Blower motor continously running... no ignition

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Old 12-29-11, 11:37 PM
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Blower motor continously running... no ignition

Trane XR80
Approx 6 years old.

As soon as the furnace has power the blower motor starts and continiously runs. With or without the thermostat attached.

Just recently cleaned the flame sensor.

The led is blinking slowly, which i understand means no call for heat.

I was told to try to jump the wires on the limit switch and tried this with no sucess.

thanks in advance for any responses...
 
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Old 12-30-11, 01:01 AM
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Take the cover off the burner compartment for the furnace. Describe in detail what happens when you turn up the thermostat to call for heat.
 
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Old 12-30-11, 04:49 AM
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blower running

what you are describing seems like there is another limit switch that may need to be manually reset. look on the furnace wirng diagram and klook for high limits. many have a tiny little button that needs pushed in.
 
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Old 12-30-11, 06:41 AM
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Please look for the nameplate of your furnace, get the model number (XR-80 is only the family group name, not a model number)....and post it in this thread.
You should be able to get to the nameplate by removing the burner compartment door.

The second limit Seabee570 mentions will exist if the unit is a "downflow or horizontal". Please indicate if yours is "upflow", "downflow" or "horizontal". Limits are safety switches of the automatic reset type, so testing them would require a jumping (as you mention), there is no re-set button in them. The ones that could be re-set are flame rollouts.

Please also count the number of pulses on the LED and let us know (example: one pulse, break, one pulse...etc. or perhaps two pulses, break, two more pulses, etc.)

But you need to post the M/N of this unit.
A bad limit is a possibility for sure, but it could also be something as simple as a restricted airflow (dirty filter? super high -efficient filter?) or a loose wire. The fact that the blower is running non-stop is an indication that the module thinks there's an over-temperature condition. Something as simple as a bad/loose electrical connection can do that. Worst case, you could have a bad module...but this is unlikely.

Other than cleaning the flame-rod, what else has been done to this furnace?, when did the problem seem to have started?
 
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Old 12-30-11, 07:51 AM
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Board inside is and Intell Ignition
Model number appears to be TUD1000c936k4

The unit is an up and down model with the filter to the bottem left of the unit when facing the removable door.

Light is blinking slowly on and off.. Which appears to me no call for heat according to whats in the door.

Only recent change was a furnace filter a few days ago.

I attempted to jump a wire across both things that were described. Both the limit switch and a little black disc with a red thing coming out of it. With no sucess at stopping the blower or sweeing ignition.

With the door off and the thermostat high enough, as soon as the the button is pushed so the machine thinks the door as closed the blower immediately startes with no flames or heat.
 
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Old 12-30-11, 08:09 AM
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TUD, your furnace is an "upflow" model

I'll look at the service manual a little later to give you additional leads, but from what you're describing, barring a wire harness or quick connector that might have gone a bit loose (let's hope that's all it is), this malfunction would seem to point to a bad module. I'll let you know later.

Try tracing the high limit wires to the point where they either connect to a harness, the board, or another component and check/attempt for tight connections.

I wonder if you did indeed jump across the limit terminals (you could have jumped a different component). Could you possibly post a photo?

The module is not getting a continuity signal from the limit, which under normal conditions is an indicator of an "overheat" situation, and that is what is getting this blower motor up and running before ignition takes place.
 
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Old 12-30-11, 11:10 AM
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As I look into this, it has occurred to me to ask for the unexpected:
Is it possible that at the thermostat the "FAN ON/AUTO" switch may have accidentally been turned to the "ON" position?

A shorted themostat wire could also cause this...highly improbable, but as we try to come down to the bottom of this problem, is good not to leave any stones unturned. Try disconnecting the "green" thermostat wire at the "G" terminal on the module's thermostat terminal strip. If this turns the blower OFF, chances are you may have a shorted thermostat cable or a defective/improperly programmed thermostat.

Wht kind of thermostat do you have here?
 
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Old 12-30-11, 11:53 AM
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The wiring diagram for your furnace shows that there is one limit switch (Trane calls them TCO), and two flame rollout (Trane calls them FUSE LINK) switches. They are connected with "yellow" wires (no other wires show as yellow there, so they should be pretty easy to spot).

This safety circuitry starts right after the Pressure Switch (note: the pressure switch has an "orange" wire on one end, which connects to the PS terminal on the board, and a "yellow" wire on the other end, wich connects to the HL1 terminal on the board).

This first yellow wire connects to the PS to the Limit, then another yellow wire connects the Limit to the 1st rollout and one more (also yellow) wire goes to the 2nd rollout, after which yet another yellow wire ends up at terminal HL0 on the board.

Here's what you need to do:
Get a jumper wire, one end of which will connect to the terminal of the rollout switch that connects to HLO on the board, and connect the other end of the jumper to the terminal on the PS where two yellow wires are connected.
With this test you are jumping them all. If the blower stops (and I hope you mean the big blower, the one bringing air to the vents thru the house), you have narrowed your problem to one of these safeties. If the blower keeps going, you have either loose connections between the wire harness and the board (fiddle with it a bit), or you have a bad board.

Note: the diagram actually shows "2" limit switches (TCO and TCOS), S= secondary. But I'm pretty sure that your unit being an upflow, will not have that second limit in place.

And please DO NOT leave the jumper in place. It is a fire hazard as you'd be byassing the safeties. Remove once you're done with the test.


Good luck!
 
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