Help needed with Trane system "open limit switch" error


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Old 11-12-20, 06:35 PM
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Help needed with Trane system "open limit switch" error

Hello -

I have a Trane XR90 system that is having a problem that I can't seem to diagnose. Here is the system...






In a so far unsuccessful attempt to fix this problem, I have replaced the control board (used KIT18110, which is an upgrade over what I had previously, and this kit included a new ignitor, which I also installed), and I also replaced the flame sensor. But the problem persists.

Here is a description of the problem: The system will be sitting there idle with a slow flashing LED. Then when the thermostat calls for heat, the LED flashes faster, the igniter lights up, the burners light, and the draft inducer starts blowing. All good so far. Then about 30-60 seconds later, the main blower kicks on, and as soon as that happens, the burners go out, and the LED starts flashing four times, then repeats. That code indicates an "open limit switch."

My understanding is that means some internal safety thermostat is over temperature, so opens (disconnects) its circuit, and that's how the system knows it's potentially overheating. I understand this can be caused by restricted airflow, or alternatively, by switches that have gone bad.

Because this happens immediately on the heat call (when the burners have only been on less than a minute), I really don't think the system is actually overheating. Airfilter is new (even tried it without a filter), and registers are all open in the house.

I could only locate two thermostat-looking switches -- one on top of the burner chamber, and one that goes into the inducer motor (maybe, not sure if that's a thermostat):




I unplugged each of those and just connected the two wires for each directly together to bypass the switch altogether, which should simulate the thermostat circuit being closed. But I get the same error... four flashing lights. (I know you'd never want to permanently disconnect these safety switches, it was just a test to see whether those switches were indeed turning off the system. I've already reconnected them.)

Are there any other thermostats/safety switches I can look for to find the culprit?

Any other ideas why this is happening in this system. This is extremely frustrating, and it's about to get cold.

I actually had an HVAC company in the other day, and they were not helpful, just said I needed a new board, and wanted like $950 to replace the board. I ordered it myself instead (they were nice enough to leave me with the exact part number of the kit) for $150 and installed it in like an hour. It was a drop in replacement, so I just labeled all the wires. The FIRST time I fired it back up, it worked fine. The second time the thermostat called for heat, this issue showed up again and the system was just blowing cold air.

If anyone could offer any suggestions or advice, I would greatly appreciate it. Thank you in advance.

Dave
 
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Old 11-12-20, 07:27 PM
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LED flashes faster, the igniter lights up, the burners light, and the draft inducer starts blowing.
Wrong. The order you describe and we read is critical. The LED flashing fast is a call for heat. The draft inducer then starts first and must prove via the pressure switch and then everything else follows. The igniter should never be on if the inducer isn't running.

The two switches you are bypassing look like burner overheat sensors. You are looking for an open hot air sensor. There should be a diagram on the back of the blower door that shows the limit switches. On Trane.... one may be located on the main blower below.
 
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Old 11-12-20, 07:51 PM
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Yes, you're right. First the heat call comes (fast flashing LED), then a few seconds later the inducer and the igniter go on, then about ten seconds later the burners go on, then about 20 seconds later the main blower comes on and... the burners immediately go out and I get four flashing LEDs (and the main blower stays running).

I understand what you're saying about the hot air sensors. Thanks a lot for your help.

Here is the diagram:




So would this be one of the switches (hot air sensors) that I'm looking for?



Is that switch accessible from there? (i.e., if I undo the screws, will I be able to remove the switch?). Should I try to temporarily bypass it, just to see if the error code and issue go away? (Then if so, I can try replacing the switch, and if that doesn't work, will diagnose what's causing things to overheat.)


I assume this is not a hot air sensor (looks like just a loop of wire, just wondering if maybe there's a sensor behind it):



Thanks again for responding. Really appreciate it.
 

Last edited by dave999z; 11-12-20 at 08:24 PM.
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Old 11-12-20, 08:22 PM
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Not 100% sure what that two black wire plug is but according to the wiring diagram it looks like "blower compartment safety interlock".

Typically limit switches on a Trane are wired with yellow wire.
Yes... that one in the picture is the one you are looking for. You can temporarily bypass it.
It's an L190-30F. Opens at 190 and closes at 160.
 
