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Randall
Randall, RVIA Certification
Category: RV
Satisfied Customers: 10336
Experience:  RVDA/RVIA Master Certified Technician with many years experience.
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Furnace runs on shore power but won't ignite on 12v

Customer Question

Furnace runs on shore power but won't ignite on 12v battery's are new and all gas appliances work
Submitted: 1 year ago.
Category: RV
Expert:  Randall replied 1 year ago.

What is the make and model of your furnace? Do you have a small digital multimeter so you can take some actual DC voltage readings here? That would make this diagnosis much easier. R.J.

Customer: replied 1 year ago.
I have not had time to get those readings for you ! Please give me a few days to get back to you on this! Thank you
Expert:  Randall replied 1 year ago.
Not a problem........just respond when you can. I will watch for your post. R.J.
Customer: replied 1 year ago.
I'm going to get that information for you today I have just been really busy withsome family issues
Expert:  Randall replied 1 year ago.

No hurry.............I am here every day off and on. I will watch for your post and then we can sort this out for you.

Customer: replied 1 year ago.
Suburban furnace model number SF 30 F and I have 11.18 volts on the wires from the thermostat to the two blue wires on the furnace. The only other wires or the 110 V lines hooked to the red and yellow wire on the furnace! With the blower running no ignition I have 144 mV a crossed the switch I don't know if that is a thermostat switch or an airflow switch .
Customer: replied 1 year ago.
Here is one more picture
Expert:  Randall replied 1 year ago.

There is no 110 VAC here.............only 12 VDC on this furnace. That was the limit switch in the photos. Take and jump across them so that it isn't in the circuit and try it. IF it works then you have the answer. Poor contact in the switch............rare, but it happens. Usually, they just go bad. Let me know what that result is.

Customer: replied 1 year ago.
I jumped across the switch and it still only runs on shore power! Also it is blowing 2amp fuse in thermostat
Expert:  Randall replied 1 year ago.

Let me give this some thought and get back to you......

Customer: replied 1 year ago.
Ok thanks
Expert:  Randall replied 1 year ago.

When you jumped across the terminals of the limit switch.......did you disconnect the wiring from the switch and then hook the two wires together?

Customer: replied 1 year ago.
Expert:  Randall replied 1 year ago.

There is no really logical reason that this works fine on shore cord and then blows a fuse on the battery only UNLESS the battery has got a short in it OR the motor is failing. Those are the only possible causes for that. Start by having the battery load tested, even if it is brand new. Even new batteries go bad and can have an internal short. Then if that checks out good and the voltage is over 12 VDC, try it again. Let me know what happens.

Customer: replied 1 year ago.
It doesn't blow fuses on the battery it blows the 2amp fuse in the thermostat in the camper, we just spent four days on shore power furnace works perfectly now are back home fusing the thermostat is blown again and the furnace will not run at all on 12 V the evacuation fan used to come on and run but the igniter would never ignite now the evacuation fan does not even come on with 12 V, I checked voltage at battery source 13.1 V
Expert:  Randall replied 1 year ago.

I know and that is what I was trying to explain to you. IF the motor is failing for bad brushes and/or bad bearings, then it will have to work harder at lower voltage............that drives up the heat in the circuit and blows the fuse in the thermostat. When you have more voltage you overcome that problem. No other explantion that I know of. I have been at this a long time.

Customer: replied 1 year ago.
Well now I can't get the fan to come on at all I appear to have absolutely no power running through the thermostat even with the fuse jumped
Expert:  Randall replied 1 year ago.

There is usually a circuit breaker in the furnace that may have tripped or it may be the main fuse in your load center. The longer it goes the worse it gets. I think you are just going to have to buy a new motor.

Customer: replied 1 year ago.
Just plugged everything back in the shore power, I still can't get it to work the furnace won't come on the a/c won't run, I have the fuse jumped
Expert:  Randall replied 1 year ago.

Did you read my last post..............there is a main fuse in the load center that controls the power for both units on the thermostat.

Customer: replied 1 year ago.
Check the fuse at load center it's fine all fuse check good, good voltage 13.1 but the blower or the AC will not come on at all
Expert:  Randall replied 1 year ago.

There is another breaker in the furnace on most models........and I hope you didn't burn up the thermostat by jumping the fuse in there. That is why they put fuses in them.

Customer: replied 1 year ago.
What tells the furnace to ignite is there a circuit board other than the thermostat
Expert:  Randall replied 1 year ago.

Yes...........and the power to the thermostat comes in from the furnace. See the diagrams on the attached link.

http://bryantrv.com/docs2/docs/SFseriesservice.pdf

Expert:  Randall replied 1 year ago.

Link to a better manual on these............

http://bryantrv.com/docs2/docs/subntnew.pdf

Expert:  Randall replied 1 year ago.

So was it the motor? Let me know if we need to dig deeper on this as all follow ups are included in our fee. I will be around all week-end.