Checking the refrigerator's air pressure protection with a multimeter: complete instructions

A sudden stop of a refrigeration unit often confuses the owner, especially if the compressor does not make the usual hum, but only quietly clicks or is silent. In most cases, the cause of such a malfunction is the failure of the start-up relay, which is popularly called a “tablet” or “comb”. This small element is responsible for starting the engine and protecting its windings from overheating, and its diagnostics do not require complex laboratory equipment.

To carry out a quality test, you will only need a basic set of tools and a working measuring device. Multimeter is an ideal device for this, as it allows you to measure the resistance of the windings and check the integrity of the contacts with high accuracy. In this article, we will analyze in detail the algorithm of actions that will help you determine whether the part needs to be changed or the problem lies deeper.

Before taking active steps, you need to completely protect yourself and the equipment. Refrigerator must be disconnected from the electrical network, the plug is removed from the retail outlet, and access to rear nodes are provided. Only by observing safety precautions can you begin dismantling and subsequent diagnostics of the electronic components of the system.

Design and principle of operation of a starting protection relay

A starting protection relay (PZR) is an electromechanical device that controls the supply of voltage to the starting winding compressor. Inside the housing there is a coil with a movable core and contacts that open under the influence of gravity or a spring when the engine reaches the required speed. In parallel with this, thermal protection works, opening the circuit during critical heating.

There are two main types of such devices: posistor and induction. Posistor relay works on the basis of a ceramic element, which is heated by current and changes its resistance, opening the circuit. Induction models rely on an electromagnetic force that keeps the contacts closed only at the moment of startup. Understanding the type of your relay is critical to correctly interpreting the device's readings.

The malfunction may consist of burnt contacts, destruction of the posistor, or breakage of the bimetallic plate. If the contacts stick, the compressor hums but does not start, which can lead to burnt out windings. If the circuit is broken, the engine simply will not receive an impulse to start.

⚠️ Attention: Residues of refrigerant or oil may remain inside the relay. Do not forcefully open the sealed case if you plan to simply replace the unit, as this may break the tightness of the system.

What is the difference between posistor and electromagnetic relays?

After switching off, posistor relays (with a “tablet” inside) require time (about 2-5 minutes) for the crystal to cool down to restart. Electromagnetic (induction) ones can try to start the compressor immediately, but they are more sensitive to voltage drops in the network.

Preparing for diagnostics: tools and safety

To successfully carry out measurements, you will need a digital multimeter set to resistance measurement mode (Ohm). Before starting work, make sure that the probes of the device are in good working order and the battery is not discharged, as an error in the readings can lead to a false diagnosis. Also have a screwdriver for removing the casing and, possibly, pliers ready.

The first step is always to completely de-energize the unit. Even if the refrigerator is not running, there may be residual voltage in the circuit or capacitors may be holding a charge. Electrical Safety is priority number one, so do not touch the metal parts of the compressor with your bare hands unless necessary.

Get to the compressor located at the bottom rear of the refrigerator. You will need to remove the protective plastic or metal casing to gain access to the black "tank" of the motor. The relay is usually put directly on three pins coming out of the compressor housing, and is fixed with a bracket or simply sits tightly in place.

Relay dismantling and visual inspection

Carefully remove the start relay from the compressor pins. Usually it holds quite tightly, but you should not use excessive force so as not to damage the plastic fasteners. Pay attention to the orientation of the part: the top and bottom should be marked UP or TOP, which is important when installing a new element.

Carry out a thorough visual inspection of the body and internal elements if the design allows you to look inside. Look for signs of soot, melted plastic, or blackened contacts. A characteristic burning smell or visible damage to the wire insulation immediately indicates that thermal protection triggered due to overload or short circuit.

Shake the relay near your ear. If you hear rattling or knocking of loose parts inside, this is a sure sign of destruction of the internal mechanisms. The whole device must be audibly monolithic, without rolling elements. Also check the condition of the terminals for oxidation.

Checking the integrity of thermal protection with a multimeter

Thermal protection is a bimetallic plate that opens the circuit when heated. In a cold state (and the relay must be cold!) it should ring, that is, show zero or close to zero resistance. Switch the multimeter to the continuity mode or measure resistance up to 200 Ohms.

Attach the probes to the contacts corresponding to the power circuit (usually these are two contacts in series with the windings). If the device shows one (infinity), then the circuit is broken. This may mean that the protection has worked and has not returned to its original state, or the bimetal is mechanically damaged.

