Suburban Furnace Problem

[sandpaper600]

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?

[sandpaper600]

Well, I FINALLY got this furnace sorted out. Turns out it wasn't the board after all. I was showing 12.6v on the hot wire, using the furnace chassis as a ground.

What I didn't know, until I stumbled upon it, was that there was no ground being supplied by the 3 wire connector. when I checked voltage between pin 1 and pin 2, I got nothing.

It should have been (pin1) 12v+ and thermostat+, (pin2) 12v-, (pin3) thermostat-. Instead of tearing out the couch to find the faulty wire, I ran 2 new leads from the house battery to the 3 wire connector.

What I realized afterward was the fan was only running on the low side when I hot wired it, due to incomplete grounding. When I gave it the new supply wires, the blower really spun up as it should have.

Sorry I'm so late getting back to this, but life got in my way with Dr appts and the such.

I hope this helps somebody in the future to not make the same mistakes as me 🙂

[dougrainer]

sandpaper600 wrote:
@dougrainer - I think you may have solved it! Nobody ever mentioned anything about this wiring needing changed and I bought the board from the same people. I have even had email dialog with them about this problem.

I'm curious about the info in caps. Where did you find that? I followed the link and looked around for other websites selling the 520839 kit, but can't find it.

Thanks very much for your efforts in this. I'm going to order this and I'll post back after I get a result.

It was simple. You stated the original P/N and the replacement. I went to my sources and NONE had the correct older replacement. BUT, the replacement you purchased did NOT state it would work for a SP model. ONLY S model. This was confirmed by various part suppliers. THAT raised a red flag for me. So, I researched and found that your new model required a extra wire kit. The caps was because that is how I copied and pasted when I found the solution. Once I found a supplier that listed the correct SP board(your new board) it stated you had to have the wire mod kit. If I was at work, I would have called Suburban and asked what was the correct Board number and they would have told me with the proviso to get the wire kit. Doug

Per Suburban, you need the wire kit because you have 2 fan relays interfering with each other. A Time delay and the built in on the module board. The wire kit goes around the time delay relay.

[sandpaper600]

@dougrainer - I think you may have solved it! Nobody ever mentioned anything about this wiring needing changed and I bought the board from the same people. I have even had email dialog with them about this problem.

I'm curious about the info in caps. Where did you find that? I followed the link and looked around for other websites selling the 520839 kit, but can't find it.

Thanks very much for your efforts in this. I'm going to order this and I'll post back after I get a result.

[GenoB]

I'm leaning towards the board. Just because your seeing 12vdc on the fan lead when its unplugged could mean a little leakage through the relay or circuit board but not enough connection to run the motor.

Since you ran a direct wire from a good 12vdc and the motor ran has to lead back to the board.

[dougrainer]

Here is the solution to your problem. Doug

https://www.rvautoparts.com/520839-Suburban-Mfg-Furnace-Wiring-Harness-For-Suburban-Furnace_p_38029....

ON THE FURNACES:
521099 CAN REPLACE THE 520741 ON THE FOLLOWING MODELS IF YOU ORDER THE APPROPRIATE WIRE KIT WITH THE 521099 BOARD:
SF20/25/30/35/42; SF20F/25F/30F/35F/42F USE 520832 WIRE KIT
NT24/30/34SP; NT40 USE 520839 WIRE KIT
NT12/16/20S; NT12/16/20SE USE 520840 WIRE KIT
DD17DSI WITH DATE CODES BELOW 9124 USE 520612.
DD17DSIWITH DATE CODES ABOVE 9124 USE 520741.

[dougrainer]

I would also state wrong board. Since the P/n's were different, then the odds of a wrong board are greater. Doug

[wnjj]

sandpaper600 wrote:
Something is causing the relay to kick out repeatedly instead of hanging in there and passing the voltage to the motor, I just don't know what. Too large of a load for the relay rating?

I'm leaning towards wrong board at this point.

If you find that’s the case, you could always get a 12V automotive relay. Power the relay coil with the board’s fan output and use a direct fused 12V source to switch to the fan. You could even try that temporarily just to see if it operates if it seems to kludgy for a permanent fix.

It’s unlikely but possible your replacement fan motor is failing or drawing too much power. Try running the motor with an ammeter in series to see how much it’s taking.

[sandpaper600]

@dougrainer - Yeah, tried that. I don't get anything. The relay just clicks repeatedly and never sends any voltage.

[dougrainer]

Have you taken a voltage reading using just the Motor Ground and positive when attempting to start? Doug

[sandpaper600]

@GenoB--I have done this and can't figure out why it fails where it fails. I have 12.6 on the feed into the furnace. I have the same on the power terminal on the board.

When the thermostat calls for heat, I have 12.4 on the blower feed from the board ONLY if the wire to the motor is unplugged. Otherwise I get no voltage and repeated clicking of the relay.

If I unplug the motor lead from the board and power the motor direct with a hot wire, the motor runs.

If I unplug the motor from the board, everything looks normal - One click of the relay and proper voltage to the motor feed. I plug the motor back in and it starts kicking the relay in and out again.

Something is causing the relay to kick out repeatedly instead of hanging in there and passing the voltage to the motor, I just don't know what. Too large of a load for the relay rating?

I'm leaning towards wrong board at this point.

[GenoB]

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]

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]

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]

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?