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-   -   Carrier furnace with spark but no pilot (http://www.diychatroom.com/f17/carrier-furnace-spark-but-no-pilot-168064/)

TGS 01-02-2013 05:26 PM

Carrier furnace with spark but no pilot
 
I have a 1988 Carrier furnace model# 394GAZ0361OC. It was having problems on startup where the gas valve opened up and igniter started clicking but it was hit or miss on if it would light the pilot. I took the spark electrode out and cleaned it and along with the gas orifice. This did solve the problem. I replaced the 3-wire pilot safety switch and the spark igniter. I am still having the same problem. I noticed that once it fails to ignite and locks out I can unplug the 6-wire molex conector on the igniter lockout control board and as soon as I plug it back in it immediately sparks and ignites the pilot. I looked at the lockout control circuit board and didn't notice any obvious burned up components. Why is the pilot lighting in this sequence and not on a normal startup?

carmon 01-02-2013 07:30 PM

let it cycle...wait for a heat call and see what happens .....

TGS 01-02-2013 08:54 PM

Here's a clarification on the original post. When I cleaned the old spark igniter and gas orifice the problem WAS NOT solved. It still randomly locked out. That's why I replaced those items. But that did not solve the problem. Even with the new safety switch and spark igniter it still has a problem that it had before, and it's the one that many people talk about on message boards. That is, the spark igniter may click away for minutes before lighting the pilot. If I put a match to it when its doing this the pilot will light immediately. Like I said, when I unplug the igniter lockout control, and plug it back in, it sparks and lights the pilot immediately. Doing this to the igniter lockout control board seems to reset it to a state where it sends enough power to the spark igniter to light the pilot without hesitation but for some unknown reason it fails to do so on one of the next several startups.

Focused2 01-03-2013 12:03 AM

Trial for ignition or timed trial for ignition.
 
Basically most units have a timed trial for ignition. On the ignition module it may say 30 sec. Purge (basically combustion fan runs for 30 seconds to remove any combustible gases from heat exchanger). 90 seconds for propane as it is heavier than air and more dangerous. So we have a purge , then we have a trial for ignition spark/ignition module basically you have the clicking sound of the ignitor trying to light the pilot line gas. Gas is flowing at proper rate out of the pilot orifice and we have a pilot. Once pilot is established flame rectification takes place. Cool term for the sensor sensing the pilot flame by sensing electrical current when pilot flame is present and the ignition module sends power to the Main Gas Valve circuit. Main flame comes on and we have heat. If the ignition module does not sense a pilot flame after say 3 trials for ignition. The module locks out to let you know something is wrong. To reset the module you have to remove power to it and it resets again for you to go through the timed trial again.
What can cause pilot flame failure:
bad ignitor cracked and sparking at support causing a weak spark at ignition gap.
Wrong size gap at ignition point.
Pilot gas pressure at wrong setting.
Orifice plugged. Orifice cleaned with metal tip tool and made larger than it needs to be. Technician using teflon tape instead of pipe dope and small piece of teflon tape once in while floating toward orifice and plugging it.
Holding circuit of the pilot gas valve not holding pilot part of valve ON to allow main gas to come on.
Well hopefully that helps, will tell you that most gas companies may troubleshoot it for you for free and is part of the service they provide. Once they point out problem , up to you to decide to call a pro or do it yourself.
Take your time and watch what is happening through the cycle and narrow it down.


Joe


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