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If you only have 10.8 volts across M & C then follow those leads back to the ignition control and probe the terminals there. If the reading is the same then the ignition control is suspect. If you have 24 v at the control then you have a bad connection
Yes 24 v at the valve and the valve should open. Valve could have been buzzing if the voltage was marginal
If the ignitor is arcing then you should have 24 v at the control but it wouldn't hurt to check across TR and GND on the control.
Only until the pilot is lit. Once it proves a pilot is present then voltage switches to MV
Also try across TH & TR
Could be a bad transformer or even a bad ground. I'd go back to the transformer and check voltage there. Should be constant 24v
You would probe it at the connections closest to the transformer. This is an old unit and my recollection of how to access the transformer isn't very good.
As long as you remove the lead on the gas (main) valve it won't damage anything. Be sure the leads don't touch each other or anything else when you remove it. You have to leave the pilot valve connected or the pilot will go out and your jump won't cause the burners to light. The only way to do this is to use the transformer on the furnace. The sensor is in the pilot flame and as long as the pilot flame is lit the main valve should open. There is no sensor for the main burner(s).
You need a 24 volt 40 VA transformer. A 40 VA is pretty common. I'd appreciate a favorable rating
Did you replace the transformer?
What is the voltage on the primary side of the transformer?
Then if you have ~120 volts on the primary and 1 volt on the secondary and this is a 24 volt transformer then the transformer is faulty.