Sometimes contact is restored after lightly tapping the relay body, but you should not rely on this. If the thermal protection does not ring, the compressor will not receive power and the refrigerator will not start. In this case, the relay must be replaced, even if the other parameters are normal.

Diagnostics of the starting coil and posistor

The starting coil creates a magnetic field to start the engine. Its resistance is usually higher than that of the working winding, and averages from 20 to 60 Ohms, depending on the compressor model. Measure the resistance between the corresponding contacts on the relay by checking the diagram on the case or in the technical documentation.

When it comes to a posistor relay, the key element is the posistor itself. When cold, it has low resistance, allowing current to flow to the starting winding. When heated, its resistance increases sharply, turning off the starting winding. A cold test with a multimeter should show the specific value specified in the specification.

Pay special attention to the condition of your contacts. Carbon deposits or oxidation create additional resistance, which heats the unit and leads to false protection activations. Cleaning the contacts is possible, but in modern sealed relays this is a temporary measure, and replacement of the entire assembly is a more reliable solution.

⚠️ Attention: Technical characteristics of resistances may differ between different compressor manufacturers (Danfoss, Secop, Embraco). Always check the ratings with the datasheet of the specific engine model.

Analysis of compressor winding resistance

Often the problem lies not in the relay itself, but in the motor windings. After removing the relay, you have access to the three compressor pins forming a triangle. You need to test them in pairs with a multimeter to find the working, starting winding and common wire.

The sum of the resistance between any pair of contacts should be equal to the resistance between the remaining two. Typically, the greatest resistance is between the starting and working windings, and the smallest is between the working and common wires. If the device shows zero (short circuit) or infinity (break), the compressor is faulty.

It is also important to check the breakdown on the housing. Press one multimeter probe to any pin, and the other to the copper tube of the compressor (clean up the small contact area). The device should show infinity. Any resistance reading indicates an insulation breakdown, which is deadly.

Typical errors during verification and their consequences

One ​​of the common mistakes is checking “hot”. If you just tried to turn on the refrigerator and the relay got hot, the posistor will have a high resistance, which could be mistaken for a fault. Allow the device to cool completely for 15-20 minutes before diagnosing.

Another mistake is ignoring the state of the contacts on the compressor itself. Even a perfect new relay will not work if the motor mating pins are oxidized. Oxides create high contact resistance, causing heating and repeated shutdowns.

Incorrect orientation of the relay during installation also leads to problems. Induction relays only work in a certain position (usually pointing up) since the core must move freely under the influence of gravity. Installation upside down will result in the contacts not opening and the starting winding will burn out.

Is it possible to temporarily start a refrigerator without a relay?

Theoretically, you can apply voltage directly, but this is extremely dangerous. Without thermal protection, the compressor will burn out in a few minutes, and without breaking the starting circuit, the starting winding will burn out. Do this only for short-term diagnostics by a technician.

When a component replacement is necessary

Replacement is required if measurements show an open circuit of the thermal protection, a coil resistance that does not correspond to the nominal value, or mechanical destruction of the housing. They also change the relay if there are frequent clicks and the inability to start the compressor, when other reasons are excluded.

Select an analogue strictly according to the markings or electrical parameters (operation current, type of compressor). Universal relays exist, but they require precise adjustment or may not work correctly with a specific engine. Original spare parts ensure long service life.

After installing a new element, be sure to check the first start-up. The compressor should start with a characteristic sound, the relay should click quietly after 1-3 seconds, turning off the starting winding. If the humming continues and the relay does not turn off, immediately turn off the device from the network.

Why does the multimeter show different resistance values ​​during repeated measurements?

This is typical for posistor relays. After the first passage of current, the posistor heats up and changes its resistance. To obtain correct readings of the “cold” resistance, it is necessary to allow the part to cool to room temperature for 5-10 minutes.

Is it possible to clean the contacts inside the relay instead of replacing it?

In modern sealed cases this is almost impossible to do without damage. In older open models, stripping is possible, but this is a temporary measure. It is cheaper and more reliable to buy a new relay, since the life of the bimetallic plate is also limited.

What to do if the new relay immediately burns out or knocks out plugs?

This indicates a short circuit in the compressor windings or a breakdown in the housing. Installing a new relay in this case will not help and can be dangerous. It is necessary to carry out in-depth diagnostics of the engine or replace the compressor.

How to distinguish the starting relay from the protective relay?

In household refrigerators, they are most often combined into one housing. However, visually the starting relay (coil/posistor) is responsible for the start, and the protective relay (bimetal) is responsible for breaking the circuit when overheated. On the diagrams they may be indicated separately, but physically they are one block.