Furnace Controller Board Repair

Friday morning, my wife said there seems to be an electrical burning smell coming from the vents. I had to go to work and I figured we'd find what it was sooner or later.

That night, it was getting colder than usual in the house, so I checked the thermostat. It was 66F inside, with the heat set to 68. I went to the basement and checked out the furnace. I opened it up and smelled a bit of burning smell. I put the front panels back on and watched the flame ignite properly, but the fan didn't come on and then the flame shut off. I figured the blower motor was bad. I looked at the flashing red LED and it was flashing code 33 - three short and three long flashes.

I took the blower motor out and checked the capacitor. The first step is to always short the terminals together before measuring continuity. I first measured voltage. It was basically at 0 volts. That meant is was safe to check resistance. As the ohmmeter charged the capacitor, it said open circuit. That was good. Then I measured about a half volt on the capacitor. Good. Final check on the capacitance meter showed 9.3 uF, compared to 10 uF on the label. Close enough.

Next, I checked the fan motor. There were several wires coming out of the motor. According to the wiring diagram, white was common, and the rest were for different speeds. Here's where my troubleshooting methods got a little sketchy. I found a spare cord with a plug on it in my electrical drawer. So I attached some terminal ends and then plugged it into the wall. As long as I was careful to not touch the terminal ends or let them contact any metal, or short together, it should be ok.

I set the blower assembly down on the kitchen floor and sat on it to keep it from moving when I connected the power. I connected the common to neutral, carefully plugged it in, then connected one of the other wires to hot. To my surprise, the blower motor ran. I checked all the other wires, and it ran on each one.

So I set the thermostat to a few degrees cooler so it wouldn't turn the heat on and set the fan to "on" instead of auto. I re-installed the blower and turned it on. The fan didn't turn. I measured the flame rollout circuit and it was low resistance, maybe a couple ohms. That checked out, so I took the controller board out of its case.

Furnace: Carrier WeatherMaker 8000

Control board: 1012-83-9410a

I think I found the problem. After a brief internet search, I decided that it was getting too cold inside to wait for an on-line order. I brushed off the burned spot to see if this was repairable. It looked like the copper trace failed and I could just solder in a wire.

Here it is. I soldered in a wire.

After re-assembling everything, I turned on the furnace and the blower motor came on and everything worked as it should. I think I'll need to select a replacement furnace for when this one finally quits. It's at least 18 years old, and it's only 80% efficient. I ran the numbers, and going from 80% to 95% efficiency would save about $80 per year. Over 10 years, that would save $800. So if the price difference is $500, it's justifiable in my mind.

I also read that the PCM (permanent split capacitor) blower motors are only expected to last about 50,000 hours, but the new ECM (electronically commutated motors) are expected to last 90,000 hours. So adding that feature will add about $400 more to the cost of a new furnace. With those two efficiency upgrades it's pushing the furnace cost to around $2063 just for the furnace alone. The additional cost pushes out the return on investment to 10 years. That's assuming I'll need to replace the furnace anyway.