Suburban Furnace Foibles

So far so good this winter, we’re warm and comfortable but we tracked down an unexpected furnace failure mode that might be of help to others in our situation.
We’ve replaced the gas solenoid valve with a newer model that draws more power but also opens positively every time it’s told to by the control board. Something important when it’s cold outside and the batteries are getting low.
We replaced the Suburban control board with one from Dinosaur Electronics that among other good features turns the furnace fan off if the furnace refuses to light. And the Dinosaur Fan 50 Plus Pins tries to light the flame three times, turns off the fan motor and waits one hour before trying again. If your batteries are low an hour’s rest may be enough to squeeze out a little more heat before sunrise.
We still had instances where we could hear the time delay relay click on and not have the blower fan start. We replaced the time delay relay first with another used unit and finally with a solidly state timed delay relay from again Dinosaur Electronics. The furnace ran great for that season and most of this one so far with the exception of the sail switch beginning to stick. A NOS sail switch took care of that but still left us with a failure to launch. After many nights of restarting the furnace rapping on it and praying to the god of heat we finally had what we considered a hard failure so we pulled the furnace and started checking for electric faults. Nothing definitive except a bad return on the control board’s edge connector. Dinette bench testing showed the furnace to be cycling properly so it all went back together. There was one more failure to start before I saw the light.
Normally in an old Suburban like ours the time delay relay starts the fan motor, and the sail switch supplies power to the control board and from then on the control board, controls. With the new board everything works the same with the addition of a fan control relay on the control board. But the control board doesn’t get power until the fan is spinning and the fan control relay needs to close before the fan will run The fan control relay on the control board closes with power from the fan going through the coil in the relay and flows out the edge connector’s 12V- return. An iffy connection at the edge connector is enough to prevent this weak current from flowing and cleaning the edge connector can help. But after many rounds of doing the same things and getting the same results I realized that with the addition of the Fan 50 Plus Pins I placed two sets of replay contacts in series. Now relay contacts rely on a small spark to keep the contacts clean and conductive every time the contacts make or close together. The small amount of current needed to close the control board’s fan control relay isn’t enough to keep the time delay relay’s contacts clean. My solution is to unplug the two fan power leads and connect them together with a jumper and cycle the fan on and off through the thermostat four or five times. Since I’ve done this I’ve been sleeping the night through with reliable furnace operation.