I have an old Thatcher furnace model 53170. The pilot light stays lit, but the gas valve will not open to ignite the burners. I have a constant 24v across the transformer terminals. When I check the terminals at the gas valve I experience an intermitant 24v signal about every 5 minutes. I can't figuare out what would cause this change.

Say... what street do you live on? Couldn't find anything on Thatcher, but this came up... can you post a few pics, esp. the gas control(s)?