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Old 10-25-2010, 01:10 PM   #11
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Lou,

Thanks for that. I was educated on the function of the sail switch when I had the problem in the Spring. My problem is that the blower doesn't even start, so as I understand it the sail switch can't be causing that. My guess is a bad board.
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Old 10-25-2010, 01:25 PM   #12
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Yes sir,

If the blower fan won't start, and the fuses are checked good, you could also look at the thermostat. The fan wire could be loose or broken.
Will the blower run reliably in "fan"?
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Old 10-25-2010, 01:53 PM   #13
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The fan in the "on" position blows from the AC unit but not the furnace. I have not checked the thermostat wires so I can do that. Does the blower itself have a fuse?
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Old 10-25-2010, 02:20 PM   #14
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Quote:
Originally Posted by grandpaofthree View Post
The fan in the "on" position blows from the AC unit but not the furnace. I have not checked the thermostat wires so I can do that. Does the blower itself have a fuse?
My setup is like that also. Unless there is a separate fuse at the unit, I don't think there is 1 in the power panel.
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Old 10-25-2010, 02:35 PM   #15
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The fan in the "on" position blows from the AC unit but not the furnace. I have not checked the thermostat wires so I can do that. Does the blower itself have a fuse?
ACK! You guys are right! The furnace blower is seperate from the AC fan. My bad. The blower fuse is most likely on the furnace control board.
I feel so stupid.
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Old 11-26-2010, 12:37 PM   #16
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the saga continues

Well, I am now calling this "the case of the intermittently working furnace". On October 31, after several attempts to start the furnace on shore power, 12 volt only, disconnecting all power, then reconnecting shore and 12v one at a time, I still had a furnace that wouldn't start. I made an appointment for service on 11/22. In the mean time, I visited my TT in storage several (at least 6) times removing items for the winter and winterizing it. Each time I reconnected the fully charged battery and tried the furnace, and each time it would not start. I am now convinced that the furnace has failed and I can get a proper diagnosis at the repair shop. On 11/22 I hook up to go to the repair shop, give the furnace one final shot, and...of course, it starts right up!! I cycled it several times and it started every time. I then turned on the gas and the heat started pouring out. I talk to the repaor shop and they tell me if it is working, there is no way they can diagnose the problem. In the interest of saving a $80 diagnosis charge, I cancelled the appointment.

The only difference between 11/22 and every other time I tried unsuccessfully to start the furnace, is that on 11/22 the TT pigtail was connected to the TV. As previously stated, I have tried with and without shore power and I have verified proper voltage each time. Is there any chance that the furnace starting has something to do with being connected to the TV? I have not yet, but I intend to test this out by testing it plugged and unplugged. I am looking for input from some members with good knowledge of 12 volt systems, which I do not have.
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Old 11-26-2010, 05:39 PM   #17
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i don't know if u said, do you have ~12v at the fan when the thermostat switches it on? i don't know how convinent that is to get to in operation. could be the motor. seems like that control board has been replaced too many times for the same problem to keep showing up.
it could also be a bad relay but that is probably part of the circuit board.
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Old 11-26-2010, 07:49 PM   #18
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i just briefly looked at a subraban furnace booklet. the thermostat throws a relay which starts the motor. once the sail switch detects air flow, the circuit board takes over and insures a purge then ignites the burner.
the cheapest thing would be to replace the relay. contacts do go bad. i have had a dc motor on the a/c heat fan in a truck act the same way.

a voltage check on the leads to the motor would tell if either was bad. if u have voltage to the motor and it doesn't run, bad motor. if all is working, u may be able to compair the voltage supplied to the relay to the voltage coming out. if there is a difference, relay contacts could be bad.

i would suspect the motor of being bad.
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Old 11-29-2010, 07:52 AM   #19
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update

I tried to start the furnace with a battery not fully charged, 12.6 volts, and the furnace blower would not start. I plugged the TT into the car, and after a few minutes the blower started up at low speed and gradually increased to full speed. I don't know what this means, but it seems to mean something.
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Old 11-29-2010, 09:58 AM   #20
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i would really be interested to know what the voltage is at the leads of the motor. if i read the manuel right, the circuit board isn't a part of it until the sail switch throws. the thermostat controls the relay that applies voltage to the fan motor. the relay that controls the fan should be a seperate item.

12.6 is full charge for a lead acid battery, i would be interested if that voltage makes it all the way to the motor. if the voltage is normal at the motor, it sounds like the motor is bad. lower voltage at the motor would have to be traced back to the last normal voltage.
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