SF35 Furnace blows fuse

I am at my Wits end.  Had a dirt dauber build a nest in the squirrel cage on the heater.  Pulled heater and cleaned out nest.  Re-installed and hooked up the heater, did not try it as it was still 90+ degrees.  the thermostat had a broken pin that changes it from cool to heat so I decided to get a new one.  Being that the thermostat I had was one for a heat pump, and I do not have a heat pump, I went online to see what one I needed. Talked to a fellow that goes by rvjunky and he had a store that sells RV parts.  I gave him the number on the control board and he matched it with the right thermostat a Dometic part no. 3106995.032.  Hooked the new thermostat up as per drawings, turn switch to furnace...15 amp fuse blows as soon as I turn it on.  replaced fuse tried again..same thing.  Went back to furnace to check for bare or pinched wires grounding out..nothing.  Pulled furnace out and set up on a bench with 12v from a battery,  worked great.  as I had it out I checked every wire to see if anything was shorting it out... nothing.  Put furnace back in and I used a 6 ft 14 GA wire hooked on the hot wire from the RV battery and on the other end I had a alligator clamp hooked to the red wire on the furnace.. The common wire I did the same way.  Hooked the 2 blue wires together and it worked fine..Put the furnace back in and hooked it up the way it should be..Turn on ..blows the fuse.  Put the 6 ft jumper with the alligator clamp on one end to the hot from RV battery and the clamp on the red wire to the furnace,,,turn on  works fine..My question is why will it run right with the alligator clamp jumper wire and without it it blows fuses.  I also tried a 6ft piece of wire and wired nut one end to the 12v RV source and the other end wired nut to the red wire from furnace..and it blew the fuse.. To help me from having to take another crazy pill what does the china made cheap alligator clamp have to do with this .  Thanks for any help and My wife will greatly thank you  Ed Edwards

Motors blow fuses when the current is too high. The cause may be stalling the motor or low battery Voltage under load  or a combination. I'd check the battery under load first and then monitor the Voltage at the motor (and then the battery) when its turned on. If necessary a larger fuse could be used temporarily for this measurement.  A significent difference may indicate an undersize wire or a poor connection. If both go excessively low, the problem is likely the battery, switch, or possibly a defective motor.

Ernie

A experiment might be to manually start the motor turning and then turn it on.

Ernie, thanks for getting with me.  I checked the voltage coming to furnace  13.7 volts  when on load from motor goes to 12.3 then back up.  I put a 30 amp fuse in place of the 15 and everything works as normal.  I am just afraid of running the 30 very long.  Any thoughts?
Ed

You may still have debris inside the blower that's increasing the drag on the motor, making it draw more current and putting it on the verge of blowing the 15 amp fuse.

As to why it's blowing the fuse when it's connected to the thermostat but not when you jumper the two thermostat wires, the thermostat may have a bit of internal voltage loss that's dropping the voltage at the motor and increasing the current flow a little bit, just enough to push it over the edge.

What voltage do you see at the furnace when you jumper the wires vs. using the thermostat?

I might note that the 'operating voltage' does not normally go through the thermostat, only the "I need heat" signal does. Thus there should be little or no voltage drop associated with the thermostat itself and that draw is nowhere near 15 amps.

I took furnace back out and took apart the blower.  the small squirrel cage was hitting the side if the cover ,  took out and adjusted it so it would not hit cover.  put back in all hooked up ,,blew fuse on 15 amp.  put 20 amp fuse in furnace did fine.  So you are saying there should be no voltage drop to 12.4 volts when you call for heat from the thermostat when the blower kicks on?  Thanks for your help

Took furnace and checked voltage to motor when thermostat calls for heat.  It drops to 11.1 volts then back up to 13.7. But this should not blow the 15 amp fuse should it?  I have never run across something like this that I could not figure out.  I guess welcome to RV repairing.

There will be a voltage drop any time a motor starts from a dead stop. The initial amp draw is 2x-4x the running amps, so the voltage momentarily falls. 12.4v isn't bad at all, but that 11.1v is enough to be serious, I think. With a motor load, the amps go up as the voltage drops.

Gary, seems it might be time for a new motor?  I don't know

You mentioned jumpering to a "red wire" on the furnace, but I'm not familiar with your specific furnace. Is that the 12v feed to the furnace as a whole, or the power wire that goes direct to the motor.  If you jumper direct to the motor using a fuse inline with the jumper, does it blow at 15A 20A?

There are two fans on that motor, one that circulates air inside the RV and another that brings in combustion air and vents out exhaust. Are both running freely?

That 11.1V reading takes you back to a possibly bad connection or battery. There must be a relay that is energized by the switch closing to apply power to the motor. High contact resistance is a possibility.  Also Have you had the battery load tested yet?

Ernie

You know I can't figure all this out.  When I tried to turn furnace to check for winter use there was a noise from the blower.  this did not blow any fuse.  I pulled furnace out and took motor housing apart and cleaned out the dirt dobber nest out of the squirrel cage and put back together.  While I had it down since the thermostat had a broken pin that controls the furnace and cool and the thermostat was one for a heat pump which I don't have I ordered a new thermostat and put it on.  I hooked all wires as they should be back up.  Turned the furnace on and 15 amp fuse blows.  All I did was clean squirrel cage and put a new thermostat on and now all these problems.  I don't understand but maybe that is just me.  I do thank all of you for your help.  I live in the 5th wheel all the time and have it set up for all year living, so it would be hard to tow to somebody to repair.  So much for the rant and back checking the furnace

Are you using the correct type of fuse? Meaning a slow blow or something along those lines since the 20A seemed to work it probably allowed for the current surge. A 15A slow blow might provide some surge protection during startup.

This is an ATC fuse , do they make a slo blow in this type?  I put a 15amp fuse holder jumper directly on the motor and it did not blow the fuse.  Put the 15 amp jumper between the 12v supply and furnace, and a 15amp fuse on the converter board like it should be...no blown fuse.  Hook all back up like it should be...blows 15amp fuse on converter fuse board.  Why would 2-15amp fuses on same circuit make a difference?

Dunno, but something is fishy. The fan draws about 5-6 amps, so a 15A fuse should be plenty to handle the starting current, even though ATC fuses are rated as a "Fast Blow" type. A typical 15A ATC will blow in about 1 second at 30A but hangs in for about 60-80 seconds at 20A (data based on Bussman brand ATC fuse spec sheet).

If the jumper with fuse doesn't blow, but the factory wire from the board does blow, it would indicate a bad connection from board to motor or weak power (low voltage) through that connection.

Had to go to Work today ...heat problems...go figure.  Tomorrow will get on furnace again.  I guess I will check the board on furnace

Well today I put a 15amp fuse jumper between furnace and hot wire.  On converter I put a jumper wire, no fuse, in the slot where the fuse goes.  Turn furnace on from thermostat comes on and run fine.  That is all I have tried.  Checked all wires and connections on furnace ,again, and hooked the furnace as it should be..  Installed 15amp fuse on converter board.. turn furnace on by thermostat..run 2 times by thermostat as it should and the 3rd time blew fuse

GIVEN UP  Calling an electrician................

Called all the electricians in this little town...all say they do not work on RV's.  So I guess I will plug on

Well finally got it fixed. I took the furnace and replaced all the wires and connectors and push connectors.  Did not really see anything but it solved the problem.  Thanks for all the fellow RVers help.