If you don't, you will likely not know you have a problem until smoke comes out your AC vents. I just had a fire in my heater core box that trashed the heater core box and took out the ac/heat control harness under the hood. The box caught on fire and darn that box is simply fuel. In short order, it melted the harness that runs above it which is connected to the AC/Heater relays. Had to grab a hose from a stranger's duplex to put the fire out, but it kept smoking until I disconnected the battery. Since I can see the heater core from the front of the firewall, I can tell there was some fine organic material in the box. That clue came from another Pennocks post that I found while waiting for a tow.

Not GM's best engineering again. This 54K mile Indy Pace car was never parked in a place where a lot of leaves were present and in most cases, was at the minimum, under a cover. The organic material as far as I can tell was pretty fine. Apparently there was enough built up to make it into the heater coils inside the heater core box. Coincidentally I had driven the car on Saturday and had turned the vents on for some passive fresh air. The heater fan was never engage/

The side story is that USAA's idea of roadside service is pathetic. After being advised by USAA's contractor that it would take about 90-120 minutes for a tow, I called the company after an hour and they said it would be two more hours.

Since the car runs fine, I just reached into my road kit for a pair of big wire cutters, cut the melted harness, applied some duct tape from my road kit and drove home.

Nothing like a Fiero emitting large quantities of smoke from the passenger compartment to give the natives a chuckle.


Yeah, fire and Fiero, not what a P-Body fan wants to experience or talk about. Large cup of water did not reach deep enough to quench all the fire.

Harness fire had potential to do way more damage than the crap in the heater core box - Immediate battery disconnect called for!

About that heater core box..

Screw waiting for a tow truck, surgically remove burnt harness and apply duct tape to wire ends.

[This message has been edited by Wudman (edited 09-19-2022).]

In my opinion, from the manufacturer, that resistor should have been installed outside the box with a protective cage around it, not enclosed in an area with dust, dirt, and possible rodent nests.

quote Originally posted by IMSA GT: In my opinion, from the manufacturer, that resistor should have been installed outside the box with a protective cage around it, not enclosed in an area with dust, dirt, and possible rodent nests.

I'd agree. GM's "Fire Bomb Engineering" almost totaled a 54K Indy 4 Speed.

https://www.fiero.nl/forum/Forum2/HTML/141959.html Heat/AC box cleaning, again...
search for more.

quote Originally posted by IMSA GT: In my opinion, from the manufacturer, that resistor should have been installed outside the box with a protective cage around it, not enclosed in an area with dust, dirt, and possible rodent nests.

Sorry but Not happening even on New cars. And covered here before.

Only thing done now is many have 1 or more Thermal Fuse(s) and maybe a Shield to prevent crap getting in the resistors.

In Short...
These Resistors are < 3Ω often < 1Ω and handle 50-100 watts or more in a Very small package and Need Constant air flow to cool.
You can't do this in normal air that often is Not moving and heats everything around the resistors.

Others have "transistor" to set blower speed using blower output flow to cool. W/o that you would need a Huge Heatsink..,

Many vehicle still use 2 speed Radiator Fans with Very Large Ceramic that somewhere in the air flow by the Fan for same reason but 1 of them are about = the space the blower 2 or 3 resistors takes up. The sealed resistor is required because has to deal w/ rain and road crap "water."

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

Wudman, nice job on the quick thinking to save your car. And thank you for the reminder to clean out the heater core box.

quote Originally posted by Wudman: Not GM's best engineering again.

This Isn't a GM problem. Is used on Millions of vehicles from nearly all brands since at least the 60's to Today.

I had old Ford cars w/ same problem w/ leaves and mice plugging the heater box causing same fire problem.

As said above, Now they often have Thermal Fuse and maybe Shields/Screens to prevent crap getting in the Resistors.
Example:

3 T-fuse are small brown w/ green tip things just bellow the resistors.

Junk inside of 117,000 mile donor Fiero. The organic material just needs a bump to get over to the side where the resistor is located

Not so much junk where the fan mounts, but if you are prone to allergies, this might be too much.


