# XE-78 furnace problem



## bigdaddywitdacaddy (Jan 29, 2007)

Hello,
I have a Trane XE78 model TUD060C924A1. My burners keep going out. I had a White Rodgers 50A50-405 Board in the unit. Furnace worked fine in November and December and nothing was changed. Then unit would start up fine but flames would go out. Then unit would retry to light 3 or 4 times then system lockout. I would have to reset system either by furance switch or at thermostat. Would start up again just fine then go out. No set time for how long it would stay on sometimes minutes other times days. Called HVAC guy he checked system said all voltages appeared fine but unit never showed problems when he was here. So tech cleaned flame sensor and left. Soon as he left it acted up of course. Called him back next day but he still couldn't figure it out. He said he noticed ignitor was bad or going bad and that maybe it was that or circuit board. So I let him replace the ignitor since it was the cheaper of the two. But it didnt help. Not soon after he left it started acting up again again. So I brought a replcement circuit board 50A55-843. Unit started up fine everything was working then a couple of days later right back to its old self flame cutting out and unit trying to relight (which you can see the ignitor turn on then a click but no gas). What else should I check? I'm thinking about changing out the Thermostat. It is a Rite Temp programmable.


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## #CARRIERMAN (Oct 18, 2006)

Hi bigdaddy

It sounds like you could have a pressure switch problem. It does not from your post sound like a constant one. When the furnace goes out, if you put a jumper wire across the two wires of the pressure switch it should fire right back up. If the little fan (inducer assembly) is making alot of noise or vibrating, this is going to be the more likely problem than the pressure switch.

Good luck
Rusty


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## rakes9720 (Dec 13, 2006)

A click after igniter warm up but no gas? Sounds like bad gas valve or gas pressure. Maybe a bad connection to the gas valve causing it to turn off seemingly randomly. There is a small chance that the igniter is misaligned, but probably not, unless someone was poking at it. Its not a bad thermostat.


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## Furnace_dude (Jan 18, 2007)

try this:

with the furnace running and behaving, use the handle of a medium sized screw driver and tap the pressure switch. dont smack the crap out of it but see if tapping on the switch makes it go out. tap the pressure switch with about the same force that you would clap your hands with, not much harder.


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## bigdaddywitdacaddy (Jan 29, 2007)

Thanks for your reply. Tried what you suggested and no luck it kept running fine.


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## bigdaddywitdacaddy (Jan 29, 2007)

Also no vibrating. Just noticed just now that it went out then tried to reignite but went right back out soon as it was lit less than a second (this is new symptom)


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## Furnace_dude (Jan 18, 2007)

next, whith unit behaving, wiggle wires around. especially near connections. see if you can get it to do something, also is the color of the IFC [if it can be seen] consistant? its the pc board and if it looks like something has burnt, have it replaced


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## jeberes (Dec 1, 2007)

*Trane XE78 Board Problems?*

I own the same unit with the same set of problems. I am curious as to the fix. What was the final outcome? I realize the post I'm referring to was nearly a year ago but now I have almost the exact same issues. I have a Trane downdraft XE78, model DD100C948A2 with a White-Rodgers 50A50-405 Control Board. My unit fires up and operates normally at times and other times runs normally except the flame will not ignite. Pre-blower comes on and runs okay, igniter illuminates, but gas will not come on and ignite thermal chamber. I have not tried tapping pressure switch yet as the unit is working good at the moment. This problem just started with my unit so in time it may worsen. I do not want to throw money/parts at it (especially Control Board at $189) If it is the pressure switch, it appears it may cost much less to repair. No fault codes appear on Control Board. Can you help?


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## cjett (Aug 29, 2007)

If you have a multimeter you can check for 24 volts ac at the gas valve. I see lots of these Trane units with intermittent gas valve problems. If you check while the hot surface igniter is glowing, and you see 24 volts at the gas valve but it still doesn't ignite, your gas valve needs replaced.


