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Old 11-18-2012, 06:56 PM   #1
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Furnace lights up, but then shuts down soon after...

Coleman Evcon Furnace/Mobile Home (downdraft).
Model: DGAT07800
Serial Number: 970704823

Has Fresh Air Intake Duct coming from roof (attic?) that opens up (butterfly valve opens) when thermostat turns furnace on. One only On/Off switch (in a box) for this, just below blower assembly.

Furnace initially wouldn't light because ignitor had a bad spot in it (about 11 years old, I was told). Replace ignitor with new - now flame comes on (appears to be full, to me - have seen a weak one before), but shuts off soon after. Within about 10 seconds, probably more like 5 seconds.

I cleaned the tang/tab? on the end of the burner tube (not the flame sensor rod, but the tab/"ground point", I was told), but wasn't very dirty - minimal rust, but cleaned it up good. End of burner tube is clean. Flame sensor rod is directly in path of flame (at the circular edge/perimeter of burner tube). Flame sensor rod was not dirty or discolored hardly at all, but cleaned it up with a dry Brillo Pad - no help - still shuts down soon after flame lights up.

Green light on control board was flashing Once, pause, the repeat (legend for One Flash, said Ignition Failure).

Insulator on flame sensor isn't cracked or discolored. Electrical connection was tight, at the flame rod (the quick disconnect type). The other end of it went to a multi-pin connector plug on the control board - wasn't loose.

I have removed the flame sensor rod to take it with me to a supply house to get a price for a new one, but wondering if an ohms check on this sensor rod is a valid test for it, when it's removed/isolated from the control board circuit (it tests, isolated, at zero ohms reading).

I discovered the microamps-dc test in an online article (Fieldpiece and elsewhere - the elsewhere may have been a copy of the the Fieldpiece article), but not sure where to connect the probes/alligator clips. I bought an Fieldpiece HS 35 which will do microamps-dc.

If this is how (the above pic) a multimeter should be hooked up, in series, to the flame sensor, I could do this.

In this pic, the white wire is coming from the control board and hooked up to the meter's black lead, and the meter red lead is hooked up directly to the quick disconnect on the flame sensor itself. Correct?

I would have to cut the the wire after it comes out of the control board box (headed to the flame sensor rod) and strip the ends, then use alligator clips connected to my multimeter to put the meter in series with the wire and get a uA's-dc reading, then wire nut the wire back together. I can use ceramic wire nuts if needed (higher heat capability). This sound workable?

The article says the mulitmeter should be capable of reading 1 to 10 microamps-dc. Any reading 1 to 10 uA's-dc is ok?


Last edited by justplumducky; 11-18-2012 at 08:39 PM.
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Old 11-18-2012, 09:25 PM   #2
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you have ignition and main flame which means pilot is proven and gas valve good..flame at rod could be distorting when main burners come on causing loss of signal..try to observe flame at flame rod...may have to adjust position..also, if main flame blows back during ignition you may have rollout switch shutting down gas valve..rollout switches located at front of burners..if so then must determine cause of rollout..dirty burners or blocked some way


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