When the refrigerator suddenly stops freezing, and strange clicks or a hum is heard from the back of the unit, which ends abruptly, this is a sure sign of problems with starting the engine. In most cases, the culprit for such symptoms is starting relay, which is responsible for supplying voltage to the windings of the electric motor. This small device is a critical element in the compressor power circuit, and its failure completely blocks the operation of the refrigeration unit.
Independent diagnosis of this unit does not require deep knowledge in electrical engineering, but requires care and compliance with safety precautions. You will only need a minimum set of tools to understand whether the part needs to be replaced or whether the problem lies deeper.
In this material we will look at how the relay works, what are the signs of its malfunction and how to carry out a professional test with a multimeter at home. You will find out why the compressor may hum but not start, and whether it is possible to temporarily revive the unit until the technician arrives.
Design and principle of operation of the starting relay
The starting relay is an electromagnetic switch that controls the supply of current to the starting winding of the compressor motor. When the refrigerator is turned on, the rotor of the electric motor is at rest, and significant force is required to move it from a dead point. It is this task that is performed by starting winding, which comes into operation only for a few seconds.
Inside the relay body there is an inductance coil and a movable core with contacts. When starting current passes through the coil, a magnetic field is created that draws in the core and closes the contacts of the starting circuit. As soon as the rotor picks up speed, the current drops, the magnetic field weakens, and the contacts open under the influence of gravity or a spring. There are also thermal relay (posistors), where the role of a switch is played by a heating element.
⚠️ Attention: There may be capacitors inside the relay that retain a charge even after disconnecting the device from the network. Before touching the contacts, make sure that the device is completely discharged.
Structurally, relays are divided into inductive (work on the principle of an electromagnet) and thermal (use a bimetallic plate). Induction models are often marked to indicate the orientation of installation (arrow "UP"), since the core must move freely under the influence of gravity. Violation of the installation position leads to the fact that the contacts do not open or close in time.
What is the difference between an induction relay and a posistor?
An induction relay uses an electromagnetic coil and a movable core to close the contacts. A posistor (thermal relay) works by changing the resistance of a ceramic element when heated: the cold element passes current to the starting winding, heats up and opens the circuit. PTC resistors are more reliable because they do not have mechanical moving parts.
Symptoms of a faulty compressor relay
You can determine that it is the relay that has failed, and not the compressor or thermostat itself, by characteristic signs. Often users confuse these symptoms, which leads to unnecessary replacement of expensive components. The main indicator is the characteristic sound made by the refrigerator.
If you hear the motor trying to start, emitting a dull hum, and then a loud click is heard and silence sets in - this is a classic picture of a “wedge” start. The compressor jerks but cannot turn over. After some time, the cycle repeats. This means that the starting winding does not receive a pulse or the relay cannot keep the contacts closed.
Other signs may include:
- 🔌 No response not being turned on when the light inside the chamber is on.
- 🔥 Heating of the housing relay or charring of contacts visible when external inspection.
- 🔊 Constant humming without switching to operating mode until the thermal protection is triggered.
- ❄️ Insufficient coolingif the relay is “stuck” and does not turn off the starting winding (the engine is running with overheating).
Sometimes the relay fails due to power surges in the network. In this case, carbon deposits may form on the contacts, which prevents the normal flow of current. If the relay has a transparent plastic body, a visual inspection can immediately reveal blackening or melting of the internal elements.
Preparation for diagnostics and necessary tools
Before starting the test, you need to prepare the workplace and tools. The main requirement is safety. The refrigerator must be completely disconnected from the power supply at least 15-20 minutes before starting work. This time is necessary for the capacitors to discharge and the compressor to cool.
To carry out diagnostics, you will need a standard home repair kit. The main tool will be multimeter (tester), which will allow you to measure the resistance of the circuit and check for a breakdown. You will also need a screwdriver (phillips and flat) to remove the compressor protective cover and pliers.
☑️ Tools for checking relays
If you don’t have a multimeter, you can use the simplest “dialing” method with a light bulb and a battery, but it is less accurate and will not show the exact resistance of the windings. Before removing the relay, take a photo of the wire connection diagram or sketch it so that you do not mix up the contacts during assembly. An error in the connection can lead to the compressor burning out.
⚠️ Attention: Never try to check the operation of the relay by applying 220V voltage to it manually, holding it in your hands. This is deadly and can lead to electric shock.
Visual inspection and dismantling of the relay
Removing the relay is usually not difficult. It is located on the side of the compressor (the black tank at the bottom of the back) and is covered with a plastic or metal cap. The cover is secured with latches or screws. Carefully remove the clips, being careful not to damage the plastic elements, which become brittle over time.
After removing the cover, you will see a relay placed on the three pins of the compressor. It can be additionally secured with a bracket or held only due to the tight fit on the contacts. Carefully disconnect the wires if they are connected through terminals, or simply remove the relay from the pins if it is held only by them.
During a visual inspection, pay attention to the following details:
- 👀 Integrity of the housing: presence of cracks, traces of melting or soot.
- 🔌 Contact condition: oxidation, carbon deposits or burning of metal parts.
- 🔊 Sound when shaking: if something rattles inside the relay (like broken glass), it means the ceramic posistor or mechanical part is destroyed.
