Originally Posted by kleinbiker1
Does this help?
Jawohl, Herr kleinbiker1!
There is a wire running vertically from the right side of the transformer 24 vac secondary down the lower right side of the diagram and it could be called the "common" lead or terminal. Put one DVM lead on this point.
You can see that the "hot" 24 vac lead goes to terminal R on your thermostat.
You see the assembly marked "36C84"? I assume this is the gas valve.
There are four switches, the contacts of which need to be closed for this assembly to be powered:
the heat switch contacts in the thermostat,
the switch marked CAL,
the switch marked SNP, and
the switch on the crease in the paper.
You already know the gas valve is shutting off so that's not the first point I would monitor. Using your other voltmeter lead, monitor the point between CAL and SNP.
If the ~24 vac stays here then the switch that is opening is downstream.
If not, it's upstream.
Continue this procedure until you find which one of these several switches is opening, or not closing, and why.
Dig your test prods into the connections: sometimes they have tarnish or invisible insulating grime on them. Watch for sharp sheet metal edges.
Your furnace does have a "computer" and this arrangement of relays is it. It uses "relay logic".
None of the relays are of the latching type so this computer does not have memory.
My '82 furnace gizzards are similar to those of your '81 furnace.