Trane XV-80 no worky

ed van

Model TDD100R9V5F4 furnace. Three zone, programmable thermos, new air filters, has worked fine for past 4 seasons. Diag LED flashing 4 times, stops, then repeats each time flame goes away. Panel says "open limit switch"

Any ideas? Air flow seems good at registers, all filters replaced. Blower hasn't been cleaned in two seasons....

Thank you all for this forum...

JohnH1

Zoning a VS furnace can be tricky. The model number says this is a counterflow furnace. If you have A/C. A dirty coil can hold back a lot of heat causing the limit to trip. Check the tempature rise across the furnace with only the smallest zone calling. The recomended range is listed on the tag with the model number.

ed van

label says 165 max with 135 factory limit setting... will check it... i do have an ac coil on "end" Do i get a coil brush and clean it all out?

I'm a maintenance type... I like manuals or procedures

another note... is there a manual or procedure for cleaning the blower motor? I know it wasn't cleaned last season....i wasn't in town, was busy overseas

JohnH1

Sorry not limit temp. Temp rise Measure the temp in the return and then the temp in the supply durring operation the difference is the rise. The rise should be in the area of 30 to 65deg. This should be listed on the tag. The furnace probly is on top of a A/C coil box. If you remove the front cover you should be able to see and clean the coil. If you pull and clean the blower there are a lot of wires to remove keep track of what goes were. Also be carefull the trane blower housing has very sharp edges. Remove the blower motor if you use water to clean that VS motor is big $. otherwise blow out with air.

ed van

Coil looks fine.... it's not a blanket of fuzz or anything like that. I did trace everything on the schematic and it goes like this
Transformer steps 120vac to 24 vac. High Limit Input(HLI) pin 6 to High Limit Output (HLO) pin 11 current path goes through 1st stage Pressure Switch (ps1), Limit Switch (tco-a), Flame Rollout switch (frs1), Flame Rollout switch (frs2), Reverse Flow Switch (tco-B). When second stage heat is called for, the vent motor speeds up, causing an increase in vacuum which, via a clear tube, closes 2nd stage Pressure switch.
the furnace goes through it's low pressure start up and flame is produced. after a short time, no more flame.
Tests:
sucked on small hose (yes I just said that) and can hear second stage switch clicking.
Question:
What should I do next?

JohnH1

It is not a limit then if the burner fires. It sounds like a dirty flame rod. On the opisite end of the burners from the glow ignitor there is a porclin based flame rod with a single wire going to it. Remove and clean the metal rod with some sand paper. It may not look dirty but it may have a coating on it. Re look at the diognostic light flash code. It should read flame failior or something like that.

ed van

It's me again...
here's what I saw now that I am looking at it more closely. Since I don't have a helper and my furnace is in the attic, I wasn't sure how long the blower stayed on after flame started.
So, here's what I did.
I hit the heat up 5 degrees from room temp, ran up to attic and observed. vent motor started spinning, ignitor glowed, flame kicked on, blower was blowing hot air out around where the ac line goes into the coil. everything was working fine. then, no more air flow! flame still rolling along, draft motor spinning away, just no blower blowing. in about 15 seconds, the flame went out. so I think everything in the limit circuit is working as it should, thank goodness, cuz it got hot really quick without airflow.
what would cause the motor to just shut off like that? since it's two stage, what can I check on it? the furnace is only 5 years old, which means we are in our 4th heating season. blower bad already?
tell me if I am crazy or not, please.

JohnH1

It could be in the motor or the board. With the fan switch in the on position. Will the blower keep running. Check two ways 1st with calling for heat and then with heat switch in off position.

ed van

if put to the on position, without call for heat, it runs forever ( or as long as I could stand cold air in the house)
if put to the on position, with call for heat, does the initial problem.... runs for awhile then shuts off, heat still blazing, then heat shuts off.

i am assuming control board?

AndrewF

So what if you put the call for heat AND have the fan in the on (not auto) position?

__________________
-Andrew
DIY hobbiest

ed van

heat on, fan to on (not auto) the motor runs for awhile, shuts off, flame goes for awhile, then shuts off and get 4 flash (limit cutout)

JohnH1

Sounds like a bad board since the motor runs by itself without fail.

ed van

So as I'm looking closer at the CNT-03078 controller board, with power applied, but nothing "required", I notice the diag led has a nice 1 per second heartbeat. DS6, now the "CFM" green LED vice what is use to be, the "RPM" LED, is flashing rapidly for 7 seconds, then off for 2 second then repeats. Also, the amber led by the y and the amber led by the ylo terminal are lit.
I did the manual check on the v/s vent motor. Good 120 VAC at motor, but no 13 vdc at pins 2 and 3 on the board connector. troubleshooting says replace the board.

1. Any TRANE gurus know what the led code for the cfm LED is telling me?
2. what does the Y and YLO LED being lit mean?
3. remember, all of this is happening in off condition with power applied and door switch depressed.

JohnH1

The CFM led is 1 blink per 100 CFM 7 blinks is the blower meaning the blower should be blowing 700 cfm of air. Any light lit means that is powered. Y and Ylo are both calling for a/c. If you are reading 24v between y and B your a/c is probly running if you have a/c. Or your board is probly bad

Edit

If you do have 24v across y and b your t-stat is calling for a/c and if the a/c and heat are calling at the same time the board may be confused and not know what to do. Check for 24v y to B

JohnH1

If you dont have a volt meter just disconect the y and Ylo wires off the board and see what happens.