Another day troubleshooting. When I woke up this morning the furnace wasn't working. When I checked it a couple hours later it was, so I went up to check more voltages. The gas valve voltage is the same as when it was not working, so that's not the problem. BTW, I misread the multimeter. It's not 1v, it's 0.1v. I disconnected the blue wire and checked voltage when it tried to start and I was getting 15v there, which is what someone was saying I should get. Curiously, as I was checking voltages, the furnace stopped working. I checked the gas valve & wire voltages again and the same as when working. Looking again at the red light on the circuit board, it glows solid red when first trying to start up the furnace. When the furnace shuts down after the brief spark, the light goes out completely and won't come back on until I reset power. Don't know if that is important, but I thought I'd throw it out there.
In response to today's comments...
When it’s working, check the voltages on the gas valve, one terminal to the other & all terminals with respect to ground. With three gas terminals this is 6 measurements.
Did that this morning. Blue wire to common was around 0.1v. My GV only has 2 contacts. One contact to the other was around .05v, but the meter kept jumping around.
Also measure all the ohms. The current into the valve will be slightly less than the valve volts/valve ohms. I=E/R.
I'm not that electrically literate. You'll have to specify which wires or connections I need to check.
Don't know what responds this fast
Me neither. I've watched literarally 30 videos on troubleshooting furnaces (including the one you posted). None of their examples of bad pressure switches, gas valves, hi limit switches, etc, match what my furnace is doing. It literally sparks for a split second and then shuts down.
Maybe try checking for bad/loose connections. I do a fair amount of furnace troubleshooting in my line of work, although I’m not a licensed hvac contractor. It’s intermittent and random, it appears, correct? Was the pilot actually cleaned?
Connections have all been checked. Sparker, Flame sensor, hi limit and flame rollout contacts have all been cleaned. Connections to circuit board have been pulled off and reinstalled. No pilot light on this furnace. Just the sparker.
It might be worth checking youtube to see if there are any trouble-shooting postings for your particular furnace or a similar model.
None that I could find. And I think by now I've viewed every video ever made on furnace troubleshooting
BTW, you seemed to have posted your AC schematic. We need the other one.
It included the circuit board connections for the gas, But that's irrelevant now. I've attached the rest of the schematic. Hope you can read it. My old eyes have trouble.
Again, appreciate all the help on this. But it is getting frustrating beyond belief. This problem started last winter. After the first tech couldn't fix it, the weather turned warm and we didn't worry about it until this year. I have a feeling this winter will end also without any resolution. In fact, they'll probably cure COVID before I ever get this furnace fixed. I can't afford $8k a new furnace right now so we'll have to get by with electric space heaters all over the place.