Trane XE78 Short Cycling-Pressure Error


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Old 01-21-16, 08:31 AM
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Trane XE78 Short Cycling-Pressure Error

Hello and Thanks in advance for the help! I've compiled the details below based on what is traditionally asked for in similar threads.

Furnance is- Trane XE78 model# TUD120c960AO, manufactured 1990.
Same specs as the TUD120C960K found on this page Tud100c972k; Tud120c954k; Tud120c960k; Tud140c960k - Trane XR80 Manual [Page 9]

Observed furnace behavior-
* Stat is a newer 3M filtrete digital stat, stat is working correctly.
1. Furnace call for heat occurs correctly, indicator light blinks at steady rate which is the code for 'call for heat'.
2. After 20 seconds igniter begins to glow and induction fan kicks in.
3. Gas valve clicks open, gas is sent to nozzles and flame ignites in the burners. Flame is a clean blue and consistent across all burners.
4. Furnace operates properly for 3 minutes- burners lit, main fan active.
5. After 3 minutes, gas valued clicks twice-short pause-clicks twice-short pause-clicks closed. This sequence happens in a span of 2seconds, the flame in the burners bobbles like its being turned off and on rapidly then shuts off. It's not the usual crisp shut off you get if I were to stop the call for heat. Main fan stays on after burners shut down.
6. Indicator light flashes 3times which is the code for 'Pressure Error'.
7. Furnace stays in this state for 30 to 60 seconds then starts the cycle over again.
After many cycles, eventually the furnace stops cycling, the fan stays on blowing cold air. The HVAC tech assumed that this is safety feature allowing for a set number of failure cycles before shutting down. Powering off and on furnace restores it, allowing the furnace to return to short cycle operations.

Testing and trouble shooting I have done so far:
I had a tech out to do maintenance and furnace checkup test-
* Heat exchanger is good, gas value check out good, filters are clean and no obstructions, inspected exhaust flue clear and no obstructions, cleaned flame sensor amp reading checked out as good, High limit switch checked out as ok.
* The tech suggested the issue is air flow related, perhaps clogged coil. A complete diagnostic and repair would be required to know for sure. This would mostlikey cost me $400 to pull the plenum and clean the coil.

Trouble shooting I performed to try to resolve the problem-
*Ruled out High Limit switch- pulled wires to highlimit added jumper wire to force continuous continuity. Furnace still short cycles.
*Pressure Switch- removed the pressure switch assembly, checked hose to furnace by blowing on it, air moved freely through hose and to furnace. Placed vacuum on pressure switch by sucking on it, I could hear the switch open and close. Squirted a little wd40 into the switch (as recommend in another thread) re installed pressure switch. No changes to furnace short cycling.

At this point I am stuck, Are these the signs of a failing pressure switch and I should try an new pressure switch part?

Any suggestions and wisdom is certainly welcome! -Dale
 
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Old 01-21-16, 01:27 PM
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>


Your repairman appears to be incompetent.

He should have checked to see if the limit switch was opening to shut off the burners, at a minimum. Based on your own test, he would have found that it isn't.

One approach would be to point out the repairman's lack of skill to the repair service and ask them to send out a senior repairman to correctly diagnose the problem under the company's guarantee.


>


In most furnaces, if a safety switch like a pressure switch or limit switch opens, the furnace shuts off completely and has to go through the entire ignition sequence again before lighting.

The usual exception to that is that short interruptions in the flame sensor current can result in the gas being switched on/off rapidly as you describe.

So measuring the DC current flowing on the wire from the flame sensor might be worth trying. a good flame DC flame sensor current is 3-5 uamps (1 uamp= 1/1,000,000 amp) Typically a flame sensor current of 1 uamp or less will cause unreliable operation of the furnace, which could be something like what you describe.


Also, it's quite likely that the circuit board is bad and causing the problem. That's really the most likely problem, in my view.


Unfoprtunately, you can't test that theory directly. The circuit board should follow the functions and procedures programmed into it. If it does, it's good. If you catch it doing something it shouldn't and there is no other explanation for that action, then the circuit board is bad.

The only other defect that seems plausible to me is that the flame sensor current isn't large enough to insure proper operation, which is why I suggested testing that.


I will look with interest to any comments PJ Max makes on this issue.
 
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Old 01-22-16, 01:12 AM
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Welcome to the forums.

2. After 20 seconds igniter begins to glow and induction fan kicks in.
That's not right. On a call for heat the induction fan should start immediately and 15-20 seconds later the igniter should start to glow.

When the flame sense value reaches the lower limit the board should go into immediate shutdown and restart.

The gas valve should never chatter like that. If it's following an incorrect pattern.... the control board is defective.

According to the pictorial diagram in the link you posted.... there are four high limit switches in a series loop. Any one could cause a shutdown so bypassing one is of no help. Two are manually resettable and two are not.

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Old 01-22-16, 07:31 AM
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PJMax good catch on induction fan sequence. After observing the sequence again, the induction fan does kick in immediately after a call for heat, then 15-20 seconds later the igniter begins to glow.

I will do a test on the flame sensor as SeattlePioneer suggested. I should run the test all the way through the short cycle, correct? If it is failing- I would expect the readings at first to be in the 2-5 uamps range, then turn to erratic readings when we hit the 3minute mark and the gas valve chatters and shuts down, correct?
(multimeter went missing or I would be sharing the test results with you now.)

The flame sensor appears to be as old as the unit, I have not replaced it in my 11 years of ownership... regardless of test I will order and new one along with a backup igniter, as a precautionary move.

To summarize your responses to this point-
1. Check the flame sensor, it has the potential to cause the gas valve to chatter then shut down.
2. If it's not the flame sensor, then it's mostlikely the circuit board. I'm a bit unclear if you are suggesting a way to test the circuit board or that I should consider the path of replacing it.
3. My own question- What's your take on a clogged coil being the potential issue? It doesn't seem logical to me that a chattering gas valve would root cause back a coil blockage.
Thanks, -Dale
 
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Old 01-22-16, 12:24 PM
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An update on furnace- (multimeter is still missing, no tests to share)
I called my local Trane dealer for parts. A. they were a block away from an errand I was about to run. B. Prices are awesome: $4.51 for flame sensor and $97.65 for a control module.

Replaced the flame sensor. Started up the furnace and was 8 minutes into the burn cycle and started to congratulate SeattlePioneer for having figured it out... then the furnace chattered to a stop, darn! The second short cycle lasted the traditional 3 minutes. I'm not sure if a cold furnace relates to 8minutes cycle and a warm furnace equates to 3minute cycle or if the times are just random.

Next step is to replace the control module this weekend. The Trane dealer was fantastic, he suggested buying the control mod now, "if you need it you have it and if you don't, we accept returns on parts." I didn't expect to go to a dealer and get prices $20 to $50 below online prices for a control mod.
Thanks for the help, I will keep you posted.
 
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Old 01-22-16, 02:34 PM
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As long as the readings stay in the 3-5 uamp range, the furnace should operate normally. If it doesn't the control module is likely bad

If the readings drop to 1 uamp or below, the ignition module SHOULD turn off the burners, and you have some other problem.

But the erratic operation you describe is more likely due to a bad ignition module than some other problem, in my experience.
 
 

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