Blowing 5A fuses, only when A/C is turned on, how to troubleshoot?


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Old 09-21-09, 07:32 PM
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Blowing 5A fuses, only when A/C is turned on, how to troubleshoot?

I've got a comfortmaker 10 sitting outside, with a Honeywell T8100c thermostat. Up in the attic, the control board is blowing 5A fuses when I turn on the A/C. The fan and heater both work fine. I would appreciate any tips on troubleshooting it. I've got an ohm meter and know my way around electronics generally, but no limited knowledge of debugging this type of thing. I'm thinking the problem is with the unit outside - would low pressure cause this? Thanks.
 
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Old 09-21-09, 07:54 PM
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I've done some more reading/research online and I think the 1st response will be that the "low voltage wires are shorted." I've opened up the A/C unit outside and see the 2 wires coming in from the unit in the attic. They seem fine going out from up in the attic and appear fine as they are tied in to the circuit on the A/C unit itself. There's appears to be a coil in the A/C and a transformer (?) - how can I verify that they do/don't function as expected? Also, how can I test these 24v wires coming from upstairs? My ohm meter continuity test requires I touch both sides of the lead
 
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Old 09-22-09, 03:47 PM
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Originally Posted by itsbilly
There's appears to be a coil in the A/C and a transformer (?) - how can I verify that they do/don't function as expected?
You just set your voltmeter to ac volts above 120 volt range, and using the red and black probes on yourr tester, test between the 2 120 "line"(incoming) wires and the 24 volt transformed "load"(outgoing) wires. And see what the coil yields either between wires or test from one wire from it to metal ground.

Also, how can I test these 24v wires coming from upstairs? My ohm meter continuity test requires I touch both sides of the lead
Set thermostat to come on, upstairs. Then with your air handler shut off, and with your test meter set to ohms, inside your air handler where the thermostat wires hook up, put your red probe to one thermostat wire and the black probe to the other thermostat wire, and you should show complete continuity which is either 0 ohms or a fraction below the number 1. If you have a digital voltmeter that shows the number 1 on the screen, and does not even jump to some otheer numbers first when testing, that means the line is open and you do not have continuity. The number 1 in that case does not really mean 1, it means dead.

Also, duplicate the test with the thermostat turned off. The same test should yield an open (dead) reading on your meter in the air handler. And if not, you have a short in the thermostat wiring or thermostat.
 
 

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