Furnace cycles (1sec on, 1sec off, repeat)

[Daniellekristopher]

2018 Cardinal 3350RL
Suburban SF-42Q furnace

When I start the furnace, squirrel cage starts for 1 second, stops for 1 second, starts for 1 second.... it tries to start 3 times. Then nothing.
All of the forums I’ve reviewed most people at least get the a few minutes before it fails or cycles. Or either get at least 10-20 second until the sail switch shuts them down. No one with a cycle this short.
Any suggestions?

[AquaMan]

I would still check the sail switch to make sure it's operational.

[ejohnson1144]

Make sure there is no critters in there

[Rich5117]

Has it just started this or is this the first time using it? Are all your other gas appliances working ok? Are you plugged in to shore power? If not are your batteries fully charged? Do you hear the igniter clicking?

[cavie]

Originally Posted by Rich5117

Has it just started this or is this the first time using it? Are all your other gas appliances working ok? Are you plugged in to shore power? If not are your batteries fully charged? Do you hear the igniter clicking?

won't start without gas. Stovetop working??

[Daniellekristopher]

Still not working.
What I’ve done so far....
1) replace the sail switch.
2) searches for critters (none), no dust build-up, but still vacuumed ducts and squirrel cage.
3) measured gas pressure with a homemade tube. 11 inWC. GenSet, Stove, and Gas Water Heater all working great
4). Tried running Furnace while on shore power. Batteries at 13.6V.

The gas does not try to ignite, because blower does run long enough to meet the 15-second sail switch contact (constant flow) call.
Cage spins for 1 second, stops, tries again. Five times (not 3 anymore), then quits until I reset.
Furnace worked great for 6 months. Typical use has been 2 days per week.

[ejohnson1144]

I would bet it's not igniting Mabe igniter needs adjusted

[Daniellekristopher]

The SF-42Q has a 15-second Purge Cycle before it requests gas. Followed by a 7-second Ingnition & Flame Sense cycle.
Blower never runs long enough to even get close to Ignition cycle.

[5picker]

I'm just throwing thoughts out...

Have you checked the voltage at the motor when it starts & stops?
I'd be interested to see if you are losing voltage at the motor wires during these 1 second cycles.

If you are losing voltage then you need to find where that voltage comes from. (maybe the control board)

If you are not losing voltage, then you have a bad motor.

[Daniellekristopher]

I think you are on the right track. It’s either a bad blower, voltage loss, or bad motherboard.
My assumption is that the 5 quick starts is actually a “fault code”. Which I cannot interprete.
If I get it resolved I will post the solution on this thread to help others.

[Rich5117]

I'll take dibs on on the board

[ejohnson1144]

Just for giggles did you try jumping the thermostat out to take it out of equasion

[TitanMike]

I'd see what happens when isolating the motor.

Disconnect it from rest of furnace and with jumper wires apply 12 v directly to it. If it runs continuously without noise or letting out any smoke, the control board is most likely at fault.

If there is any load protection of the circuit board, the one second on, one second off, could be a circuit breaker/short protection cycling due to high current draw by failed motor. Test the motor and get it out of the picture if it works OK.

[Daniellekristopher]

I’d like to attempt to “jump the thermostat out”. Attached is a photo of the thermostat wiring. What are the steps?

I will do the Motor isolation & jumper-run tomorrow evening.

[ejohnson1144]

Find which two wires go to the funace and jump these wires at thermostat . should start the furnace also was mentioned to check your blower. Should be able to run the fan if you take thermostat out of auto this will test your fan.

[Larry-NC]

Originally Posted by Daniellekristopher

I think you are on the right track. It’s either a bad blower, voltage loss, or bad motherboard.
My assumption is that the 5 quick starts is actually a “fault code”. Which I cannot interprete.
If I get it resolved I will post the solution on this thread to help others.

I just saw this same symptom a few weeks ago. It turned out to be the converter (makes 12v DC from 120v AC). You can prove this by turning off the furnace and putting an equivalent load on the converter by turning on a lot of lights. When you do, they will blink at the same 1-second rate.

Larry

[koller1509]

You may want to check your high temp limit switch. I had the same trouble once and the switch was stuck open. Try to jumper it out.

[Daniellekristopher]

QUICK UPDATE: the thermostat could not be jump-tested as it only has three wires which feed a relay module (which is hidden somewhere; with what appears to have lots of in/out wiring on the manual diagram). HOWEVER, I re-initialized the living-room thermostat and associated relay module. The Dometic CT thermostat operating manual suggests that owners Initialize the thermostat(s) EVERY TIME the RV is moved. Furnace works now!! Ejohnson1144 wins... if we’re playing price-is-right rules.
From reading forums, this has also helped people with other issues (AC, Heat Pump, temperature dead band, quick cycling).

Below are the steps to reset the Dometic single zone thermostat....

“System Initialization:
A system initialization will need to be performed by the installer after the system is installed.
• Make szure the Single Zone LCD thermostat is in the Off condition. See page 4, “Quick Reference To Control Buttons”.
• Press the “+” button and, while holding it, also press and hold the On/Off Mode button for three seconds. LCD will show ― ― .
• Press the On/Off Mode button again to turn system off.
This completes the initialization. The furnace On/Off temperature differential should be set at this time. See “Mode Description - Furnace” on page 9 for further information on furnace mode differential setting”

I will send another update in a week or so. After I’ve done a few drives with the 5th wheel.

[ejohnson1144]

Yes and even I forgot about that . good job