A digital or analog multimeter is used to test capacitors. There are two types of capacitors used in window AC units — run capacitors and start capacitors. Some manufacturers combine both the run and start capacitors in one unit. The start capacitor provides the extra starting torque needed by the compressor to start under-load and is connected only during the start-up cycle. The run capacitor, on the other hand, remains connected at all time and serves to increase the efficiency of the compressor evaporator/condenser fan motors. Testing these capacitors is an easy process that even a beginner can do correctly and safely the first time. Unplug the window air conditioner from its dedicated wall receptacle. Remove the unit from its window mounting and place on the floor or a sturdy table. Some of these units can be quite heavy so you may want to enlist the aid of a helper. Remove the screws holding the sheet metal, wrap-around cover in place. Use caution when sliding the cover off. The edges of the sheet metal are very sharp and can cut your hands.

Replace the the screws in the frame for safe keeping. Locate the run and start capacitors. Run capacitors are oil-filled capacitors while start capacitors are electrolytic capacitor; run capacitors are round or oval shaped, silver-colored metal cans, while start capacitors are round, tubular-shaped, black-colored units. These capacitors, as a rule, are located on or very near the component they serve i.e. compressor or fan motor. Discharge the capacitor by shorting its terminals together using the metal shank of a screwdriver. Place the metal shank of a screwdriver with a plastic handle between the terminals connecting them together. Keep your fingers away from the metal shaft while discharging the capacitor. A capacitor in good condition can hold an electrical charge for several hours, even months, after the unit is unplugged from the power source. Remember, electricity is dangerous, and a shock from one of these capacitor can kill you. Take a picture of how the wires are connected to the capacitor with a digital camera, or make a sketch of the connections.

Remove the wires from the capacitor by pulling the female terminals from the male terminals on the capacitor using the long-nosed pliers. Remove the wires from the capacitor and short the terminals together again with the metal shank of the screwdriver to make sure no residual charge remains on the capacitor. how to replace a capacitor in an ac unitA residual charge is a minute charge that will affect the test but is not a danger to humans. replacement coil for ac unitSet the function switch on your digital multimeter to it capacitor test position and touch the meters test probes to the capacitors terminals. what to do when your inside ac unit freezes upThe meter's LCD should display the capacitor's actual capacity in Microfarad.

Most capacitors have an allowable tolerance of ±10 percent. If the reading falls within this range, the capacitor is good. The rated capacity is marked on the capacitor's case. Reverse the meter probes and repeat the test. If the reading varies more than 10 percent of the rated value, replace the capacitor. Remove the capacitor from the circuit and remove the bleeder resistor from across the capacitors terminals to prevent a false test result. An analog meter will read the resistor's resistance and will make a good capacitor appear to have a high resistant internal short. Set the meter's function switch to the “R X 1000” scale. Touch the meter's test probes to the capacitor's terminals. Watch the meter's pointer. The pointer will quickly snap all the way over to the right and the slowly drift back over to the left for a good capacitor. Reverse the meter probes and test again. A meter reading of infinite resistance, the pointer remaining at the left of the scale, indicate an open circuited capacitor and it needs to be replaced.

A constant meter reading of zero resistance, the pointer snapping to the right and remaining there, indicates a shorted capacitor and you need to replace. Things You Will Need Phillips screwdriver Flat-blade screwdriver Long-nose pliers Digital multimeter with capacitor test function or analog multimeter References Capacitor For Motor: Test Capacitor Photo Credits Polka Dot Images/Polka Dot/Getty Images Suggest a Correction Had a checkup on my AC/ furnace a couple weeks ago. The technician said the capacitor leaking oil and not pulling enough amps. He recommended me getting a whole new system. (My house was built 6 yrs ago.) I said no I cannot afford, he replaced the capacitor. I asked so you tested and the amps are all good now. He responds that he did not test anything after he replaced it. That struck me as odd but what do I know I'm just a woman. Since replacement, the lights dim when unit kicks on, which it has always done that, no worry. While unit is running the lights will dim several more times.

Each time they dim there is an audible pause in the operation of the unit. Very very short pause like of about a second. I would be very appreciative of what could be the issue and what I should check on my unit. If the lights are dimming several times while the unit is running, the only thing you can check is listen to the outdoor unit to determine if the compressor is unexpectedly starting and stopping when it should be running continuously. Set the thermostat to a low setting and listen outside to see if you can hear the compressor starting and stopping at about the same interval as you notice the lights dim. If abnormal starts and stops are occurring, a competent technician is needed to evaluate and repair the problem. Whether or not the compressor is cycling unexpectedly, installation of an appropriately sized 3 wire hard start kit can sometimes alleviate lights dimming on startup due to the electric service being marginally small for the unit. You may want to try a better technician than the one who advised an entirely new system and then replaced a leaking capacitor.