Testing Fridge Compressors, Klixons, and Start Relays: A DIY Guide
⚠️ SAFETY MESSAGE: Always unplug the refrigerator from the mains before touching any electrical components. Compressors and capacitors can hold a charge—handle with extreme caution. If you are not confident with a multimeter, call a professional.
If you’ve checked the thermostat and the fridge still isn't cooling, the problem likely lies in the compressor assembly. Usually, you’ll hear a "click" followed by silence—this is the Klixon (Overload) tripping because the compressor can't start.
1. Testing the Compressor Windings
To see if the compressor motor itself has failed, you need to check the resistance between the three pins (Common, Start, and Run).
Set your multimeter to Ohms (Ω).
Measure all three pairs: C-S, C-R, and S-R.
The Golden Rule: The resistance of C-S + the resistance of C-R should roughly equal the resistance of S-R (within 0.5 ohms).
Ground Test: Touch one probe to a pin and the other to the metal casing of the fridge. If you get any reading (other than "OL"), the compressor is shorted and must be replaced.
2. Checking the Start Relay (PTC)
The start relay gives the motor the "kick" it needs to spin.
The Shake Test: Remove the relay and shake it. If it rattles like broken glass, the ceramic disc inside is shattered. It’s dead.
Multimeter Test: Measure the resistance between the two holes where it connects to the compressor. You should see a low reading (usually 3–30 ohms). If it’s "Open Loop" (OL), replace it.
3. The Klixon (Overload Protector)
This is a small safety switch that cuts power if the compressor gets too hot.
The Continuity Test: Place your probes on both terminals. It should show continuity (0 ohms). If it is "Open," the internal fuse has blown, and the compressor will never get power.
4. The Capacitor (The Power Booster)
If your fridge has a large plastic cylinder near the compressor, that's the capacitor.
Visual Check: Look for bulging, leaking, or burn marks.
Multimeter: Use the "Capacitance" setting (µF). If the reading doesn't match the number printed on the side, it's weak and needs replacing.
Fisher-Style Analysis: Replacing a £15 relay is a "Wide Moat" move for your wallet—it saves you from buying a £1,000 fridge. In investing, we look for companies like Halma that provide these "mission-critical" small components that keep massive systems running.
If you having problems with your fridge you compressor is a major part of any fridge freezer refrigerator system and its control mechanisms so knowing how to check them over is very useful for a DIYer.
Firstly the compressor is just a motor really it has winding for running and starting so checking the windings are intact is my first check if course this will not show a seized compressor but it will tell you if the windings are damaged and or there is a short to earth.
Then there is the capacitor this one is only a 2.5Mf capacitor and my cheap multimeter does not read that low so a bit of a fail but if your compressor is humming then its usually a sign your capacitor has gone but it could also be seized but luckily a capacitor is only a few pounds so worth trying.
The there is the control relay and overload device this get a signal from the thermostat to tell the compressor to turn on the relay operates and the compressor functions if the capacitor or even the compressor was seized then an overload cut out device prevent power being applied to the compressor to long to to prevent overheating, damage or worse.
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Here are a few photos on what i do to test a compressor.
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