- 📏 Model compliance: check the markings on the housing, they must match those required for your type of compressor.
If, upon external inspection, obvious signs of overheating or melting are visible, further checking with a multimeter may be unnecessary - the part requires replacement. However, to be completely sure and exclude other causes (for example, an interturn short circuit in the compressor itself), it is better to carry out electrical measurements.
Checking the relay with a multimeter: step by step instructions
The most accurate diagnostic method is to measure electrical resistance. Switch the multimeter to resistance (ohms) measurement mode, selecting a limit of 200 ohms or 2 kohms. First check the relay itself, disconnected from the compressor.
For inductive relays (with a coil), the resistance between the main contacts should be close to zero (if the contacts are closed at rest) or infinity, depending on the design. The coil resistance is usually between 30 and 100 ohms. If the device shows one (open) or zero (short circuit) where it should not be, the relay is faulty.
For thermal relays (posistors) in a cold state, the resistance between the contacts should be small (3-50 Ohms). If the multimeter shows an open circuit (infinity) on a cold posistor, it means it has burned out and will not start the engine. If the resistance is zero, a short circuit has occurred.
It is also important to check the circuit for a breakdown to the housing. Touch one probe to any relay contact and the other to the metal frame or housing. The device should show infinity. If there is any resistance, then the insulation is broken, and using such a relay is dangerous.
Additionally, you can check the operation of the mechanism. By applying voltage to the induction relay coil from a safe source (for example, 12V, if the coil is low-voltage, which is rare for household refrigerators, or carefully 220V, taking precautions), you should hear a click of the contacts closing. But it is safer and easier to simply replace the questionable part, since the cost of the relay is low.
Table of typical resistance values
For ease of diagnosis, below is a table with approximate resistance values for various types of relays. Remember that the exact parameters depend on the specific refrigerator model Indesit, Atlant, Beko or LG.
| Relay type | Coil/posistor resistance | Contact resistance (closed) | Symptom malfunction |
|---|---|---|---|
| Induction (RT, RTP) | 30 - 100 Ohm | < 1 Ohm | Coil breakage, contact sticking |
| Thermal (posistor) | 3 - 50 Ohm (cold) | Depends on the circuit | Destruction of the ceramic element |
| Combined | Checking according to the circuit | Check according to the circuit | Malfunction of the thermal or starting element |
| Any type | Infinity | Infinity | Full open circuit (burnt out) |
When taking measurements, take into account the instrument error and the ambient temperature. The resistance of a posistor strongly depends on temperature: a heated element can show hundreds of kOhms, which is normal for its operating state, but not for starting.
Replacement and performance testing
If diagnostics confirm a malfunction, the relay must be replaced. You can buy a new part in spare parts stores for household appliances, indicating the model of the refrigerator or the marking of the old relay (for example RT-21, 100N0021). Installation is carried out in the reverse order of removal.
Place the new relay onto the pins of the compressor until it ends. Make sure it fits snugly and doesn't wobble. If the relay is inductive, strictly observe the orientation along the arrow UP (up). Connect the wires to the terminals according to the diagram or photograph made earlier.
After installation:
- 🔌 Put the protective cover of the compressor in place.
- 🔌 Plug in the refrigerator.
- 👂 Listen: it must followed by a confident start of the motor without prolonged buzzing.
- 🌡️ Check the cooling after 30-40 minutes of operation.
⚠️ Attention: If after replacing the relay the compressor buzzes for 5-10 seconds and clicks again, the compressor itself may be faulty (interturn short circuit of the windings or wedge of the piston group). In this case, further attempts to start may finish off the engine.
Is it possible to temporarily start a refrigerator without a relay?
Theoretically, it is possible to apply voltage directly to the working and starting windings, but this requires the skills of an electrician and carries a high risk. Without a relay, the compressor will not turn off the starting winding, which will cause it to burn out in a matter of seconds. This can only be done for a short-term check of the compressor.
Successfully replacing the relay often brings the refrigerator back to life for several more years of service. However, if the problem was caused by a power surge, it is recommended to turn on the unit through a stabilizer or surge protector to protect the new part and electronics.
Why does the relay burn out?
The main reasons for failure: frequent power surges in the network, contact wear due to a large number of on/off cycles, compressor overheating due to contamination of the condenser at the back of the refrigerator or incorrect installation (too close to the wall).
Is it possible to repair an old relay?
Repairing the relay (cleaning the contacts, restoring the coil) is possible, but not economically feasible. The cost of a new part is low, and the reliability of a restored unit will be questionable. It is easier and safer to replace.
What is the difference between relays for Atlant and Indesit?
The difference lies in the design of the seats (distance between pins), type of connection (connector or pins) and operating currents. A relay from one brand may not fit the compressor of another physically or electrically.
What to do if a new relay is not on sale?
You can choose an analogue. The main thing is the coincidence of the operating current (indicated on the case, for example, 1/4 HP or 1/5 HP) and the type of connection. Universal relays often have additional adapters included.
Is the compressor humming after replacing the relay - is this normal?
A short-term hum (up to 5 seconds) before starting is acceptable. If the humming is prolonged and the startup does not occur, perhaps the relay is selected incorrectly (cut-off current is too weak) or the compressor requires repair (oil change, wedging).