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The compressor is the heart of any refrigerator. This is the motor at the back of the fridge that is responsible for circulating the refrigerant in the fridge. When it is not working, the fridge does not cool or freeze things as it should with the thermostat set correctly.
One of the signs that can tell you the refrigerator compressor is bad or needs your attention is when it runs without cooling. You can tell this if ice starts to melt and won’t form or if the fridge compartment gets a little warmer. Also, food going bad in the fridge can be an indicator that something is wrong.
But before you call an HVAC guy to come and fix your fridge, it is good to troubleshoot it yourself to find out what could be wrong. It is also good to not be clueless about what needs to be fixed. And if you are a handy guy like me, you can attempt to fix some of the small problems and save some money.
In this post, I show you how to test the compressor of your refrigerator with a multimeter so you can tell if it is time to refill, replace, or get a new fridge altogether.
Why would you want to test the refrigerator compressor?
As I have mentioned, the refrigerator compressor is the heart of any fridge. When it is not working properly, the fridge will not cool things. In other words, a fridge with a bad compressor is as good as a storage box.
So, one of the reasons you might want to test the compressor of a fridge is if it is running without cooling. Other reasons include overheating or the compressor not running at all.
How to test the fridge compressor
Tools needed:
- Digital Multimeter
- Phillips screwdriver
Instructions
Step 1: Unplug the power cord from the wall outlet.
Step 2: Pull the refrigerator from the wall to access the back side. You can even turn it around to get good access to the backside.
Step 3: Use a screwdriver to remove the back cover or water tray (for small fridges) to access the compressor. The compressor is the metallic cylinder you see at the back of the fridge. Remove anything else that may prevent you from accessing it.
Step 4: Unscrew the set screw on the relay cover to access the relay assembly. This is where the electrical cords terminate on the side of the compressor.
Step 5: Pull the relay assembly from the compressor. If it is hard, use a flat screwdriver to pry it out gently to avoid breaking or bending the prongs.
Step 6: Once the relay assembly is out, you will see three prongs sticking out of the compressor. They provide an electrical connection to the windings of the motor in the compressor.
The prongs are arranged in a triangular manner. Either one on the top and two at the bottom or vice versa. The single prong at the top or bottom is the common pin and the other two connect to the starter coil and main/run coil respectively.
Step 7: Test the compressor windings with the digital multimeter. Here is how to do it:
How to troubleshoot a refrigerator compressor with a digital multimeter
- Plug the black probe or jack into the port labeled “COMMON” and the red one into the port labeled “V/Ω” on your multimeter.
- Set your multimeter to measure resistance or ohms
- Measure the resistance of the starter coil by connecting the black probe tip to the common prong and the red probe to the starter winding prong on the compressor.
- Measure the resistance of the main/run coil by connecting the black probe tip to the common prong and the red probe to the Main/Run winding prong on the compressor.
- Determine the resistance of both the Main/primary and starter/secondary coil by connecting one probe to the starter coil prong and the other probe to the Main/Run winding pin.
- Measure the resistance between the compressor windings and ground by connecting one probe to the body of the compressor and the other to each of the two motor winding probes. You might have to scratch a little bit with the probe to expose the metallic surface if your compressor is coated.
- Note down the measurements.
Step 8: Compare the measurements with the manufacturer’s values. You can get the values from the compressor repair manual which you can easily find by searching its serial number online.
Step 9: Interpret the readings. The table below is a summary of what to make about the resistance measurements.
How to interpret the readings
| Measurement | Purpose | Correct reading |
|---|---|---|
| Between Starter pin and common | Measures the resistance of the starter winding | should be within 5% of the manufacturer’s value |
| Between Main/Run pin and common | Measures the resistance of the main winding | should be within 5% of the manufacturer’s value |
| Between main and starter pins | Measures the resistance of both the starter and main winding | should be a sum of the top 2 values |
| Between main pin and compressor casing | Determines the resistance between main winding and ground | Should be an open circuit or (OL) |
| Between starter pin and compressor casing | Measures the resistance between starter winding and ground | Should be open circuit or (OL) |
If the values are not within 5% of what is in the repair manual, then you know your refrigerator compressor is starting to give in. Furthermore, if there is continuity between the windings and the casing, then the motor is shorted and you should replace it.
If the values are correct and the compressor is running but not cooling, it is time to refill with Freon. Contact an HVAC guy to come and refill it for you.
Lastly, if the values are correct but the compressor is not running, then the relay or the main power supply to the compressor has a problem. You can try to test the compressor without the relay assembly just to be sure it is ok.
Well, that is how to troubleshoot a refrigerator compressor with a digital multimeter when it not working properly.
