Help me troubleshoot this furnace

Hi:
I have this ongoing problem with our RV furnace which I posted once months ago when the furnace stranded us in Yellowstone when it was below freezing. I have read any thread that I could have found here or online including the very good one in the library section of this forum, but I am not sure what would be the next step to figure out the problem.
The problem is the common issue of fan coming on and the furnace tries to kick in but it doesn't stays on. This problem has gone away twice when the weather was warmer! but now that we are below 30F again, the problem is back. The unit (the RV) is 2016 Atwood. What I have found is: My house batteries (two of them connected to each other) read at 12.35V. When I check the power at the fan, it reads 10.6V which I believe (please correct me if I am wrong) is enough to run the fan but not enough to push the sail switch to open the circuit. So, I have checked all the connections that I could see and I know of and I have found no rust or loose connections. Still, I took them off, clean them and put them back together. Still 10.5-10.6 at the fan. Then I moved to the reset switch on the unit. I found that the power at the reset switch is 10.6 but when I remove the connection from the circuit board to the switch I read 12.1 at the switch! This is where I need help. What does this mean? I thought maybe the batteries may be bad and they don't hold the charge under the load but when I tested them, while I was reading 10.6 at the fan, the batteries were at 12.1.
Does this mean the switch is bad? Does it mean there is excess resistant through the board? Or something else? Is there a sail switch that open up at lower voltage?
Thanks for the help and happy holidays.

Just a thought.....take a piece of wire[10,12]and hook it to the case of the furnace ,hook the other end to good ground.Sometimes the problem is on the ground[negative side].

If what Ronster recommends does not clear the problem, try running jumper cables from battery to furnace to see if you can get full voltage. You can connect both ground and hot this way. Once you get good voltage to the furnace, disconnect one wire to see where the voltage drops. Then you know which side to look for the problem. If you have to, make jumper cables out of a heavy extension cord.

Both are good suggestions.  Seeing the voltage drop when a load is applied means there is resistance somewhere in the circuit.  It can come from a wire being too small for it's length or from a bad connection, including hidden corrosion where the wire is crimped into a lug.

What is the voltage at an intermediate point when the furnace is running , like at the 12 volt fuseboard?  If it's good there, then the problem is after that point, i.e between the fuseboard and the furnace.  If it drops similar to what you see at the furnace, the problem is upstream between the fuseboard and battery.

You can verify the furnace ground is good by using a length of wire to extend the meter's negative lead to a known good ground, like at the fuseboard or directly to the negative battery terminal.  The size and length of the wire doesn't matter, because the meter doesn't draw any current when reading voltage.

Start the furnace and see if you measure any voltage between the known good ground and the furnace ground wire, i.e with the meter's (-) lead connected to the good ground, put the (+) meter lead on the furnace (-) power connection.  If you see voltage, the furnace ground has a problem.

Now measure the voltage between the hot lead at the furnace and the known ground, again while the furnace is running.  You should see close to what see at the fuseboard, if not you have a problem along the hot wire going from the fuseboard to the furnace.

I agree with sending power directly to the furnace to bypass potential problems. I'd then check for a voltage drop on the motor itself. Does the motor turn freely? Possibly lube the motor bearings, if it makes a big difference in freeing up the motor then I'd consider a new motor. Maybe maybe not?  The biggest and most likely item in the system to draw high amps is the motor. 

You are measuring 12.1V at the battery. That, if accurate, is a nearly dead battery.

Ernie

Okay, here is where I am. I plugged in the RV over night. The house batteries are reading 12.5-12.6 now.
I checked the power at the reset switch The red wire #1 in the picture and I get 12.5 but II still have the issue. Then I check the power at the other pole of reset switch (blue and red wire #2 in the picture) and I get 10.5. I take the blower wire off of the circuit board (the red wire #3) and I connected it directly to 12.5 on another battery, the blower runs but the unit doesn't kick in. I guess because the blue wires which goes to sail switch don't have power at this point.
I guess if I had problem with fuse of bad connection on wiring before they come to the unit, I wouldn't' have 12.5 v at red wire #1 at reset switch.
I checked the switch with voltmeter and it seems to be fine!

HRDWRK said:

I checked the power at the reset switch The red wire #1 in the picture and I get 12.5 but II still have the issue. Then I check the power at the other pole of reset switch (blue and red wire #2 in the picture) and I get 10.5.

I checked the switch with voltmeter and it seems to be fine!

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Those two statements are contradictory.  If you're losing 2 volts through the switch (12.5 volts on one side, 10.5 volts on the other) the switch isn't fine!

If your furnace draws 7.5 amps, it only takes 0.25 ohm of resistance through the switch contacts to lose 2 volts.  Your multimeter might not be able to accurately measure resistance that low.

Pull the wires off of the switch, use a piece of wire to jumper them together and see what happens.  Verify that you have full voltage on the blue/red connection with the blower running and see if the furnace fires up.

Pull the wires off of the switch, use a piece of wire to jumper them together and see what happens.  Verify that you have full voltage on the blue/red connection with the blower running and see if the furnace fires up.
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Okay, I did the hard wire. I get 12.5v at the main wire (#1 on my picture) but as soon as I connect to #2 (red and blue combined) the voltage drops to 10.4 and fan keeps running and furnace tries to kick in with no luck.
I have been reading the wiring diagram that I found online but I can't see where the blue wire from the #2 connection in my pictures go to. I don't see that on wiring diagram that I found. I thought it goes to Thermostat but I pulled the thermostat off and I don't see anything blue wire at that size behind it. The blue wire from connection # 2 goes in to the house and then another blue wire comes out from the house and connect to another blue wire from circuit board and both together connect to one of the poles on sail switch. I guess the last place is fuse panel.
Could this be the electronic board? IF it is bad ad creating resistance?!