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Old 11-12-20, 09:05 PM
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Well, I thought that was it. Bypassed that switch (unplugged the two yellow wires and jumpered them together), and the first time I turned the system on it worked - burners stayed on even when the main blower came on. Lasted through a full heat cycle. But next time the thermostat called for heat, the problem was back -- four blinking lights. Uggh. It is tough to diagnose this thing when the problem is intermittent like this.

Any other place in the unit I should look for a similar switch?

Thanks again.
 
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Old 11-12-20, 09:47 PM
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Follow those yellow wires. Additional sensors will be inline.
Ultimately they will end up on the large plug on the control board.
 
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Old 11-13-20, 01:14 PM
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Thanks.

I think I found the culprit. It's the "TCO-B Reverse Flow Switch" on the diagram. It looks like this:



When I bypass that, the problem goes away. When I check continuity across that switch, the circuit seems to break if it gets even the least bit hot. I'm going to try replacing this switch. If the problem persists with the new switch, then I guess I have an airflow problem.

I ordered a replacement. Hopefully this does it.
 

Last edited by dave999z; 11-13-20 at 01:54 PM.
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Old 11-13-20, 04:07 PM
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I have a related question...

When a heat call ends, my furnace control board (Emerson 50M56-495) is supposed to keep the inducer fan on for a little bit (maybe 10 seconds) and keep the circulator fan on for another minute or so (it can be set to 60, 100, 140, or 180 seconds via dip switches) to dissipate the heat.

That is not happening currently. When a heat call ends, everything shuts off immediately--burner, inducer, circulator. I think this may be one of the reasons these limit sensors are tripping. Everything gets super hot when it doesn't clear all that hot air at the end of a cycle.

Any idea why this may be happening? Is it wired wrong? Is there something I have to program on the control board?

Wires from thermostat are connected to control board terminals as follows:
blue-->C
green-->G
red-->R
white-->W
yellow-->Y

The air conditioner is also wired to C and Y of the control board terminals.

Any help would be greatly appreciated. Thanks!
 

Last edited by dave999z; 11-13-20 at 04:23 PM.
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Old 11-13-20, 04:17 PM
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The burner and the draft inducer are supposed to stop immediately at the end of the heat call.
Running the inducer longer just sends all the heat in the exchanger out the exhaust.
The blower is time delayed and is user settable.

Where is that switch mounted...... on the draft inducer housing ?
 
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Old 11-13-20, 04:55 PM
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That switch was inside the main unit near the main blower. (Right behind my humidifier, which is how I was able to access it.) Unfortunately, I don't think that switch is the problem. Thought it was, but I think the real problem is how much friggin' heat immediately builds up as soon as a heat call ends and everything shuts off.

The furnace control board (which is brand new) manual says: "When the thermostat is satisfied, the gas valve is de-energized. After proof of flame loss, the heat delay-to-fan-off period begins and the inducer blower remains energized to purge the system. When the purge is complete, the inducer blower is de-energized. After the delay-to-fan-off-period ends, the circulator fan is de-energized."

https://ng9ns1dgijc23gl7u480isox-wpe...structions.pdf

I'd really like to find out why mine is instantly turning everything off when a heat call ends, rather than letting the circulator fan continue to run for another 60, 100, 140, or 180 seconds.

I really appreciate your time helping me try to figure this out.
 
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Old 11-13-20, 05:30 PM
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I'm not following,.... you said the burner lights and then after 20 seconds it goes out and you get 4 flashes and the blower keeps running. We need to find out which safety loop or sensor is opening.

As far as the main blower running after the call for heat has ended is strictly a timed function on the control board. If that is not happening.... that is a board issue.

Yes.... I see that has a 5 second post purge. That won't do much to cool anything.



