]

Well, quick update on nothing... I haven't cracked it open yet, but I did manage to get it into the garage and in position for the tear-apart. More info to come SOON! I want to drive this thing, not stare at it...