Trane xe80 not always lighting

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  #1  
Old 02-01-12, 11:12 AM
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Trane xe80 not always lighting

Hello,
I need some advice or a confirmation of my suspicions.
I have a trane xe80 forced air furnace with an intermittent problem. This furnace uses an hsi. When the thermostat calls for heat the draft inducer kicks on. after a short time the hsi heats up and there is a loud click. then the hsi cools down and it repeats the process two more times. The burners are not lighting. after the third try it goes into lockout and the blower and draft inducer run continuously until i manually shut it off. This happens once or twice a day. Most of the time it operates normally. Also, sometimes the burners will light but will shut off after 5-10 minutes of operation althought that is not as common. I am leaning towards a sticking gas valve as i was told the hsi would not even ignite if the pressure switch was not functioning. I have replaced the flame sensor and put a new filter in even though the old filter was just over a month old both with no new result. I also have checked for bad grounds and tested the voltage at the pressure sensor. What do you guys think?

Thanks,

Chris
 
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Old 02-01-12, 11:24 AM
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The probability is that the ignition control board is failing to energize the gas valve. The probability of that is probably about 80% or so.

Maybe a 10% chance of a bad gas valve --- they tend to be very reliable.

another 10% or so chance that something else, such as the pressure switch, thermostat or something else is shutting off the ignition sequence sequence.

I'd start by measuring the voltage across the gas valve. After the HSI has a chance to heat up, the circuit board should switch on the 24 VAC to turn on the gas valve. If it does and the gas doesn't turn on, you have a gas supply issue or a bad gas valve.

If it doesn't switch on the voltage, you need to check the voltages at the pressure switch and thermostat to make sure nothing else is shutting off the ignition sequence. If all the inputs to the circuit board are correct and the gas valve isn't being energized, you have a bad circuit board.
 
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Old 02-01-12, 11:34 AM
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I already replaced the control board too. forgot to put that in the post.
 
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Old 02-01-12, 11:38 AM
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at first i was leaning toward pressure switch or vent fan but every time i have tested it the voltages have been good. Plus i was told the hsi wont light up if the pressure switch isnt working. do you know if that is true? i dont believe i have tested the voltages on the gas valve.
 
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Old 02-01-12, 02:10 PM
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Originally Posted by planeswalker1 View Post
at first i was leaning toward pressure switch or vent fan but every time i have tested it the voltages have been good. Plus i was told the hsi wont light up if the pressure switch isnt working. do you know if that is true? i dont believe i have tested the voltages on the gas valve.
Yes it is true. You can illustrate that yourself by pulling a wire off the pressure switch after the HSI has started to heat up.

It's unlikely that the pressure switch, thermostat or other input will open at the critical moment each time, but it's worth checking.

Probably it's the circuit board, but you don't want to replace an expensive circuit board and have the same problem because you failed to check other possibilities.

And low voltage to the gas valve could be a problem. A relay on the circuit board might fail to make a good connection and not energize the gas valve properly.

Still, 80% or so chance of the circuit board being the problem would be my estimate.
 
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Old 02-02-12, 09:52 AM
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Ok here is the latest. It was working fine when i got home from work last night. i tested the voltages on the gas valve. There are two wires 1 red and 1 blue. The red one shows 24-25v when the burners are lit. the line becomes hot after i hear a click which i believe is the relay on the control board.
What i found this morning is more interesting. It locked out again sometime during the night last night and wouldnt re-light this morning. the voltage to the red wire on the gas valve was still showing proper voltage during the lighting process. What i found though was if i press the line down into the terminal block the burners will light. as soon as i let it go they shut off. I believe there is a bad connection in the terminal block. It is held on with 2 t-15 screws i am going to try removing it tonight to see what it looks like underneath. Maybe there is a broken solder connection or something... I will let you know what i find.

here is an image that shows the same valve.
http://i.ebayimg.com/t/White-Rodgers-Gas-Valve-36E98-206-/00/s/MTYwMFg5NTY=/$(KGrHqF,!lcE4590!w9-BO,D2LzLVw~~60_35.JPG
 
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Old 02-02-12, 11:07 AM
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You are doing well!

If you are lucky you will be able to identify the problem with the poor connection at the gas valve and be able to correct it. If not, the gas valve would need to be replaced.


Good luck!
 
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Old 02-06-12, 08:22 AM
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Well thursday night i pulled the terminal block off of the gas valve. It consisted of a small circuit board that you could only access from the bottom. I saw one solder joint that looked suspect but i reflowed them all just to be sure. Since I reinstalled it thursday night my furnace has not locked out once. Im still keeping an eye on it but i think it may be fixed. Thanks for your support!
 
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