Sail switch in 1976 Duo-Therm how to?

Thanks for all the comments. The battery is brand new deep cycle off the shelf from Costco. I have a 7 amp solar panel that works that recharges the battery. The wiring looks to be a 8 gauge and looks very clean. There must be a bottleneck somewhere in the system. If the wiring is the same then the current should keep it tuning at the same rate. One of my problems is that all of the wiring seems buried and not easily accessible to do andy tracing. I tried pulling the unit out to see if I could examine and maybe clean it out but I can't even seem to budge it and it appears to but be built in place and I would have to disassemble all the woodwork to be able to see anything.I can't see any other fasteners locking it down in place The furnace seems to have had some rewiring done to it and looks a bit jury rigged and perhaps the gauge of wiring and the type of connections are not sufficient to carry the load. Who ever did the work must have had better access then what I have had so far. I have only found 1 screw holding the kick plate in place and I removed that but it is still locked in place.

If you really want to tear into the furnace.......
Here is a service manual for older Duo-Therm Furnaces......LINK

BUT

If it works on shore power......then it will work on battery IF your battery is any good.

Same wiring goes to furnace......either converter supplies the necessary 12V DC power or the battery does.

Get your battery system fixed.
Either your converter (charger section) is not fully charging battery
AND/OR
your battery is not able to hold a good charge and voltage drops off quickly.

You should be able to run furnace all night on just battery power.

I had the same problem with my old 5ver, turned wires that fastened to the battery were very corroded. It has 2 batteries and the wiring was a mess. 2 10 gauge wires was what was used to tie the 2 batteries together. I replaced and rewired the wiring and the furnace fan ran great.

Check the size of the wiring to the furnace. Most manufacturers back then they used smaller wiring than they should have. Many times you can't get the amperage from the batteries to make the fan blow hard enough to activate the sail switch. Often it will work when plugged in, or when the engine is running, but not on battery.

If the sail switch works, leave it alone and fit the battery. It's there for a reason and if the fan goes to slow you don't want the gas to come on.