So I'm doing battle again with my furnace. Suburban NT34SP. The unit has worked well for the last 16+ years, well except for that whole motor replacement thing 2 years ago. We're in bed and I'm thinking that it's starting to get cool in the house. I check the thermostat and it says the temp is 68°. Not good, as I keep it @ 72°.Turning the thermostat off and back on yields only a single click, then silence. No blower. Doing some online troubleshooting tends to point me in the direction of the board, since I just replaced the motor year before last.I checked for continuity through the sail switch and the over limit switch, both of witch were fine. So, I ordered the board.I received my new board and installed it today. Now when the thermostat calls for heat I get a repeated relay clicking. Still no blower. I can spin the blower cage by hand, so I know it's not stuck. I also have spun it manually while trying to start it, in case it has a dead spot. No joy. If I run a hot wire to the motor it runs fine.I have 12.6 v at the furnace. I found that if I unplug the wire from the board to the blower motor, or if I unplug the ground wire from the motor, I get just the normal one relay click.If I manually close the sail switch, the relay stops clicking. At this point I normally would start looking at the motor again, but if it spins using a hot wire, why doesn't it work when the board feeds it voltage? (I know the board is giving it voltage at the motor because I checked.)I'm not a furnace repair man by any means. I'm trying to dope this out with logic, but this dope has hit a wall.Anybody had anything like this?

72 at night when sleeping in an RV? Lucky it has lasted 16 years IMO.

Not a pro by any means but since you know the motor is OK and it's getting +12V, maybe it's the negative side that's missing (ground).Can you measure across both connections to the motor?

Are you positive that you have the Fan motor wires connected to the correct spades on the new module board? Your model board should have the built in FAN RELAY. When you close the contac wires to start the furnace, the relay on the module board closes and outputs 12 volt positive to the Fan motor. The fan motor starts to run and the sail switch closes and starts the Ignition sequence.1. You may have the wrong module board. Does the Malfunction happen with the original board installed?2. You have the motor 12 volt pos wire on the wrong spade of the module board3. You have lost the ground for the furnace, hence the clicking. Doug

dougrainer wrote:Are you positive that you have the Fan motor wires connected to the correct spades on the new module board? Your model board should have the built in FAN RELAY. When you close the contac wires to start the furnace, the relay on the module board closes and outputs 12 volt positive to the Fan motor. The fan motor starts to run and the sail switch closes and starts the Ignition sequence.1. You may have the wrong module board. Does the Malfunction happen with the original board installed?2. You have the motor 12 volt pos wire on the wrong spade of the module board3. You have lost the ground for the furnace, hence the clicking. DougI'm sure they are hooked up to the right terminals. I marked the power wire before old board removal. The wires and the board have different size spade connectors also. It's easy to follow the wires on this unit and see that they are correct.The board does have the built in relay. That's what's doing the repeated clicking.With the old board I only had one click from the relay, then nothing. I put the old board back in just as a check and it still gave just the one click.As far as loosing ground...here's the kicker...if I unplug the ground wire for the motor, the relay clicks once. If I put a hot wire on the + side of the motor, it runs (using the existing ground.)I have a spade connector inline from the board to the motor (from when I replaced the motor), if I unplug this and call for heat, I see 12.4 v on that wire. When I plug it back in, there is no voltage there and the relay starts clicking.Suburban discontinued the original part number for this board and the new board is supposed to retrofit all models of furnace and water heaters. I'm wondering if my furnace, being about 18 years old, is somehow not captured in this update. The original part number is 520741 and the new board is 521099.

