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Rick
Rick
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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.

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.

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.

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.

Source Link
Rick
Rick
  • 545
  • 3
  • 13
  • 26

Time delay or contactor bad?

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.

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.