Gas and Oil Home Heating Furnaces - Pilot Burner Problems?

I'm a first time poster, so please forgive any errors or omissions. I have recently replaced a pilot burner on a Payne 395 induced draft furnace w/part #lh680005 (3 wire - yellow, green, white).

The operation is as follows: When energizing in heat (jumpering R to W on PC board & door switch pushed in) clear relay on PC board clicks, induced draft motor comes on, spark sequence begins and pilot lights. Within 30 seconds, pilot goes out and then begins a short light and relight sequence where the pilot only stays lit for a few seconds.

My Ohm meter shows Yellow and Green is closed before the pilot heats up . . . . when it gets warm my meter begins beeping and then stops. The problem seems to me to start when the NC relay on the pilot burner opens. Jumpering Yellow (24V) wire to Green wire or from R on PC board to the T1 black ignitor box eliminates the problem and the furnace works normally. The NO relay (Yellow to White) never opens because it appears that the pilot never gets hot enough - or that the NC gets hot too fast before the NO can close.

The gas valve is a new 5 wire (White Rogers) and I have verified wire for wire that all of the wiring on the factory schematic is correct. I have recently repaired another furnace with a similar problem and the pilot burner did the trick.

This is my second pilot burner on this furnace. I experimented with the first pilot burner to see what happens when you heat it. I found that there is a time delay between the NC and NO of about 15-30 seconds if I heat the flame contact surface with a flame (in this case a birthday candle). By that time, the NC contacts have opened and the NO have yet to close. Seems to me that this is my problem, but I am not 100% on this. I replaced the pilot burner on the furnace today, but have the same symptoms. Is it a defective pilot burner?

Side Note: Just to see what was going on, I have disassmebled an old 3 wire pilot to see what is going on. There is a bimetal switch with two small contacts for the NC and NO relays. It would make sense to me that the NC relay stops the spark ignitor after it opens, and the NO relay opens the gas valve and keeps the pilot on. Seems to me one should close at the same time the other relay opens.

Help!

For the pilot burner to light, power must initially be on to terminal 5 through the NC contact of the pilot switch. So you know that is happening.

Once the NC contact of the pilot switch opens, the pilot burner should remain on as long as terminal 4 (the hold coil) is energized by the pressure switch.


The problem you have implies that either the power to 4 is being shut off or you have a defective hold coil between 4 and 2 or 3 (the ground side of the low voltage circuit).

Do you have terminals 2 and 3 properly wired to the ground side of the low voltage circuit? Is the pressure switch closing to energize terminal 4?

It is that circuit you need to analyze for defects or problems.

It sounds like you have been expending a lot of time and parts trying to figure out the carrier three wire pilot system by a process of deduction. It's a system ingeniously designed to defeat do-it-yourselfers.

SeattlePioneer; Thanks for the reply. I am not in front of the furnace, so from memory, the last terminal to the back of the valve is #5 and connects to T1 on the black ignitor box. If I jumper to get 24v from Yellow to Green on the pilot burner (or from R on the PC board to T1) then #5 is energized and all is well. When I disconnect it, the burners go out.

After reading your reply it leads me to believe that I am only temporarily energizing the pilot because the minute I remove my jumpers it stops working.

I have checked and rechecked the wiring schematic for correctness. I even reversed 2 & 3 on the gas valve just in case (they have a small jumper that connects them). I also checked T2 on the black ignitor to furnace for proper ground and also on both screws of the "black box" to ensure good ground (I think those screws connect to the ignitor ground by looking at the back of the ignitor).

I believe I also checked to see if the pressure switch closed on a call for heat (yellow to black on the P/S if I remember correctly) . . . but now I am suspicious of the pressure switch because if I remember right, #4 is a small terminal connector in the middle of the valve. I think it comes from the inducer circuit board. Although I have checked continuity of the wire between the inducer circuit board to #4 terminal, I do not believe I have 24V to it when system is energized. I will check and recheck the pressure switch to ensure it is closing and then check terminal 4 again.

My burning question (literally) is . . . does this pilot burner use flame rectification ground for the pilot valve to stay open? If so, how critical is flame adjustment? I can see that the flame comes in contact with all surfaces of the pilot burner, including the electrode. If so, is there a way to check, adjust or measure this in terms of voltage? Yes, I have invested a great deal of time into understanding the 3 wire pilot, but I am determined and feel that I am close. (Thank God the house is being remodeled and is vacant).

Lastly, does polarity matter on this furnace? They recently did some electrical work, and although I don't think the electrician changed anything would reversed polarity matter on this furnace?

Thanks to someone like you, I saved $450 by fixing my Mitsubishi big screen TV myself last weekend . . . had to buy 7capacitors from Radio Shack for $1.50 apiece and solder them onto the PC board. I found step by step instructions from a guy who had the same problem and solved it. My TV guy had no idea. I really want to understand how this pilot circuit works. Thanks again.

SeattlePioneer; Thanks for the reply. I am not in front of the furnace, so from memory, the last terminal to the back of the valve is #5 and connects to T1 on the black ignitor box. If I jumper to get 24v from Yellow to Green on the pilot burner (or from R on the PC board to T1) then #5 is energized and all is well. When I disconnect it, the burners go out.

After reading your reply it leads me to believe that I am only temporarily energizing the pilot because the minute I remove my jumpers it stops working.




By applying power to terminal 5 on the gas valve, you are bypassing the pilot switch, which normally would shut power off to the pick coil (terminal 5) after the pilot switch heats up.

The pick coil is only supposed to provide power temporarily to open the pilot valve ---- the hold coil (Terminal 4) is what should keep the gas on, which only happens as long as the pressure switch is closed.

So no---- flame rectification is not used to keep the pilot burner on. The circuit through the pressure switch to terminal four does that, which is why that is where your problem no doubt is to be found.

Sounds like a burnt out HOLD coil to me.
If it is you have to replace the gas valve.

Try this:
unplug the pilot at the molex plug..push pilot wire aside and using a jumper in the opposing wire install it in the green and yellow holes, which will open the pilot valve and energize the spark generator..next move the jumper to the white and yellow hole and wait for the main burner to come on..If the pilot goes out when you are transferring the jumper, install two jumpers, one in green/yellow one in yellow/white. If the main burner comes on you have just proven the problem lies with the hold portion of the valve.