Well what a few minutes of fire can do for your Fiero. The fire that I had was likely "energized" by the fact I had the fresh air Vent button energized. The fan was not on and the heat was set to "Cold". I probably put 250 miles on the car since bringing it out of storage this summer.

Anyone that is driving a Fiero is likely looking at a potential fire under the hood where the resistor is in the box that contains the AC evaporator. In taking apart a donor car, it is clear that debris builds up on the left side of that cavity, where the AC evaporator is closest to the external cover of the box. The organic material appears to get pinned by the AC evaporator and the external housing cover, eventually funneling material into the resistor. By the time that happens, you likely have quite a tinder box of super dry organic material ready to burn. That area is beyond the access of a fire extinguisher. By the time I saw smoke, it was likely too late to do anything by wait till I could see the fire from under the hood.

The pictures above are from a donor car I am taking parts off. If I had know this could be an issue, I'd have removed the external cover as part of the process of getting my low mileage Indy back on the road. Instead the fire was robust enough to melt part of the inner box frame, so my restoration will involve completely replacing the AC/Heater box. That will necessitate removal of screws and connections from under the dash to pull that box out through the front.

Replacing the harness is relatively easy, the box looks like a total be-atch, but since the car is a 54K Indy, it will get done.

If anyone has pulled this box out and knows any shortcuts, I'd love to hear them!

Also, if anyone has a box from an 84-85 (estimating those and others will fit), in Northern California or the Central Valley, I'd be interested. I'd rather not tear into the donor anymore because it is an Indy and as far as I am concerned, potentially restorable regardless the weathering.

One thing for sure this incident tells me is that we throw way too much stuff when it comes to Fieros. The older they get, the more valuable even minor stuff becomes.

If there is any upside to this situation it is that I had an appointment at a shop to go through the AC system. It is likely that shop would not have pulled the front cover off to clean out that box unless there was a cause to replace the evaporator. I can't imagine that the AC would have been working as designed with a pile of debris covering 1/3 of it.

[This message has been edited by Wudman (edited 09-26-2022).]

To pull the inner box need to pull the whole dash out first.

Can leave the console frame but almost everything back to shifter needs gone or loose.
Console frame have tabs to link w/ the dash but the frame can flex just enough to clear the dash.

Start at the top removing speakers then follow rest of hardware down and back.

Depending on damage... Inner box maybe fixable in several ways.
Outer as well but harder to plastic weld several pieces and be strong enough to hold relays etc.

I think box sections are polypropylene and "easy" to weld. (Most glues and more won't stick to PP.)

quote Originally posted by theogre: [QUOTE]Originally posted by Wudman: Well what a few minutes of fire can do for your Fiero. The fire that I had was likely "energized" by the fact I had the fresh air Vent button energized. The fan was not on and the heat was set to "Cold". I probably put 250 miles on the car since bringing it out of storage this summer.

Note: AC Blower is On unless the Off button is pushed and stay pushed.
If any other button is touch and just pops Off button out then the Blower is On.[/QUOTE]

I pretty much figured energizing the system by engaging the "Vent" button was the match. It was the first cool day in Sacramento in several weeks, hence I decided to open the vents. They probably had not been engaged in ten years or so..

quote Originally posted by theogre: To pull the inner box need to pull the whole dash out first.

Thanks! Pulling the whole dash is what I was expecting, but one can hope! First stop to a local shop to recover any Freon that may still be in the system.

I had contemplated grinding down the burn on the lip of the box and creating a new seal between the cover and the box with something like RTV. As long as I get a good seal, it is unlikely the repair would be noticed since all that area is buried under a panel. That is still an option and it would save me a ton of time. The front cover is done, no saving that.

As previously noted, there are plenty of posts on the "smoke in my vents". Consider this a cautionary tale, especially if you don't know the history of your Fiero.

Thanks again Ogre! You rock!