Instructables

Step 7: Your air conditioner unit

Air conditioning systems always seem to fail during the hottest, most miserable weather. No one wants to spend a muggy night trying to sleep with no air conditioning. If the problem is only a fuse, your meter can save you the cost of a service call by a technician, and you can have your system up and running again before the house has even warmed up inside.

Your air conditioning system may have more than one set of fuses protecting it. There may be a set of circuit breakers in your main electrical panel. Check to see that the circuit breaker toggles have not moved to the "off" position due to a sudden overload. Go to the air conditioner unit outside your house. Look for any cable conduits (metal pipes, some flexible). Follow them with your eye and look for any metal boxes that might contain fuses. The fuses will likely be mounted in a fuse block that can be pulled from the box. Use the continuity setting to check the fuses. The first photo shows a control box near our heat pump/air conditioner. It looks like it might contain fuses, but it contains only a disconnect switch.  

See the second photo. It shows the inside of the fuse box that controls our air conditioner. The yellow text box contains information on the ON/OFF switch. I overlaid the photo with capital letters as markers. Those in red make a "hot" circuit even when the switch is "off." Keep your hands safely away from these terminals.

Set the meter to the 400 volt AC setting. If you place one probe on A and the other on B, the meter should read about 230 volts. That is true whether the switch in the box is "on" or "off." Place one probe on A and the other on C. The meter should read about 115 volts. Place one probe on B and the other on C. The meter should again read about 115 volts. Readings between F and C or G and C should also each give a reading of about 115 volts when the switch is "on." These readings indicate the fuses are good. With the switch in the "off" position, readings between F and C or G and C should be zero volts.

If you wish to check the fuses without them being electrically charged, move the switch lever to the "off" position. Set the meter to CONT. Touch one probe to D and the other to F. You should hear the meter's shrill chime tone. Now place one probe on E and the other on G. You should hear the tone again. This also indicates the fuses are good. If one of the fuses does not test good, be certain the switch is in the "off" position. With your fingers or a pair of pliers or a wooden stick to pry, remove the bad fuse. Take it with you to a hardware or building supply store and get a replacement. Use a fuse. Do not use a piece of metal, a piece of copper tubing, or a wire with alligator clips to bridge the space for the fuse. In the days when most homes had screw-in plug fuses some people sometimes put a copper penny behind a blown fuse when they did not have a new fuse. Then they often forgot about the penny. Pennies do not burn away like a fuse does when there is an overload. Pennies used this way often caused house fires. There was a frequent saying that, "The words 'In God We Trust' were placed on pennies for the benefit of those who use them as fuses."

 
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ac-dc3 years ago
If your A/C has blown a fuse DO NOT replace the fuse before you have repaired the damage that caused the fuse to blow. A multimeter isn't all that handy for homeowner A/C diagnosis unless you have in depth understanding of the components, it usually isn't a circuit or fuse problem, more likely ("IF" electrical at all) a failed motor start capacitor, motor short, compressor seized, control board, etc.