AC not working - did some troubleshooting

Old 05-29-15, 11:51 AM
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Cool AC not working - did some troubleshooting

Hi - I'm a little handy but not experienced with air conditioning. The problem - no AC and no power to thermostat. Here's what I've done so far, all with the help of online forums/videos:

I've tested the thermostat - it has no battery, runs on 24V, and it works on my other air handler/compressor. It is not the problem.

I've tested the transformer. It is a 115V system. Power in reads 22 ohms and power out reads 1.0 ohms. I think this is within proper limits but would like reassurance.

I've checked the breaker on the electrical panel - no problem.

I've checked the fuse on the low voltage - no problem.

I've checked the capacitor on the air handler - no problem.

I direct wired the air handler (bypassed the thermostat) and it ran fine, no short.

I bypassed the thermostat and connected the low voltage yellow wire (cooling) with the low voltage red wire. If I understand this correctly, it should bypass the thermostat and call for the outdoor compressor to start. This did not start the compressor.

What could be wrong that causes the compressor to not start and the thermostat to be without power? Thanks for any suggestions you might have and, remember, I'm learning as I go.
Old 05-29-15, 12:44 PM
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It appears that you've lost the 24VAC. This control voltage runs the thermostat and controls the compressor/condenser and air handler. Instead of measuring resistance on the transformer, measure AC voltage. Typically, you should measure 120VAC on the input (primary) side, and 24VAC on the output (secondary) side. Some control boards have a fuse to protect the transformer in the even of a short on the 24VAC line. It's possible that you have a blown fuse. You need to figure out why you don't have 24VAC at the thermostat.

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