Compressor / Fan not running

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 wouldn’t 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.

You have established that high and low voltage are present at the outside unit, good start. I have addressed and corrected the same problem a few times in the last month. Contactor -you replaced-. Tripped pressure switch will break low voltage -locate within cabinet and see if there is a reset, usually a red button-, this will require a later refrigerant pressure check. Low amps on the low voltage -check connections on either end of the wire as well as any splices between the two, it can be pinched and still look good so total replacement with new 18/2 is something to concider. After checking the above I would be suprised if the system doesn't work.

OK, finally got it. Connecting the transformer directly to the contactor I found the contactor would activate correctly. At this point I suspected your suggestion about the wiring was correct. Fortunately the next step I took was to connect the transformer directly to the contactor using the 24AC wires fromt he inside unit to the outside unit. It still worked. Turns out, in the outside unit, there is a time delay relay. Once I wired this back into the circuit it failed again. From what I've read it delays the start of the compressor after shutoff to prevent restarting too soon. Anyway, this had failed. This device was in series with the 24Vac to the contactor and the net result was the 24Vac was grounded to the chassis and pulled down too much to activate the contactor. Wired around the delay relay and it works.

The thermistat in use has a compressor time delay feature also. This seems redundant. At the moment the system is working and I'm assuming the thermistat will provide the same protection. This may be a bad assumption I don't really know for sure.

you are correct. The stat will provide the same protection as the delay on break timer.

Thanks ozone and hvactechfw for the help. Feels good to fix it myself and learn something along the way.