farmkid

I have a Ducane MPGA100B4B furnace that quit working yesterday. When the thermostat calls for heat the inducer motor starts and runs for ~15 seconds and then quits. It does nothing else.

Here's what I've checked so far:
Ignitor: I pulled out the ignitor and it looks fine. My multimeter reads 80 ohms across it. I also checked to see if the furnace was sending any current to it, and it is not. The voltage across the wires going to the ignitor stays at 0 from the time the thermostat calls for heat until it stops.

Pressure switch: The light on the circuit board shows a 3 flash code, which apparently means the pressure switch is stuck open or closed. I checked that according to directions I read somewhere else. Those directions said to take the wires off and the switch should be open when the furnace is off and should be closed once the inducer starts. That checked out fine. It was open when off and closed after the inducer started.

Any other ideas what it might be, or what I should do next?

keepinitcool

Hey farmkid,

On a call for heat the inducer motor should start up, prove the pressure switch, energize the ignitor for a period of time, energize the gas valve which in turn should light the burners. The flame should come in contact with the flame sensor which proves the flame. After a delay period the fan will come on and deliver warm air to the residence. Since you say the pressure switch is closing after the inducer motor comes on and the ignitor isnt bad, it sounds like the problem is in the circuit board since no current is being sent to the ignitor. Hope this helps ya out.

farmkid

I spent a bit of time looking at the wiring diagram and checking continuities/voltages/etc. trying to figure out how it all works. The diagram showed two optional limit switches in the same circuit as the pressure switch. I found one on the heat exchanger and eventually found the second one on the blower. I too was starting to suspect the circuit board because both limit switches seemed to be okay. When checking voltages, I found that I had only ~9 volts at the pressure switch, but 25 volts at the circuit board. Upon further checking, I found that one of the wires going to the limit switch on the blower had no continuity through it. (It must have had passed some current, but just had very high resistance, because I still got some voltage at the pressure switch.) How a wire goes bad when it's in a dry environment and doesn't move is beyond me, but nevertheless I guess it must have had some corrosion or something build up at one of the connectors or something. A piece of wire and a couple of spade connectors later and my furnace works again. And now I know a lot more about how a furnace works.

keepinitcool

Glad you got it figured out. Enjoy the heat!