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airheatman
airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 8260
Experience:  Furnace, A/C and heat pump specialist.
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I have a Trane xe70 furnace with AC. It's about 26 years

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I have a Trane xe70 furnace with AC. It's about 26 years old. The ssy fuse (type T, 12amp) keeps blowing up. This is what I did so far. First time I replace the fuse, I turn on the fan at the thermostat, it did not blow the fuse, but after turning to cool it blew the fuse. Second time, I removed all the wires from thermostat, and only connect the green and red wire to test the motor. It blew the fuse. I open the circuit breaker panel and 120v was present at furnace breaker. I carefully inspect all the wires inside the furnace to look for cuts or loose wire. Everything seem ok. Also, I was able to remove the blower motor with the cage to test the motor.There were a little corrosion at the connectors to the motor harness, but I was able to clean and removed connector to the motor harness. I measure the resistance of each color wire with the white color wire as common. This is reading I got: black and white is .9 ohm, red and white is .9 ohm, yellow and white is 1.5 ohm, blue and white is .6 ohm. Note: Only yellow and blue wires are not connected to any components. I believe this readings shows the motor is bad. I am about to order a new motor and capacitor to repair furnace. Is there anything I need to test or be aware of before ordering parts. I am handy in removing and installing components, but need help in troubleshooting the real cause of the problem. Thanks, Phillip
Customer: replied 8 months ago.
I'm editing the first post. "First time I replaced the fuse, I turn on the fan at the thermostat, it did not blew the fuse, but turning the fan to auto and system to heat, it blew the fuse." If this is any help?
Customer: replied 8 months ago.
Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Help via Email or Text Message. Let me know if you need more information, or send me the service offer(s) so we can proceed.

Do you have a picture of the wiring diagram on furnace showing this fuse ?Due to age I may have a hard time finding a diagram online.

If you have a diagram, take a clear picture and post it here.

If you have a paper clip icon on your page you can attach using that link.Or

You can upload the pictures using the tiny pic site .

Here is a link to the site.

http://tinypic.com/

Click on Browse , select the file from your folder.then click UPLOAD NOW.

They will give you a quote to enter in the box to verify you are a human.Enter the quote and click UPLOAD NOW

You will see your picture on right side of screen.

Then copy the link URL for email & IM

Paste that link here on the site.

Customer: replied 8 months ago.
There's a picture of the wiring schematic on the inside cover. I'll send you a pic when I get home after work.

Sounds good. Look forward to helping you .

Customer: replied 8 months ago.
Here's pic of wiring diagram and furnace. Let me know if you need more pic of furnace.
Customer: replied 8 months ago.
Here's pic at the site Tinypic.com
http://tinypic.com/r/2unvkhw/9
http://tinypic.com/r/2vtnnm0/9

Can you point out on diagram and in pic where the fuse is located or are you referring to the fuse outside of the furnace at power supply?

I am not seeing such a fuse inside unit so I suspect you are referring to power supply.

There are 4 components that are different in HEAT mode than in FAN On mode .Fan limit. tdr,blower relay fan speed on motor.

So if fuse does not blow in FAN ON but does in heat we need to determine which is causing fuse to blow.

The link below shows the fan circuit in heat mode. Start by examining the wires to make sure none are cut, pinched or burned allowing wire inside of sheathing to touch cabinet.

If wiring appears ok, lets start with motor.High speed winding could be good but low speed shorted so on fan call it would run but on heat it would blow fuse.

Locate the red wire coming down from fan limit and move it over to a different speed wire on motor and see if fuse still blows.If it does not blow we know we have a bad motor winding.If not move the to fan relay.We will go from here.

Customer: replied 8 months ago.
Here's pic of the fuse that blew above the Switch that is on the wall next to the furnace. I'll try following the steps to troubleshoot the furnace you send me.

Thanks for verifying this. The only thing that will make that fuse blow is high amp draw or a dead short in high voltage circuit.Lest check the wires to make sure no skinned place .I have seen the wires get hot near burners and melt, etc,

If wires look ok, Move that red wire from fan limit over from red motor wire to black since we know high speed in motor is ok.f furnace works that way we know the low speed motor winding is shorted.If not we will go from there.

