Step One: Hook up your gauges

You want to make sure that the unit you are troubleshooting has refrigerant in it. You may find that your compressor has overheated and blown the run capacitor. Only to replace the run capacitor, to find out that the compressor is running in a vacuum. This can cause further damage to the compressor if it’s not already damaged to begin with. You also want to make sure that the pressures are equal on both the low and high side. Compressors don’t like to start with un-equalized refrigerant pressures. 

Got refrigerant and equalized standing pressures? Is the condensing fan motor running but the compressor isn’t? If so, you can skip to step 4, but should still check for loose wiring.

Step Two: Check the contactor for low voltage

Take off the service panel and check to make sure that the contactor is pulled in. You can normally hear the buzzing sound. If the contactor has a cover plate, pull this off and check it. Most residential air conditioners have contactors with 24-volt coils. Check the contactor for 24 volts. If you have a condensing unit that has a low voltage wire going through the condensing unit, it could be going through a high pressure switch. Bottom line: if you don’t have 24 volts coming into the condensing unit itself, then your problem is outside of the condensing unit. Find out where you’re losing voltage.

While you are testing the contactor, keep your eyes open for burnt wires, loose wires, signs of arcing, and of course a swollen run capacitor.

Step Three: Check the High Voltage across the Contactor

Check the bottom terminals of the contactor. L1 and L2. Record the voltage you get and compare it to the operating voltage on the name plate. If you don’t, then check the disconnect at the condensing unit for correct voltage and backpedal until you find out why.  If there’s a fuse at the disconnect, check the fuse.

Note: If the breaker to the condensing unit trips immediately upon resetting, you should check the compressor for an internal short to ground, as explained in step 5.

Check the voltage at the top of the contactor from L1 to L2. Tighten the connections with your screwdriver and recheck. If one side has power and the other doesn’t, replace the contactor.

Pro Tip: If you have an infrared thermometer, point it at the compressor and check the temperature. If it’s hot and overheating then it is more than likely off on internal overload.

Step Four: Check the Run Capacitor

Disconnect the power and check the run capacitor with your multimeter. Read the capacitor and make sure that it’s within specs. If you have to replace the run capacitor don’t assume the correct size is in there. Some manufacturers are nice, and the run capacitor will be displayed on either the nameplate, or the nameplate of the compressor. Some manufacturers won’t display the information on the nameplate, and some units will be old and worn. If possible, get the M# of the compressor and call your supply station or tech support to get the correct sized run capacitor.

If your compressor’s run capacitor is good and is not blowing breakers, go to step 6, then 7.

Step Five: Compressor Keeps Blowing the Fuse/Breaker – Checking for Internal Short to Ground

Before jumping straight to this, I have to reiterate: Check your wiring for loose connections and arcing. A famous spot for this to occur is right at the seal tight coming into the access panel, where the vibration of the metal cabinet against a wire can sometimes eat through the insulation.

Checking for a grounded compressor:

Remove the wires or plug from the compressor terminal. Inspect the terminals. Set your meter to OHMS, the highest resistance. Test your meter for continuity. Put one test lead on the winding, and the other on grounded metal, such as the discharge line, and sand it down if you want to. If you are reading any resistance, then your compressor is grounded. Replace.

Another way to test this is to use a tool called a Megger. It is more accurate than your multi-meter and can tell when your compressor insulation is beginning to get weak.

Step Six: Check the Compressor Shell Temp

If the compressor gets too hot, it will go off on internal overload. By reading the wiring diagram, you can see which points to check. Or you can cool the compressor off and see if it starts. The fastest way to cool off a compressor is with a garden hose. If you don’t have access to a garden hose, get a bucket and some wet towels. Be smart about it though. Don’t spray the hose at the compressor terminals with the wiring harness disconnected. Think rain, just spray the top of the compressor shell and it should cool off in a couple of minutes. The wet towel method will take longer. Once cooled the internal overload should reset. Restore power.

Step Seven: Check the Motor Windings

Set your meter to Ohms. Measure the resistance between the windings. There’s a little formula we use to test the windings that goes: Common to Start + Common to Run = Run to Start. For example, if Common to Start reads .02 ohms and Common to Run = .04 ohms, then Common to Run should be around .06 ohms.

Step Eight: Compressor May be seized

If you performed all the prior tests and still can’t get the compressor to start, then you can add a compressor saver hard start kit. Hard start kits are only good on PSM motors, which most residential compressors are. If you’re not sure, call tech support.

Lots of the 14-16 Seer condensing units, especially ones with TXV’s and located on roofs, will need a hard start capacitor to turn it over. Sometimes I may perform this step before checking the motor windings.

Step Nine: Condemn the Compressor

If you’ve thoroughly completed all the steps below and the compressor is still not starting, then it is time to condemn the compressor. Sometimes though you really just want to know why the compressor won’t start or what caused it. If you have a good cutting wheel and a bunch of free time after replacing the compressor with a new one, take the old one home and cut the top off and inspect it.

Should you get the compressor running by completing one of the steps above, check the system operation to ensure the problem won’t happen again. If you have odd refrigerant pressures, then you should troubleshoot the refrigerant charge and get the problem fixed. Compressors are designed to run at specific pressures with ideal duct work, along with many other conditions.

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