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## jeberes (Dec 1, 2007)

Thank you - it has been approximately 24 hours since the unit failed to properly operate so I have not been able to check the gas valve for 24 vac with the igniter glowing. When it does fail I will check. Question: If 24 vac is not present at the gas valve with the igniter glowing, would I then suspect the pressure switch as being faulty? (This is considering that the mother board is not showing any fault codes.)

Thanks again for your help!


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## cjett (Aug 29, 2007)

Right, if there is no voltage at the gas valve at that time. I would try putting a jumper across the pressure switch long enough to see if it will operate, if so that is the problem. And of course do not leave that jumper on unless you are at the unit during testing.


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## kien555 (Feb 21, 2009)

I had the same problem. when you said "I would try putting a jumper across the pressure switch long enough to see if it will operate, if so that is the problem. And of course do not leave that jumper on unless you are at the unit during testing. Can you please tell me where it the jumper go to exactly.


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## JSievers (Nov 24, 2009)

*Tap on Gas line... Sticky Gas Valve?*

My girlfriend is having a problem similar to the above described. I think it is an intermittent sticky gas valve. When we tap on the gas line during the call for fuel, the furnace ignites and will run ok for the next few times. After reading this forum, I will verify there is 24volts but I am ready to take the plunge of ordering a new valve. Does anyone have any recommendations for the most inexpensive, reliable American company to purchase this part from... and the part number?

Thank You.


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## jamiegrahams (Aug 24, 2011)

*XE-70 Furnace start up problem*

I also have the same problems as above this last spring. My air has worked all summer with out a problem. I would love to know how other people sloved this problem. 

The only difference in my situation is that sometimes when my flame goes out it codes a open temp switch. I can shut it off and on and sometimes it lights and sometimes it codes a glow soid as a board faliure. My flame sensor works fine(replaced it with a used one), if i uplug the pressure switch it codes as it should. My blower and exhaust fans both turn on. Exhaust fan is a little noisey and may need replacement in the future. My blower motor and cap are only 6 months old. Should I replace the board before the cold wether?


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## Mcaralis (Dec 28, 2011)

*Trane xe 78 furnace issue*

Hi I have a trane xe 78 burner that is 19 years old. The issue that I am having is the system willbtry to start and the flames may or may not light. The issue is very intermitent. When the flames do not light the blower stays on a cools off the house until the power is shut off. The unit will try to light 4 times then lock out. A tech came here and sadly everything checked out. We have changed the flame sensor, ignighter, control board, and gas valve. The high limit switch checks out as does the pressure switch. The unit seams to work better since all the updates but still has the issue once or twice a day. Any ideas? I was thinking of changingbthe pressure switch tomorrow to see if that does it but I am tired of shooting in the dark


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## harleyrider (Feb 20, 2007)

#CARRIERMAN said:


> Hi bigdaddy
> 
> It sounds like you could have a pressure switch problem. It does not from your post sound like a constant one. When the furnace goes out, if you put a jumper wire across the two wires of the pressure switch it should fire right back up. If the little fan (inducer assembly) is making alot of noise or vibrating, this is going to be the more likely problem than the pressure switch.
> 
> ...


 unit does not have a pressure switch..........its natural draft


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## tmmak12 (Jan 13, 2017)

I have a similar unit (so it is over 20 yrs old). The symptoms described here are the same - cold start up OK, ignitor on and flame blowing for 5+ min. The all of a sudden, the flame stopped. The rest of the motors are all humming. The ignitors turned on a few times, but no gas, no flame. The main blower keeps running and blow cold air. The LED flashes then dump the error code of 2 flashes, which essentially tell me nothing - system lockout. Checked the pressure senor, closed upon startup, so there is vaccumn. I even tried shorting it when the flame went out, but no effect. I checked the flame sensor current at about 3.x uA, so this is normal. I checked the limit switches (both the one on the exchanger wall and the one facing the flame. Both are low ohmic. I have DMM on them looking for opens when the flame cut out. The voltage on them is clamped at 0 so they are close. 

I am out of easy to check options. If the gas flow valve is faulty, why would it drops out in the middle of it? I did not check the AC voltage that are supposed to be driving the valve body, as the wiring connection is not quite clear to me. 

Any other suggestions?


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