Help, No Blower - "Open High Limit Device"

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  #1  
Old 11-11-05, 09:06 PM
magnumeagle
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Unhappy Help, No Blower - "Open High Limit Device" - Replaced Motor & Still No Blower

I have a 2 Trane Furnaces with A/C units one in basement and one in Attic. Both have been working fine, but due to recent stretch of warm weather have not frequently been needed. Went to use the Attic unit for 2nd story and it turns on, ignites, but no blower. Also no blower in A/C mode now. I took covers off and held down cover switch and cycled system. The unit started with the gas evacuator blower then ignitor came on, then gas burner ignited, then it clicked and no blower, 30 seconds later it shut down the burner. The red LED on the circuit board next to the blower was flashing 4 times indicating "Open High Limit Device" according to the sheet glued to the blower door. Is this major or can I get a replacement limit switch, if this is the issue, where is the limit switch for the blower?

Please help, I don't want to call someone to come out and replace some stupid switch, when I can do it myself.

Thanks to all for any help.
 

Last edited by magnumeagle; 11-15-05 at 09:37 PM. Reason: Update
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  #2  
Old 11-12-05, 05:04 AM
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Sounds like the blower motor has gone out if it didn't come on with a/c or heat. or it's the board..
 
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Old 11-12-05, 06:55 AM
magnumeagle
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Thanks Jay, not what I wanted to hear though. Is it common for a blower motor to go out? Seems like something that would last for a while the house is only 5 years old!

Scott
 
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Old 11-12-05, 08:07 AM
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it should have a parts warranty of 5 years.
 
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Old 11-12-05, 06:20 PM
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Originally Posted by tinner73
it should have a parts warranty of 5 years.
A lot of times, those furnaces that have the diagnostic guide by flashing the led light a number of times?...the high limit that is open can be reset. It is usually a thermodisc type switch that is about the size of a dime and has two wires going to it. It could have a red color pin-like button in the middle of the disc, between the two wires. If you find such a switch, push down on the pin. This will "close" (and reenergize) the open circuit. If that is your problem, you will hear the limit switch make a little click. Then you will know that is it. (If it doesn't click when you push it in, then that wasn't your problem.)

Now..then comes the question as to WHY it limited? Could be as something as simple as an electrical glitch that did not let the blower motor to come on. Naturally if the blower does not come on, the heat exchanger will get too hot and the furnace limit switch will click "open"(which shuts the furnace off). Or, it could be that your blower motor burned out or is shorted out or the bearings are shot. With the furnace off, see if you can freely rotate the fan with virtually no resistance....and it will keep spinning when you leave go. If you can, that is good.

But you are not necessarily out of the woods. You need to run the furnace and do the same test if the furnace shuts down again, and do it rapidly before it cools down. Sometimes then you will notice how the blower wheel drags. You could have a motor bearing problem. Motors are carefully designed so that the armature has only thousanths of an inch clearance around the outer perimeter. If the bearing gets sloppy, the motors armature (the part that turns) takes a shift and it causes a heavy pull towards the internal magnet, and this like seizes up the motor. If you know how to check for sloppy bearings by wiggling the fan blower from side to side, that would be good. Motors always have "end" play, but virtually no side-to-side play. If you notice side to side play, the motor bearings are shot. Sometimes, the bearing is so bad and the armature shift so severe that when the fan tries to come on, all you will get is a hum and you will see the fan spin but a smidgeon and stop, as it is seized in it's tracks by not from the fact that the bearing is metal on metal, but because the magnetic draw of the armature to the side of the motor is holding it there. Sometimes in emergencies, yo can help the motor get going by spinning the blower wheel by hand and it will get going. But obviously you have to be careful and watch your hands/fingers.

You can pretty much test to see if a motor is plain old shorted out also, if you are a handy electrician. Either disconnect the two main wires that feed the blower motor, or snip them if you have to. But if you do, you will have to wire nut them back together again later. Then take a voltmeter set to ohms resistance and put one lead on each motor wire and see if the needle goes almost all the way to the right. If it don't the motor is for sure burned out in the sense that it isn't shorted out, but rather disconnected internally burned out. (But to be sure, always double check to make sure your ohm meter is working by touching the two probes together to see it the needle goes all the way to the right as it should. Then, retest the motor wires.)

To see if the motor is actually shorted out, you put one test lead on any of the two motor wires and the other test lead to the motor housing itself. If the ohm reading makes the needle go over to the right, you have a dead short. Either way, the motor has to be replaced. To know that the motor is most likely good requires that when you put one test lead on each mortor wire (when set to ohms), that the needle goes almost all the way to the right. AND when you put one test lead on one of the motor wires and the other test lead to the motor metal housing, the needle never moves from it's far left location. Then, the motor is probably good.
 
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Old 11-12-05, 08:21 PM
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if your hearing the click from the board, but not getting the blower to come on it is likely to be the motor, the capacitor, or the board.

To check the capacitor turn your fan switch to on while the power is off to the unit. with the door offreach in and spin the motor by hand while it is still spinning turn the power back on. if it is the capacitor the motor will probably rstart and run in this case albeit not for ever, and it is not good for the motor to run like this for extended periods.

If this test shows the capacitor is bad replace with capacitor with the same uf rating, and equal or greater volts rating.

If you are handy with a volt meter to check if it is the board trace the wires from the blower back to the board, again with the fan switch on, and the power off. with a volt meter check the voltage to the motor leads that should be energized in the fan cycle when power is applied. should read 120 volts. or you could check the amps on the blower leads, but thiswould only show is the fan was locked up,cause if it had open windings you would show 0 amps.

If these two test show ok then it is most likely the motor.

Just sum Ideas.
 
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Old 11-15-05, 09:27 PM
magnumeagle
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Unhappy

Thanks to all of you for your help, I have replaced the blower motor - $175, and it still won't start. When I pulled the old fan assembly out, there were some pieces of the chopped insulation from the attic in the fins of the blower fan. These pieces were not pink, but black and burnt. I decided that must mean that the blower motor went out. Replaced the motor - not too bad a job, but the price of the motor sucked. I put everything back together and was excited to fire up the furnace. First thing I did was turn the fan from auto to ON - nothing, same for auto A/C and Auto Heat. The fan is not kicking in - all works, burner fires, no error signals on the LED and then when the fan doesn't kick in it shuts down the burner and gives the Open High Limit Device message. I believe this is probably because at least one of the high limit devices opens due to excessive heat in the burner chamber. What is causing the blower not to work? Worked fine and just quit out of the blue. Replaced the motor, still doesn't work. I took the suggestion and tried spinning the fan during the fan kick-in and that didn't make it spin. What else is there?

Please help, you guys have given some great suggestions, but I still can't make it work.

Anybody want a used blower motor?

Thanks for your help.

Scott
 
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Old 11-16-05, 10:40 AM
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Some where on that unit should be a wiring schematic, trace the wiring from the blower. You can buy a 115 V test light at any hardware store for a buck or so to find out where you have lost voltage. Just put the fan on run and start looking.
 
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Old 11-17-05, 05:37 AM
magnumeagle
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Thanks slls, I'll try that. I have been checking components by removing them from my unit in the attic that doesn't work and replacing them on the identical furnace in the basement that does work and running it to see if I have a fan fail to start. So far I have tested the limit switches and the capacitor and they all work fine on the basement unit. Now I have a new blower motor, the capacitor is fine, what's next, the circuit board?
 
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Old 11-17-05, 11:40 AM
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If the blower wires trace to the circuit board I would say yes. I tried to find a Trane wiring schematic online but Trane don't publish one, there has to be one on the furnace.
 
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