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Old 11-02-2011, 11:33 AM   #1
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Day & Night (Carrier) Natural Gas Furnace Lights Sometimes


Day & Night (Carrier) Natural Gas Furnace Model 394JAZ036065AAFA with intermittent pilot (3-wire model 740A).

Problem 1: Sometimes it works fine, but more often than not the pilot will not light unless I blow on it with a blow dryer while the electrode/ignitor is sparking. After just a couple of seconds of blowing, the pilot lights and then everything usually works ok.

Problem 2: Sometimes before the pilot has a chance to ignite or even shortly AFTER it has ignited, the ignitor shuts off after a fairly loud "clunk" sound.
This can happen before the first spark or after just a couple sparks. Whatever makes the clunk sound seems to be shutting off the ignitor and the gas
before it should.

I've already replaced the thermostat, the electrode/ignitor, the pilot assembly along with the aluminum supply line and compression fittings on both ends, and the gas valve, but none of that helped. I also inspected the circuit board inside the ignitor/lockout control box, but found no obvious burnt out components. I measured the spark electrode wire resistance at 0.5 ohms, which seems pretty good.

What could be causing these problems?


Last edited by scotth1981; 11-03-2011 at 05:14 PM. Reason: clarification of symptoms
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Old 11-02-2011, 03:58 PM   #2
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Day & Night (Carrier) Natural Gas Furnace Lights Sometimes


Back connection in the molex plug.

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Old 11-02-2011, 05:03 PM   #3
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Day & Night (Carrier) Natural Gas Furnace Lights Sometimes


BeenThere, thanks for the suggestion. I just rechecked the molex plug that plugs into the ignitor control box, and all the wires feel tight. I've checked it before. I'd be very surprised if the symptoms could be explained by a loose wire that never gets moved except maybe when I'm troubleshooting.
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Old 11-03-2011, 03:36 PM   #4
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Day & Night (Carrier) Natural Gas Furnace Lights Sometimes


Here are a couple easier related questions: How long should it take for the pilot to ignite once the spark starts? And
how long should it take for the lockout control to shut off the spark if the pilot fails to ignite?
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Old 11-03-2011, 03:47 PM   #5
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Day & Night (Carrier) Natural Gas Furnace Lights Sometimes


If blowing on the pilot does allows it to light and it doesn't light when not blown upon, are the igniter electrodes directly in the pilot stream?
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Old 11-03-2011, 03:56 PM   #6
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Day & Night (Carrier) Natural Gas Furnace Lights Sometimes


How, the electrode should be in the correct position and adjusted according to the model 740A installation instructions (http://www.xpedio.carrier.com/idc/gr...39740dp1-a.pdf).

When the pilot lights, the flame splits on either side of the electrode, which a gas company technician told me was good.
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Old 11-03-2011, 05:14 PM   #7
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Day & Night (Carrier) Natural Gas Furnace Lights Sometimes


Alrighty then. Are you seeing the spark going directly through the pilot flame or is the pilot flame over to one side of it?
I'm only asking because the hair dryer might be redirecting the pilot gas closer to where the igniter can reach it.

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