Have a Trane XE90 furnace, when turning on heat this year for the first time the furnace blows an inline fuse that is on the output of the 24v transformer. Disconnected all wires from thermostat to furnace and then jumpered the red to white posts on the control board to turn on heat and still blows the fuse. Does not appear to be external to the AC unit or thermostat. Having difficulty reading schematic as it is very old, Does the 24V feed the sensors on the furnace such as the pressure switch? flame sensor? gas valve?, Where else does the 24V power go to that could cause a short?

Probably stuck blower relay

Sounds like you didn't turn the power off and blew the low voltage fuse. Typically a 5 amp blade style (like a car) inside furnace. TURN THE POWER OFF!!

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My first thought is, if you have to ask this question, you should not be playing with your HVAC because you will likely hurt yourself. But if you insist; What does the humidifier require 120 or 24V? If it is 24V, then you take it after the thermostat at the relay that turns on the rest of the system. If the humidifier wants 120V, take it from the switched portion of the above mentioned relay. Your heat pump is 240V, so one side of the humidifier goes to the relay, the other side goes to ground, not the other side of the 240V return. If your heat pump is connected through a GFI, this will likely trip the GFI, shutting down your heat pump.
Go download http://www.trane.com/webcache/un/split%20system%20air%20handlers%20%28ssa%29/product/12-1054-05_04012001.pdf

Go to page 12. 24V should be terminal 6 and A on the relay. 120V should be between B and ground on the fan motor. Before opening anything, turn all the breakers to the heat pump, likely 3 dual gang breakers. Have a couple spare automotive ATO 5A fuzes in case you blow one. Have an shut-off valve for the water supply. Please be careful!!! What you are doing has a high potential of hurting yourself or blowing up something in your heat pump, leaving you without heat.

on older tranes if 120v power is reversed it would do that try switching leads on pigtail to board. make sure to unplug first!

hello Miss/Mrs/Mr,
I have the furnace trane xl 80, when I turned for heat everything working but only blower motor was not running, I wait for 1 minute all system suck down? I guess the capacitor blow out, that the reason why blower motor was not running

Please email me that is the

dodgy relay possibly

The short is on the 24V side...I would put a 3 amp fuse in series with the Hot 24v line before it gets to the circuit board from the transformer you will have to replace again....have a few spare fuses....after having done that./...turn the power on and call for heat...Observe what happens and what stage of the cycle you're in when the fuse blows...Exhaust fan starts...Ignitor starts to warn up....etc...I need more info...shorts are hard enough to find as it is.......does it blow the fuse immediately...usually it will go through a few stages before you get to the problem piece....Make and Model of the furnace could also be helpful to me

How about the Heating/cooling thermostat. Sounds like you better check it.

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Check to see if the green thermostat wire is conneceted to the "G" terminals on the thermostat and on the furnace.

Trane is very picky about being a Trane dealer before they will even talk to you. But as far as trouble shooting a module there no guides. If the flame comes on then the fan does not shutting down the furnace it could be a fan relay failure or the fan module failure. The modules propose is to allow the furnace time to heat up before bringing on the fan so as to not blow out cold. Or the fan motor it's self could failing