Customer: replied 8 months ago.
Ok, I'll go that route.

Let me know what you find

Customer: replied 8 months ago.
This is what I did so far. Check wires close to frame and burners. Everything seem ok. Measured continuity between red wire from motor to fan limit. I decided to verify again that the fuse doesn't blow when fan is turn on. I install new fuse at walk. With system off and fan off at thermostat, I then turn only the fan on, it blew the fuse. So my first assessment was wrong. Should I still switch the red wire from the fan limit? Is there other steps I need to take.
Customer: replied 8 months ago.
Found melted wire! I decided to look behind the junction box! Found a red and black wires melted together. When I separated the two wires, saw bare copper wires. Attached is pic. Anything I need to do (beside installing electrical tape at the bare wires) or check for other components that might have shorted.
Customer: replied 8 months ago.
All the wires were long and stuff inside the junction box. Found more cut wires and burn mark behind the blower relay. Attached is pic. Is there a way to test the relay or should I just replace it.
Customer: replied 8 months ago.
I just shorten the wires, reconnected the connectors and tapped any bare wires. Is it safe to turn on the motor now to test or should I wait for fan control center with relay to install.

AS long as all wires are taped up you could try motor but since that relay had been hot in the plastic plug I would not run long

Customer: replied 8 months ago.
Ok! I'll give it a shot. Will turn on motor for a short time. I will be putting parts back in.Let you know what happens.

Let me know

Customer: replied 8 months ago.
Tape all wires and connectors. Carefully reinstall wires in junction box. Turn on fan at thermostat. Fuse blew again! I guess I need to order fan control center and relay. Is there anything else I can try or order?

You might want to run a hot wire to white and black or white and red and see if the fan trips breaker or will run.If the red and black touched very possible motor fried

Customer: replied 8 months ago.
On the first post, I mention I did resistance test on the motor to verify the motor is bad. Is this resistance test accurate to see if motor is bad. I'm not too comfortable in connecting hot wires to motor and have never done it. Should I just order a motor since it's more than 25 years old?

I think I would replace motor

Customer: replied 8 months ago.
I will order fan control center and relay (the one you posted on the link) , the fan motor and capacitor . I found a site that shows a new motor is $149.00 (http://keithspecialty.com/k/Trane-MOT12767.htm#5092). What do you think of this motor at keithspecialty.com) Is there a better site that sells motors and other furnace parts. Any more parts that you would replace for this furnace?

I do not have specs on motor in front of me but as long as horsepower,amp draw, votage and rotation or reversable match that is fine.Youu can probably beat the price by using a generic motor like the trip saver.

http://www.ebay.com/itm/PROTECH-Trip-Saver-Blower-Motor-51-23012-41-1-6-1-2-hp-1075-RPM-Reversible-New-/222206722793

Customer: replied 8 months ago.
I will look into trip saver motor. Again I will order the three parts (relay, motor, and capacitor). Any good site, I can purchase a hub puller tool, I had a hard time removing the motor shaft from the cage when I first removed the motor. Thanks for all your help and knowledge. I will update you when I get the parts.

Sounds good.I buy when I can from an Amazon seller.

Customer: replied 8 months ago.
I should get my blower motor today. I'll be installing the motor, capacitor, and fan control center with relay tomorrow, Saturday. Anything I should be aware before installing parts and then turning on motor?

Just make sure to turn power off before starting.If you have any questions whatsoever about which wires go where, do not guess,let me know and we will figure it out together.

Customer: replied 8 months ago.
Thanks. I'll take pictures of the wiring hookup before disassembly.

Sounds good

Customer: replied 8 months ago.
Successful! I was able to turn on fan right away and later tested the heater. Thanks for all your help!

If you have further questions, please let me know.

Thanks for letting me work with you.If my answer has been polite, professional,and thorough, please click one of the ratings before you go. If you still need help let me know!.Expert is not paid for assisting you until you rate answer.Feel free to follow up for 30 days at no additional cost. Bonus always appreciated if you feel justified.

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