 
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Old 11-13-20, 05:53 PM
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The problem with the four flashes and the burner going out doesn't happen if I bypass all the sensors. But I'm saying I don't think the sensors are necessarily bad. (I've ordered replacements regardless, but won't have them until next week.). I think the system is actually overheating, particularly at the end of a heat cycle when everything abruptly turns off. It heats up so much that even if I try a new heat call a few minutes later or even 10 minutes later it's still too hot and that is probably what is opening that limit sensor and giving me the four flashes. That limit sensor then needs to get back down to 85C to close, which takes a long time. Whereas if I just manually do a fan call for a minute after a heat call ends, the system doesn't get scorching hot like that. So I really need to figure out why the delay-to-fan-off procedure is not happening. Was wondering if perhaps there is something about my thermostat wiring. This is a brand new board, so I wouldn't expect it to have this problem right out of the box.

I do not recall if the previous board had this issue. I think the system used to take a minute or so to turn off when I'd turn the thermostat off.

I appreciate your help and apologize that these issues are getting jumbled together. This all seems related though.
 
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Old 03-20-21, 08:48 PM
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Did you figure out what your issue was? I have nearly the same model but mine is branded American Standard. My problem is nearly the same too.
I have tried many of ​the same things you have to remedy the issue. I upgraded the board Kit18110. Replaced the reverse flow switch and still the same issue. One difference is, I have set the dip switches on the board to run the circulation fan delay 180 seconds after the burner is shut down. That part works properly. From a cold start it works fine. When it has ran once already the reverse flow switch opens and it cuts the burners and runs the circulation fan. I noticed many of the reverse flow switches for other Trane models are rated to open at 130 F to 140 F. Instead of 95 F.
I honestly can't say that it ever ran properly since I had it installed. I just never paid very close attention because it always heated the house.
 
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Old 03-21-21, 11:50 AM
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Hello. Yes I did figure out the problem (or, rather, figured out that my solution was incorrect). The replacement board I had used (from KIT18110) on the first attempt was the wrong part. I got that part number from the HVAC guy who quoted me $950 just to replace the board and wrote the part number on the quote. What a joke. Not only was that quote a ripoff, but it wasn't even correct information. The board in that kit was not wired correctly for my furnace system so would not run the fan at the end of each heat call, which would then cause the whole system to heat way up, tripping those temperature switches, which wouldn't cool down enough to reset before the next heat call.

I actually called Emerson White Rogers, who manufactures these control boards. They answered the phone on a Saturday and were incredibly nice. They helped me for like 30 minutes, determined the correct board (50A65-5165) and ignitor (768A-815) part numbers for my Trane system, and emailed me the manual. Fantastic service (and I didn't even purchase anything directly from them). I ordered the correct part numbers from Supply House (correct board was only $87), installed them, and everything works great.

So ultimately I replaced the board, ignitor, flame sensor, and three different temp limit switches. Probably like $175 total in parts. The problem was some combination of those. Everything works great now. Installed an EcoBee thermostat too, which now directly controls the humidifier.

(BTW, when I finally installed the correct board, I had to get the correct ignitor for that board too. The ignitor that came in that incorrect kit was not compatible with the correct board.)
 

Last edited by dave999z; 03-21-21 at 12:10 PM.
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Old 03-21-21, 03:07 PM
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Great deal. I'm glad you were able to figure it out. I may give White Rogers a call myself. I have checked with two different parts suppliers that say the 18110 kit is the correct one for my system. But it behaves different than my old board. Like the initial fan on delay is now 45 seconds instead of 10 seconds like the manual states it should be. So it developes a lot more heat before the blower kicks on.
Do you know what the delay is on your new board. Just curious since it is a downdraft model too.


 
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Old 03-21-21, 03:18 PM
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You can view all the sequence times for the 50A65-5165 board (which is correct for my Trane system) on page 2 of this PDF: https://climate.emerson.com/document...us-3583534.pdf

BTW, if you call White Rogers, you'll want to have the part/model number of the original board that came with your system. That part is likely discontinued, but they will tell you what the correct current replacement is.
 
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Old 03-21-21, 03:19 PM
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Awesome. Thank you for the info.
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Old 03-21-21, 03:28 PM
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No problem. Good luck. Hopefully you'll be able to solve whatever the issue is.

BTW, the board in the 18110 kit looks practically identical to the 50A65-5165 board, but the pins on the 12-pin connector are wired differently. That's why the 18110 kit's board wasn't properly controlling the fan on my system.
 
 

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