Furnace help please!


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Old 12-06-13, 11:02 AM
C
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Furnace help please!

At the very end of last winter, my furnace blower motor died. I could smell it, it was obvious. Entire thing seized. Fine, so I shut it off, and in October I called around to get an appointment to have it repaired. Told the lady what was wrong, model/serial #, etc, and she said the tech would bring the parts with him. Mid november was the first available date. I used Sears because I have a relative who used to work there, and I have zero experience working on a furnace.

Sears guy shows up, says yep, you need a furnace motor, capacitor and a fuse. He doesn't have any parts. Gives me some outlandish total for parts(about $800 for a new motor, $150 for cap, etc). Says it will be 3-4 weeks to get parts. Awesome.


So I just ordered the parts myself, used all Lennox stuff, exact match to my versions. Sears didn't charge to come out since they didn't do anything.

Motor shows up, and I took the old housing out. The motor shaft was seized to the blower wheel, and even with a torch and PB Blaster I couldn't get them apart. So I ordered a new blower wheel from midwesthvac, because they had my exact oddball size in stock.

Put everything together last night, no problem. New motor, new blower wheel, new cap, new fuse. Turn it on, motor starts right up. No heat. Blows nothing but cold air. Reset the breaker and the tstat, and nothing.

So I start to check, and find that there is an auxiliary limit switch on the blower housing, and that it will trip if the blower motor quits moving air, which makes sense since I had a blower motor failure. So I bypassed it by shorting it, and the furnace started right up and was working great. It looks like a regular limit switch, but the wiring diagram on the PCB says the big orange wire runs to the aux limit switch, and back again.

Enter 4 hours later, and nothing works. Fuse is good. Board and everything looks fine, all wiring intact. Switch still bypassed.

This is a Ducane Furnace, model MPHA125B5, serial 5979939932.
Aux limit switch is part # 36T01 B2. There is an extra limit switch on the opposite side of the blower housing that wasn't connected, so I am just going to swap them.

Here is a photo of the setup with the dual switches. Both are the same, but only one was connected:
http://www.rocksportsusa.com/chris/furnace/IMAG1870.jpg

This is the old motor still semi installed in the housing. I took photos of every major step so I wouldn't put anything wrong.
http://www.rocksportsusa.com/chris/furnace/IMAG1728.jpg


This is my circuit board:


The two orange wires below are the ones for the aux limit switch. They run into that white plug near the PCB. it used to have ALL of the blower motor wires running through it, but the new motor didn't use the plug, so I just removed the old wires.



So where do I start?
 
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Old 12-06-13, 02:21 PM
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Figured it out. Both aux limit switches are open. Main limit switch is fine. PCB looks fine, and both fuses looked fine.

Started testing every wire, found an open circuit in the connection for the neutral wire running from the main switch to the PCB. I guess I must have wiggled it loose when moving that part of the housing to get access to the blower motor and housing. Hence why it ran for four hours, and then the vibration caused it to lose contact.

Repaired that, replaced the buss fuse, and all is fine. On my way to get a new aux limit switch.
 
 

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