Equipment: Gas Heat unit: Lennox G12 Series Outside compressor Armstrong Concept 10 Experience: Novice at HAVC. Some electrical experience (house wiring and digital electronics) Problem: Outside unit stopped working (both compressor and fan). Blower motor works ok. With the blue and yellow wire to the contactor coil leads disconnected and the thermostat commanding Cool on there is ~ 27Vac across the blue and yellow wires. Yet the contactor does not close the switch when the wires are connected to the coil leads. There is 246Vac across the input side of the contactor although I wouldnt think this would affect whether the coil closes the switch. If the contactor switch is closed manually, the compressor and fan start and run ok. Step 1: Replaced the contactor (which visually appeared to be in good condition). With a Furnas 45EG10AJA. Same symptoms. Step 2: Replaced the thermostat with a used one laying around. Same symptoms. Step 3: Shorted an 18v battery across the old contactor coil leads and the switch closed correctly. This is the limit of my diagnostic skills. Is there a specification of the amperage the transformer must supply to the contactor coil? Is there a way to test the transformer? Any ideas how to further diagnose? I cannot find a service or parts manual for the heat unit (Lennox site has an owners manual). Is there an online place to find these? Is there a place to find the part number for the transformer? Any ideas or help would be greatly appreciated.