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My AC unit has the fan working but not the condenser and I found that one of the two 50 amp fuses was bad. Unfortunately after replacing it, I saw and heard a spark and the new fuse was blown immediately after replacing it. Then I replaced the capacitor (just in case) and checked if there are no loose wires touching around the capacitor area, and pulled some wires going into the fan/condenser unit back and forth just in case some of them were touching around there as well. Then I replaced the fuse again with a new one, and that one was blown immediately again. Thus I've lost 2 additional fuses in 30 minutes just trying to replace the bad one. I measured resistance at 0 by using a multimeter to confirm that the two replacement fuses were good before being blown. What can I do to further investigate the problem without the power/compressor working?

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    If the fuses keep blowing, the compressor motor is damaged and is shorted to ground. You said you measured resistance as zero - resistance of what? Measure the resistance of the compressor. – JPhi1618 Aug 23 at 17:53
  • Thanks. I measured the resistance of the fuses to check whether they're good or bad, and that's how I identified the original problem. How would I measure the resistance of the compressor? – Kon Rad Aug 23 at 17:55
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    OK, Google to the rescue, and indeed I seem to have a short within the compressor. I guess I may need to replace the compressor, and will contact my HVAC guy. Thanks for the tip that I need to check the compressor resistance - that's what I needed to know. – Kon Rad Aug 23 at 18:32
  • Reposting comment as an answer then so this has an answer... – JPhi1618 Aug 23 at 18:38
  • 50 amps?!? It's time for a new unit anyway.... That's like 7.5~10 tons, or a unit from 1985. – Mazura Aug 24 at 2:09
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The next step is to see where the short is, and for that you will need to check the resistance of the compressor and fan motors. The exact wiring can vary by model, but basically, you need to check the resistance between the wires going to each motor and see if any are shorted.

As a first step, you can probably just check all the wires going to each motor and see if they are shorted to the case of the motor. If a wire breaks internally it can ground out to the motor housing. Beyond that, you need to understand more how the motors are wired.

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You do need to check the resistance of the motor windings to ground. A standard ohm meter usually will not show a problem, we use a meter called a megger this is a high voltage ohm meter. What happens with the compressor motors is moisture gets in the system and the oil becomes acidic and etches the varnish off the motor windings and when enough if the insulation is gone the motor shorts to ground and blows the fuse. If this turns out to be the case the tech should recover the refrigerant passing it through a filter dryer then flush the system and add a new filter dryer on the suction line. There are acid test kits that if you have had a positive acid problem your system should be checked 3 days to a week after replacing the compressor and adding the new filter dryer. I have only had the test be positive 1 time but it was a good thing I checked because the compressor in that unit was almost $3000. , I had to pull the system down again and flush it then replace the filter dryer and on that system I put a second filter dryer on the liquid line and I checked that unit a week later then again a month after that. Since I checked and found the acid, flushing and new filter dryers were all that was needed. If I would not have checked that compressor would have failed it may have been a week or a few months but the time and $ saved for a quick test is worth every penny.

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