Are you saying you see 12.5 volts at one side of a short jumper wire and 10.5 volts at the other end?

Lou Schneider said:

Are you saying you see 12.5 volts at one side of a short jumper wire and 10.5 volts at the other end?

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I am saying when the jumper is not connected to the red and blue combined connection (#2 on my picture) I read 12.5 but as soon as I connect the jumper to this #2 connector the voltage reads (drops) to 10.5. Since the red wire in this #2 connection goes to the circuit board, I am wondering if the board is bad or ether, since the blue wire of it goes back up to the unit, there might be something going on there.
Having said all these, this issue of ignitor not being able to stay on went away twice when the weather was warmer on two different occasions. Does the weather do anything to propane regulator? Condensation? excess resistance?
Thanks for all the help.

I had the same issue. There seems to be a short somewhere. Not sure where.. I worked and worked and inally there is a wire I think it is Yellow goes from the control board to a "T" flag fitting that fits on a ground tab just above the board,, Disconnected it and all works fine.. NOTE the blower ground is hooked to that same 4 Tab Block abobve the board. I assume that yellow wire is grounded elsewhere as well but why it caused an issue I have yet to figure out.. I WILL, but I've not gotten to it yet.

HRDWRK said:

I am saying when the jumper is not connected to the red and blue combined connection (#2 on my picture) I read 12.5 but as soon as I connect the jumper to this #2 connector the voltage reads (drops) to 10.5. Since the red wire in this #2 connection goes to the circuit board, I am wondering if the board is bad or ether, since the blue wire of it goes back up to the unit, there might be something going on there.
Having said all these, this issue of ignitor not being able to stay on went away twice when the weather was warmer on two different occasions. Does the weather do anything to propane regulator? Condensation? excess resistance?
Thanks for all the help.

Click to expand...

Here are some pictures:

Thanks John.
This is my other issue that all the wires on this unit are red and blue, besides the black ground wires. No white or yellows! I have not seen any wiring diagram that just has red and blue. I guess that doesn't matter since I am just assuming where they are going based on the diagrams but I would be nice if they were color coded.

Have you run just the motor? Power up just the motor and see how many amps it pulls. I may have missed it up above if you already did just this part.

QZ said:

Have you run just the motor? Power up just the motor and see how many amps it pulls. I may have missed it up above if you already did just this part.

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Finally rain and work stopped! I just ran the motor. It draws 6.5 amp.
This what I did today. I have left the RV plugged in. The house batteries now read at 13.4v. I ran the unit and at the motor I read 12.4 but it still struggles to fire up. I bypassed the sail switch by hard wiring the wires. The same thing, it tries to fire up but it doesn't stay on. Please correct me if I am wrong, by bypassing the sail switch regardless of the voltage drop, I am ruling out the motor and the sail switch? Is this a correct assumption? So, what I have left with is the circuit board and propane supply (regulator at the thank and at the furnace) and the ignitor. Since it ignites but doesn't stay on, I assume the ignitor is okay. The only way to test the circuit board is to take it out and take it to an RV center to test it which may cost me more than just buying a new one at $70. How do I purge or check the propane regulators?
Thanks for the help and happy new year.

Did you figure out your problem? I have the same symptoms as you and wondering if you solved the furnace problem.
Thanks
Ted

HRDWRK said:

So, what I have left with is the circuit board and propane supply (regulator at the thank and at the furnace) and the ignitor. Since it ignites but doesn't stay on, I assume the ignitor is okay.

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The tip of the ignitor doubles as the sensor that lets the the board know the flame is burning.  To do this it has to be in the flame itself, not to get red hot but because the flame produces an ionization path that allows a trickle of current to pass between the tip of the ignitor and ground, telling the board that the flame is on.

Dirt on the tip of the ignitor can act as an insulator, blocking the sensing current flow.  Is it nice and clean?

HRDWRK said:

How do I purge or check the propane regulators?

Click to expand...

The best way to check the regulator is using a manometer to measure the pressure in the gas lines.  It should be 11 inches water column, or about 0.5 PSI.  To make an manometer, take a piece of clear tubing, bend it in a U shape and hold it vertically.  Fill the bottom of the U with water and connect one end to the test port at the furnace valve or to a stove burner and leave the other open to the air.  A piece of wood or a yardstick makes a handy support to hold the tubing in place.

If the gas pressure presses one side of the water down 5 1/2 inches while the other side rises a like amount, you have an 11 inch difference in height between them, or 11 inches water column.

Another way is to light a stove burner and observe it while the furnace ignites.  If the burner flickers or goes out, there isn't enough gas flow.  If it burns steady, the gas supply is probably OK.

Google "Testing RV propane pressure with a manometer" - there are several short YouTube videos showing how to do this.

tedandcandy said:

Did you figure out your problem? I have the same symptoms as you and wondering if you solved the furnace problem.
Thanks
Ted

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Hi Ted: I have not yet. I have been very busy with work. What I have though is taking the propane regulator and the ignitor out. I found a very small piece of what seems to e the ignitor insulator inside the furnace. I have ordered a new ignitor which I received yesterday. I keep it posted when I install the ignitor. This is what I have learned from all this issue: This is a very general problem which can be caused by many underlying causes. I feel there is no one fix for it. Based on what I have read, which I have to say is a lot since the issue started, the problem can be variety f things. Depend upon the age of your unit, you may want to start with simple things such as checking your connections and the power to the unit.
Keep you posted.

Thanks for the update,my furnace has your exact symptoms found this out at Grand Canyon a few years ago. I haven?t really pursued the problem, because living in San Diego not much use for heat. Now if the AC was acting like this I would be on it! My MH is an 2001 safari zanzadar the furnace is located under the refer with access behind in the bathroom cabinets , a real pain to work on.
Ted