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briankstan · 09-05-2014, 05:44 PM

I had searched the internet and the lots of different forums but I can't figure out what is going on.

I have a Suburban FS-42 Furnace in my 1998 Winnebago. the furnace works fine when on shore power.

When I'm on batteries the blower will come on but it won't light.

I have replaced the Sail Switch, High limit switch and the Main Board.

here is what I'm getting for voltages at the furnace.

On Shore, I get 13.6v no load, and 12.5v when the blower kicks on, Furnace lights no problems.

On Batteries, I get 12.6v no load, and 10.5v when the blower kicks on, Furnace won't light and blower continues to run.

I know the voltage drop has something to do with it, but I can't figure out why. the sail switch is fully triggered so it should be passing the signal to light but doesn't at the lower voltage.

I thought that maybe It had a bad ground and ran a new ground wire, but this didn't solve the problem.

Also note that the batteries are new and fully charged, I have 4ea. 6V batteries. Everything else in the coach work fine with the exception of the furnace lighting when on batteries.

any help or suggestions is greatly appreciated.

Skip426 · 09-05-2014, 06:40 PM

10.5v under load is an indication of a battery or connection problem.
New batteries doesn't mean good , have them tested, one bad cell could cause your problem.

NeilV · 09-05-2014, 09:12 PM

At 10.5 volts the blower is not going to move enough air to satisfy the sail switch. Is the voltage dropping to 10.5 at the batteries or at the furnace? If at the batteries then you need to get them checked along with possibly the charger in the coach as it may not be bringing the batteries up to a full charge. Be sure to check that you have a good ground connection to both the batteries and the frame as this could impact charging and running the furnace. Yes also check the positive leads.

briankstan · 09-06-2014, 09:54 AM

I'll continue to chase It down. I really don't think it is the sail switch because it is fully triggered on both batteries and shore power. I have visually confirmed that. As well as actuating it by hand to see if that was the issue.

The trouble shooting continues.

Let me know if you guys have any other suggestions I can try.

"007" · 09-06-2014, 12:52 PM

Have you cleaned the ceramic igniter, spark cable electrode, point for spark, wipe clean the ceramic point.

SF FURNACES

Shellback1 · 09-06-2014, 01:07 PM

As I see it the only difference between 110 and batteries is the source. The 110 is converted to 12v by your converter and the heater works. The batteries supply 12v to the same PC board but doesn't fire the burner. Trace the battery wires and do a load check or hydrometer check on the batteries. The blower should not draw that much current to dran the batteries ot 10.5v.

briankstan · 10-16-2014, 07:01 AM

so here is my latest update. After checking everything out on the furnace itself I moved to the wiring.

I thought I had it solved when I found the main ground wire coming from the house batteries hanging on just barely connected. the end had been bent and broken and was only just holding on. I replaced the battery cable with a new one. I checked the voltages out and I was seeing a slight improvement but still too big of a drop.

Last night I decided to run a new power wire to the furnace. (I already had run a new ground). I did a test with the new wire before I running it and checked the voltages. I was getting 12.7 volts at the furnace connection. I turned on the blower and it dropped to only 12.5 volts.

I set the thermostat and the furnace fired right up.

It's the first time it's fired on battery power for me.

We are headed out this weekend for a couple of nights so I'll report back when I get back with a final update that indeed it continues to work as it should.

Skip426 · 10-16-2014, 07:22 AM

Thanks for posting that your fixed up, and the details of the repair required.
Glad you've got heat.

Safe Travels.

briankstan · 10-19-2014, 03:50 PM

Latest update. Furnace is working great now. We had the RV out thia weekend and with the nights getting down in the 30's we stayed warm with the furnace working great now with new wiring. Thanks to all those that helped wirh the troubleshooting.