I just went through a bunch of trouble shooting on a water heater in my RV. I'm a very good low voltage tech and worked tech support for many years for a electronic security mfg. However my experience with RV's are limited but I do suggest the following:1) Connect one lead of your volt meter to a good known neg or ground source and set it for DC range around 12vdc2) Since we know the sequent of the start up for a furnace, follow that with all the wires connected3) Turn thermostat to heat and put the other lead of your meter to the motor pos+ lead and see if voltage is present. If so then why isn't the motorr moving ? Search that. If relay clicks, then what is feeding the relay common side ? See why there is no voltage coming across the relay contacts ?4) If the motor starts then check both sides of the sail switches to see if they are working correctly. If OK then why isn't burner starting.5) Ck pos+ leads to gas valve and see if there is voltage there, etc* Just follow the logic trail and see where it fails is my suggestion ...

Suburban Furnace Problem | Good Sam Community

GenoB
Explorer
Jan 10, 2020
I just went through a bunch of trouble shooting on a water heater in my RV. I'm a very good low voltage tech and worked tech support for many years for a electronic security mfg. However my experience with RV's are limited but I do suggest the following:
1) Connect one lead of your volt meter to a good known neg or ground source and set it for DC range around 12vdc
2) Since we know the sequent of the start up for a furnace, follow that with all the wires connected
3) Turn thermostat to heat and put the other lead of your meter to the motor pos+ lead and see if voltage is present. If so then why isn't the motorr moving ? Search that. If relay clicks, then what is feeding the relay common side ? See why there is no voltage coming across the relay contacts ?
4) If the motor starts then check both sides of the sail switches to see if they are working correctly. If OK then why isn't burner starting.
5) Ck pos+ leads to gas valve and see if there is voltage there, etc
* Just follow the logic trail and see where it fails is my suggestion ...
sandpaper600
Explorer
Jan 10, 2020
dougrainer wrote:
Are you positive that you have the Fan motor wires connected to the correct spades on the new module board? Your model board should have the built in FAN RELAY. When you close the contac wires to start the furnace, the relay on the module board closes and outputs 12 volt positive to the Fan motor. The fan motor starts to run and the sail switch closes and starts the Ignition sequence.
1. You may have the wrong module board. Does the Malfunction happen with the original board installed?
2. You have the motor 12 volt pos wire on the wrong spade of the module board
3. You have lost the ground for the furnace, hence the clicking. Doug

I'm sure they are hooked up to the right terminals. I marked the power wire before old board removal. The wires and the board have different size spade connectors also. It's easy to follow the wires on this unit and see that they are correct.

The board does have the built in relay. That's what's doing the repeated clicking.

With the old board I only had one click from the relay, then nothing. I put the old board back in just as a check and it still gave just the one click.

As far as loosing ground...here's the kicker...if I unplug the ground wire for the motor, the relay clicks once. If I put a hot wire on the + side of the motor, it runs (using the existing ground.)

I have a spade connector inline from the board to the motor (from when I replaced the motor), if I unplug this and call for heat, I see 12.4 v on that wire. When I plug it back in, there is no voltage there and the relay starts clicking.

Suburban discontinued the original part number for this board and the new board is supposed to retrofit all models of furnace and water heaters. I'm wondering if my furnace, being about 18 years old, is somehow not captured in this update. The original part number is 520741 and the new board is 521099.
dougrainer
Nomad
Jan 09, 2020
Are you positive that you have the Fan motor wires connected to the correct spades on the new module board? Your model board should have the built in FAN RELAY. When you close the contac wires to start the furnace, the relay on the module board closes and outputs 12 volt positive to the Fan motor. The fan motor starts to run and the sail switch closes and starts the Ignition sequence.
1. You may have the wrong module board. Does the Malfunction happen with the original board installed?
2. You have the motor 12 volt pos wire on the wrong spade of the module board
3. You have lost the ground for the furnace, hence the clicking. Doug
ScottG
Nomad
Jan 09, 2020
Not a pro by any means but since you know the motor is OK and it's getting +12V, maybe it's the negative side that's missing (ground).
Can you measure across both connections to the motor?
Lwiddis
Explorer II
Jan 09, 2020
72 at night when sleeping in an RV? Lucky it has lasted 16 years IMO.

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