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SF-25 furnace troubleshooting


Cranky
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Hi, I have an older RV with a SF-25 furnace in it. It has outside access and is pretty easy to remove. The furnace was working fine, but making a lot of noise/whining when running. So I pulled it out to see if I could lube up any moving parts and to make sure nothing was rubbing. When I put the furnace back in place, it will now not turn on. I have good voltage all the way to the time delay switch. But not going from the time delay switch to the motor. When I jumper the two blue wires on the furnace, I can get it to come on. But it will shut off shortly after removing the jumper. It does not have a no spark shut off and will run without ignition. The 4 wire connection from the rv to the furnace has 3 purple wires and one green(ground). I have good voltage (13V) coming from the one purple wire that matches up to the red on the furnace. Should there ever be voltage from either of the other two purple wires that match up to the blue wires on the furnace harness. What went bad on me from pulling it out and putting it back in? What other troubleshooting should I do? Thanks for your time and input. 
 

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Unless you flipped that plug connect in reverse the wiring should be fine . You should only have 1 positive wire when un plugged.

  Did you by chance turn the on off switch on the left side of the furnace off.

 If nothing was rubbing, then the bearings may be going bad.

 I will check back later as I have some travel to do today.

 

   Vern in a T-shirt  

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12 hours ago, Cranky said:

When I jumper the two blue wires on the furnace, I can get it to come on. But it will shut off shortly after removing the jumper.

The two blue wires are to and from the thermostat and that is what turns the furnace on and off. Have you tried connecting them to the thermostat and turning it on and off with the thermostat? If you look closely at the electrical diagram in the service manual, page 16, figure 17, to the lower left side are those two wires. It is supposed to work like that.

12 hours ago, Cranky said:

It does not have a no spark shut off and will run without ignition.

I am not sure what you mean by this? I can tell you that the furnace you have does have a lock-out when it does not sense the heat of burning propane that will turn off the spark and shut the propane valve, but it does not turn off the blower. Only the more recent furnace models will shut off the blower when they go into lock-out.

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3 hours ago, Wrknrvr said:

Unless you flipped that plug connect in reverse the wiring should be fine . You should only have 1 positive wire when un plugged.

  Did you by chance turn the on off switch on the left side of the furnace off.

 If nothing was rubbing, then the bearings may be going bad.

 I will check back later as I have some travel to do today.

 

   Vern in a T-shirt  

Didn't flip the plug, it does have two D shaped receptacles to prevent the accidental of plugging it in backwards.

Mine is old enough that it does not have an on/off switch on the furnace. Wish it was that easy, lol.

Couldn't find any rubbing, pretty sure at this point that the motor bearings are going and maybe an imbalance on the squirrel cage fans is contributing to the noise. The old give it a smack on the side would quiet it down for awhile. But the noise always comes back.

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3 hours ago, Kirk W said:

The two blue wires are to and from the thermostat and that is what turns the furnace on and off. Have you tried connecting them to the thermostat and turning it on and off with the thermostat? If you look closely at the electrical diagram in the service manual, page 16, figure 17, to the lower left side are those two wires. It is supposed to work like that.

I have not tried connecting the blue wires directly to the thermostat. Only when I plug in the 4 pin harness plug between the coach and the furnace. I will give this a shot when I get back to it.

3 hours ago, Kirk W said:

I am not sure what you mean by this? I can tell you that the furnace you have does have a lock-out when it does not sense the heat of burning propane that will turn off the spark and shut the propane valve, but it does not turn off the blower. Only the more recent furnace models will shut off the blower when they go into lock-out.

I just meant that the blower will run even if the gas does not ignite. 

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