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TL; dr

My HVAC has an electronic circuit that delays energizing the compressor contactor for 8.5 seconds upon cooling demand. I can't tell if the time delay is bad, or the contactor. Once the delay passes, the contactor is energized but buzzes loudly and doesn't draw in the contacts. Voltage across the coil is 3-4V, and the 24 V rail droops to ~15 V.

Detail

When I bought my house a couple years ago, it came with a Lennox AC/gas furnace unit on my roof, model GCS20R-461-75-2P. Heating and AC both worked, but I couldn't get the fan to run by itself. I started experimenting at the thermostat by bridging wires, and noticed a spark when I tried to start the AC. After that, AC wouldn't work, only heat.

So I went up to the roof to investigate, and found a tangled mess of wires and relays. I’m an electrical engineer, so when I found the circuit diagram inside the unit, I started tracing out the fan circuit, and found it to be completely mis-wired. I restored it to the factory configuration based on the circuit diagram (photo below), and voila, fan-only works now.

Heating still works, too, but AC still doesn’t work. The wiring seems correct, but when cooling demand is activated, the contactor (K1) buzzes after an 8.5-second delay, and doesn't suck in the contacts. Measuring the voltage across its coil, it’s only about 3-4 V. The lightly-loaded1 24 V transformer gives about 27 V.

When in this state, the 24 V supply droops to around 15 V, and K123 begins to oscillate rapidly. I assume this is due to the drooping voltage.

I can’t tell whether the time delay circuit (DL15) is bad, or if the contactor coil is bad. If I disconnect the coil from the time delay, its output measures at 27 V, making it hard to test its output. In truth, I'm doubting this is what I saw, since the time delay uses a relay, I would have expected its output to be disconnected during the delay period.

Is it typical that a failed contactor end up drawing a lot more current (i.e. being a short)? I’m gonna pick one up tomorrow, but replacing it is a pain because of its location, and if it turns out to be the time delay…I’m not sure where to get that part.

Aside from replacing the contactor and testing, does anyone have any other recommendations for diagnosing the issue?

Circuit:

HVAC circuit diagram

Time Delay:

enter image description here

Contactor:

Image of contactor

The mess I inherited:

Image of the mess of wires and relays

1Relay K123 is normally energized when the unit is energized.

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  • the 24 volt transformer should not drop to 3-5 volt, either it is bad or someone is drawing over the limit current
    – DIY75
    Commented Jul 11 at 7:34
  • 1
    If you energize the contactor coil directly from the 24V source using a jumper, you will know if the contactor is good or bad. If the contactor works and the compressor starts, and if the only thing presently between the 24V source and the contactor coil is the time delay, then the time delay is bad. Don't let the compressor run too long without cooling air from the condenser's fan. A minute is OK.
    – MTA
    Commented Jul 11 at 13:32
  • I'd look for an issue in the 24 VDC supply, as well. Electrolytic capacitors often go bad, and diode rectifiers can fail. In either of those cases, the line could be a normal 24 VDC or above without load, and drop greatly with load. If you briefly place a 50 Ω resistor across the 24 VDC line, voltage should fall to no lower than ~18 V. If it drops to 5 VDC, the power supply is definitely at fault. BTW, +1 for good documentation.
    – DrMoishe Pippik
    Commented Jul 11 at 22:51

1 Answer 1

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This is a common problem with relays that causes buzzing or chattering. When the contacts become carboned up or pitted after many cycles the relay begins to chatter and will not hold. If the relay is able to be disassembled it may be possible to burnish (polish) the contacts to remove the carbon and pitting and return it to service. This may give you a few more months/years of use but ultimately the relay will probably have to be replaced. This would be my first step to remedy this situation.

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