Originally Posted by LC Garage
Voltage at furnace connector with engine off and not plugged into shore power is 10 volts, plugged into shore power it is 12 volts. Since almost any testing or diagnosis requires pulling the furnace, I've ordered a sail switch (odds are that is the problem) as well as a limit switch, when those arrive will pull furnace, check and replace switches, clean everything and re-install. Hopefully the next report will be with a working furnace. Thanks again for the tips and help!
Pulled the furnace, everything was dirty and crusty as expected;
The picture doesn't do justice to the amount of built up crud on the fan;
Tested the sail switch and sure enough it was bad (very intermittent continuity);
New switch on top, old one on bottom. You could feel the difference in the two switches, old one felt "crunchy" and only achieved continuity if held shut, new one was smooth and continuity was achieved at about 66% closeure of sail arm.
Just to be safe, went ahead and replaced limit switch (it's 27 years old);
Spent some time cleaning out all the nooks and crannys with compressed air and also gave the fan a good cleaning;
Reassembled and back in the hole;
Outside exhaust before and after;
Hooked everything up, turned on the gas, switched on the thermostat and everything worked as it should. After fan ran about 10 seconds, burner lit up and we had heat!
Performed a leak test with soapy water in a spray bottle and the compression fitting at gas shut off valve was leaking, took it apart and found a couple tiny burrs on the compression joint. Cleaned it all up, reassembled and still had a minor leak. Pulled it apart and will look for new parts today. Will reassemble and if it passes leak test will consider job completed.
Also decided it would be a good idea to install a gas alarm and the motor home had one in the spare parts, so began installation;
After getting it partially installed, hooked up power and ground and the alarm came on and stayed on. We even took it out to the battery and tried it direct, same result. Must be a bad alarm and may be why it was found in the spares. Will look for another one, any recommendations for a gas alarm (or a combo gas and smoke alarm) that will run primarily off of 12 volts?
Thanks again to all who chimed in with good solid advice.