Trane XV80 won't start


  #1  
Old 11-06-10, 07:24 PM
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Trane XV80 won't start

Trane XV80 gas furnace, 9 years old.

~30 second initialization with red LED flashing slowly.

Then red LED flashes fast and duct fans come on. Ignitor is energized with 115V for about a half second, not enough time for it to heat. No gas smell, no flame.

Red LED flashes 9 times (indicating an ignitor problem?). Yellow LED (DS3?) comes on steady. Green LED comes on steady dimly and then flashes brightly 8 times (low flame indicator?). Main blower stays on.

The furnace did operate once normally since the problem started. Another time (I think) it fired on all burners for a few seconds and then shut down.

Trying to start with the ignitor wired to an external switched outlet brightly heated the ignitor but did not produce any flame.

I removed and inspected the burners and they look fine.

I removed and cleaned the flame sensor with 0000 steel wool.

I inspected and cleaned whatever the Texas Instruments device is that goes into the combustion chamber, and it looks fine.

I tried lightly tapping the gas valve. I also noticed I get the same sequence of events with the gas valve disconnected.

Any idea what the problem is? I'm particularly interested to know how to determine if the gas valve is opening properly and/or what conditions would keep the controller from opening it.

Thanks in advance for your help!
 
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Old 11-07-10, 11:12 AM
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Does the draft fan motor come on, when you first call for heat? That has to occur to stage up everything for the ignition. The ignition module or controlboard is fed that information, through a safety circuit, that can only work if that draft fan comes on.
 
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Old 11-07-10, 11:27 AM
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Yes, the draft fan comes on when the hearbeat speeds up. The ignitor and gas valve both get energized for a fraction of a second, but not long enough enough for ignition-no glow and no gas, that I can detect. After that, the blower fan runs, and the indicator lights report errors. To my amateur eye, it appears something is telling the system to abort ignition immediately after it starts.
 
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Old 11-07-10, 11:55 AM
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Originally Posted by IBStramezi View Post
Yes, the draft fan comes on when the hearbeat speeds up. The ignitor and gas valve both get energized for a fraction of a second, but not long enough enough for ignition-no glow and no gas, that I can detect. After that, the blower fan runs, and the indicator lights report errors. To my amateur eye, it appears something is telling the system to abort ignition immediately after it starts.
About the only two things that can abort it that fast is the thermostat and the pressure switch. Even if the ignitor were burned out, it would TRY to ignite for like 20 or more seconds. Not like 2 seconds. It could also be a bad board. But we have to rule out the stat and pressure switch shutting down. Do you know how to do that with a voltmeter? Also make sure every single connection/jack connection is good, even if you have to pull off wires one by one and reinsert.

Does the draft inducer motor also quit in 2 seconds, or keep running? If that stays running, but only sends 120 volt signal for 2 seconds, the ignition control box, or board, containiing the wire that goes to the ignitor, is probably bad.
 
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Old 11-07-10, 12:34 PM
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The draft inducer motor also quits immediately. If I turn down the thermostat, turn on the furnace and then raise the thermostat, I get the same failed sequence. I don't see any indication of loose wires, and resetting them doesn't help. The schematic mentions manual flame rollout reset switches, but I can't find them. Is the pressure switch actually the pair of membrame switches, high and low? I have a voltmeter but don't what I should check for.
 
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Old 11-07-10, 12:47 PM
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Okay, now I know we HAVE TO check that thermostat circuit.

You use the voltmeter set to say the 200 VAC scale. Put the red probe on the W terminal in the furnace where the thermsotat wires hook up, and the black probe to either a good metal contact on the furnace or the C terminal. Hold it there during the call for heat and see if the 24 volt reading goes away when the inducer shuts off. OR, you can use a piece of wire to jumper between the R and W terminal, temporarily, for testing purposes. Temporary because if the furnace runs that way it will keep running and never shut off!
 
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Old 11-07-10, 01:01 PM
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Hope I did this right: I jumpered R and W1 and the furnace immediately tried to fire, but failed. Then jumpered R and W2 and got the same results I have been all along. I also tried disconnecting the pressure switches hose from the flue, and got the error message for a bad switch instead of the error message I have been getting. So that suggests the switches are not the problem.
 

Last edited by IBStramezi; 11-07-10 at 01:21 PM.
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Old 11-07-10, 01:24 PM
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Okay. So it is not the thermostat.

I am fearing it is the board. I'll have to review the thread. Hard to keep all the symptoms in my mind when responding to so many people's furnace symptoms.

Find out how much a new board will cost you. The one that the inducer motor wires plug into. Make sure the wires are tight! If no issue there, then consider what a furnace repair person would charge you to come out to diagnose. This might cost you 1/2 or more of what a new board would, if you get it and put it in yourself. IOW, let's say it turned out to NOT be the board (but I actually think it is), you would not be out a lot, when compared to the service trip.
 
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Old 11-07-10, 01:40 PM
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Yes, that is exactly what I thought, that, if it comes to it, it's probably cheaper to put in a new board than call in a serviceman. The one wildcard in all this is that I vacuumed the furnace compartment this fall for the first time, and the problem started soon after. I stayed away from the circuit board, but there's always a chance I loosen a wire connection and can't tell. I'll remove each wire, spray the connector with WD-40 and reinsert. Also, I want to say I really appreciate the time you've put into this, and your willingness to share your skills.
 
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Old 11-07-10, 02:06 PM
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And pay attention to molex multi-pin jack connectors, as these can be a source of problems. I have had problems associated with these, and have found the split pins to have their splits totally closed, and the diameter of say one of the 6, 9 or 12 pins inside, to be just small enough to not be making good enough connection with the female counterpart in the mating jack recepticle, and have had to widen the split with a razor knife, and got it to work again, that way.

You can also easily enough remove the board, after marking the wires so you know where they all go back ,and inspect the front and back of the circuit board to see if you see any brown or black flash marks at solder connections - because if you see that, that will indeed confirm a bad spot in the board.
 
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Old 11-07-10, 03:04 PM
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Maybe I'll be able to find a heating contractor that can bench test my circuit board.
 
 

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