Air Conditioning Capacitor Troubleshooting
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If your air conditioner is not working properly, you may be considering calling a repairman to help fix it for you. Unfortunately, this could cost up to several hundred dollars. But with just a multimeter and a little bit of troubleshooting knowledge, you can diagnose and fix the common problem of a failed air conditioning capacitor in your unit.
An air conditioning capacitor (also known as a run capacitor or a dual run capacitor) supports the compressor and fan motors in an air conditioning unit. Over many years, they break down internally and the capacitance reduces. This causes them to lose their ability to start the fan or the compressor in your air conditioner.
A dual run capacitor is two capacitors in one case, and these are commonly used in air conditioning units. If you are unable to source a 3 pole capacitor, you can use two separate capacitors that match the ratings on the dual run capacitor. For example, the dual run capacitor you need to replace has a microfarad rating of 40/5 uf. You can get two separate run capacitors (rated at 40 uf and 5 uf respectively) in place of the one rated at 40/5 uf.
What to look for when diagnosing your air conditioner
Check to see that the contactor in your AC unit and the fuses are working. If both of these components are working, it is very likely that the problem lies within a defective air conditioning capacitor.
Use a multimeter that is set to the "capacitance" rating to determine whether or not the run capacitors in your unit are working. If the readings you get are lower than 10% of the capacitor’s rating, you will need to get a replacement air conditioning capacitor.
View our troubleshooting video tutorial below to see how to diagnose these problems.
How to Replace an Air Conditioning Capacitor
The two most important specifications to look for when replacing a capacitor are the capacitance and voltage ratings.
Capacitance - This will be indicated by the microfarad rating on your capacitor. The unit used for this is "uf" or "mfd." The replacement air conditioning capacitor should have the same rating.
Voltage - Make sure the capacitor’s voltage rating is at or above that of the system voltage. It is perfectly fine to have a capacitor with a maximum voltage rating that is higher than your system voltage. For example, using a 370V or 440V air conditioning capacitor in a 240 V system voltage is perfectly fine.
Form factor/case style - Capacitors come in two case styles: oval or round.
Oval and round air conditioning capacitors are completely interchangeable as long as they can fit within your air conditioning unit. There may be a mounting strap within your unit that is either oval or round, so the capacitor needs to fit within that area.
Run vs. Dual Run Capacitors
Run - If your capacitor has 2 terminals, it doesn’t matter which wire goes to which terminal.
Dual Run - If your capacitor has 3 terminals, the condenser motor should have a wire on the fan terminal (usually the lower uf/mfd rating) and the compressor "start" wire is connected to the "HERM" terminal (usually the higher microfarad rating). The third is labeled as "C" or "common."
Place one of the probes on either the HERM or FAN terminal (depending on what you're checking) and the other on the "common" terminal.