I have a Nest thermostat that I believe is the culprit of this issue. I've already replaced the contactor once and now the new one is chattering and not kicking the AC on. When the AC is turned off, what should the voltage at the contactor be? I'm getting 27 volts (24 volt transformer), which seems reasonable with no load. However, if the AC is turned off shouldn't there be no voltage? And when the AC is turned on, it still remains 27 volts, but the contactor won't stay locked in.
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What's the make and model of the A/C unit, and the make and model of the furnace/air handler?– Tester101Commented Aug 1, 2015 at 15:05
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1 Answer
If the transformer does not have one leg grounded, then you could measure 24 volts at the contactor when the thermostat is not calling for cool. This is because you're measuring between one leg of the transformer (24 volts), and a floating wire (0 volts).
When the thermostat calls for cool, you'll still measure 24 volts if the leads are not attached to the coil. Because you're measuring between each leg of the transformer.
If the leads are attached to the coil, and the thermostat is calling for cool. Then the circuit is complete, and you should measure 0 volts. Because both leads are now at the same potential.
If the coil is bad, or there's a loose connection or broken/damaged wire somewhere. You'll see the voltage "bounce" between 0 and 24 volts, but probably only if you're using an analog meter.
Check all the wiring for loose or broken connections. Make sure the contactor is the proper size, and that the coil is still good.
answered Aug 1, 2015 at 15:46
