A modern car is impossible to imagine without a complex electrical network that operates dozens of life support systems. From headlights to fuel pumps, all of these consumers require reliable current management. That's where the game comes in. motor-relayIt is a key element of protection and switching.
Many motorists face a situation when the window lifter or cooling fan stops working, and the reason for this is often this small black cube. Understanding that, how the relay worksIt allows you to quickly diagnose a malfunction and avoid expensive repair of wiring in the service.
In this article, we will analyze in detail the physical principles of the device, consider the connection schemes and learn to check its performance using a simple multimeter. You will learn why this component is called the “heart” of automotive electrics.
The Physical Principle of Action of the Electromagnetic Switch
At the heart of any standard automotive relay is a simple but ingenious physical law of electromagnetism. The device is a switch-offIt allows you to control powerful currents with weak signals.
When a small voltage is applied to the control coil, a magnetic field is created inside. This field attracts a metal anchor that mechanically closes the contacts of the power circuit. Thus, a weak current from a button or control unit “allows” the passage of a powerful current to the consumer.
The main task of such a mechanism is the uncoupling of chains. Thin wires from the headlight switch should not withstand the tens of amperes that the lamps consume. The relay takes this load on itself, protecting the driver and wiring from overheating.
⚠️ Attention: Never attempt to close the power relay contacts directly with wire ("screw") to temporarily solve the problem. This can lead to wiring melting and fire as the chain remains unprotected.
There are different types of devices, but in cars the most common ones are open-ended (NO) relay. At rest, their power contacts are open, and the current does not arrive to the consumer until activation.
The history of the relay
The first electromagnetic relays were invented in the 19th century for the telegraph. In cars, they began to be used in large numbers with the advent of electric starters, when it was necessary to control huge currents in a safe way.
Interior design and marking of contacts
To understand how the relay works in the car, you need to look inside its body. A standard 4 or 5 contact device consists of several critical components that ensure stable operation.
The basis of the design is electromagnetic coilIt's wound on the core. It creates the necessary effort to move the mobile contact. The spring returns the mechanism to its original position after the stress is relieved.
On the product case there is always a diagram showing the purpose of the conclusions. Standardized DIN marking helps you quickly navigate the diagnosis of:
- 🔌 30 - general power input (power from the battery).
- 🔌 87 - access to the consumer (normally open contact).
- 🔌 86 - control input (plus from the coil).
- 🔌 85 - control output (minus coils per mass).
- 🔌 87a Normally closed contact (occurring in 5-pin versions).
The contacts inside are made of special alloys resistant to sparkling. When the chain is opened under load, an arc occurs, which gradually destroys the metal. Silver spraying or special composites allow you to extend the life of the device.
When replacing the relay, pay attention to the current strength indicated on the body. Installing a lower-value element (e.g., 20A instead of 40A) will cause it to fail quickly.
Functional purpose in the onboard network
The use of relays in the car is dictated by the need for safety and resource savings. Without these components, you would have to lay thick wire harnesses to each button in the cabin, which would increase the weight of the machine and the risk of short circuit.
One of the key functions is chain-protection. Comfort, alarm or ECU units of the engine operate with microscopic currents. Directly connecting powerful consumers would knock expensive electronics out of service in seconds.
The relay also allows you to implement the logic of “or”. For example, passing beams may be switched on either by turning the key or automatically when the engine is started. Multiple control signals can activate a single power relay.
In modern systems multiplex The role of classical relays is assumed by solid-state keys in control units, but in power circuits (starter, headlights, gasoline pump) electromagnetic devices remain uncontested.
Typical malfunctions and causes of failure
Despite its simplicity of design, the relay is not a “perpetual” element. Over time, under the influence of vibration, temperature and electrical loads, irreversible changes occur in it.
The most common problem is contact-burn. When powerful consumers are included (for example, xenon lamps or a fuel pump) on the contacts, soda is formed. This increases resistance, causes heat and eventually leads to a complete lack of contact.
The second common cause is interturning in the coil. If the insulation of the wire inside the coil is broken, the resistance falls, the current grows, and the winding burns. In this case, the magnetic field is not created and the click does not occur.
Mechanical damage is also common. A burst spring or a stinging anchor will not allow contacts to close. Often this is due to the ingress of moisture inside the body through the looseness.
⚠️ Attention: If you smell a persistent halogen or melted plastic in the area of the mounting unit, check the condition of the relay immediately. This is a sure sign of overheating contacts.
Diagnosis should begin with a visual examination and voltage check. Often the problem lies not in the relay itself, but in oxidized connectors or a bad “mass”.
Practical Diagnostics of Multimeters
To check the operability of the device, you will need a regular digital multimeter. The diagnostic procedure is quite simple and does not require a deep knowledge of electrical engineering.
First you need to ring the coil. Switch the device to resistance measurement mode (OM) and connect the probes to the terminals 85 and 86. The device should show a value in the range from 50 to 100 ohms.
If the resistance is infinitely large, there is a cliff in the coil. If close to zero, the interturn circuit. In both cases, the relay is replaced. Next, check the power contacts at rest.
☑️ Relay verification algorithm
Exercise 12 volts to the coil terminals. You should hear a clear click. At this point, the resistance between contacts 30 and 87 It should be close to zero (vertebral mode).
It is also important to check the absence of a circuit in the off state. Between contacts 30 and 87 without supplying power to the coil, the device must show infinity (break).
| Verification parameter | Normal value. | Sign of malfunction |
|---|---|---|
| Resistance of the coil | 50 - 120 Ohms | 0 Ohms or ∞ (infinity) |
| Contacts 30-87 (calm) | Gap (∞) | Closing (0 Ohms) |
| Contacts 30-87 (work) | Closing (< 1 ohms) | Rip or high resistance |
| Inlet voltage 30 | 12.0 - 14.5 B | No tension |
Replacement and features of the choice of analogues
If a fault is detected, the relay must be replaced. Fortunately, in most cars, they are installed in easily accessible mounting units and are fixed by a simple effort of hands.
When choosing a replacement, focus not only on geometric dimensions, but also on electrical parameters. It is critically important that nominal The new element was equal to or greater than the current of the old.
Pay attention to the location of the conclusions. Although the DIN standard is universal, some manufacturers may use non-standard numbering or leg arrangement. Compare the diagram on the case of the old and new device.
For chains with high inductive load (sirens, powerful motors), it is recommended to use a relay with built-in resistor quenching the spark. This will prolong the life of contacts and protect electronics from power surges.
⚠️ Attention: Do not use relays with less switching current. If it was on 40A, you can’t put on 20A, even if it’s physically fitting.
After installing a new element, be sure to check the operation of the chain. If the problem has not disappeared, you should look for a malfunction in the wiring or the energy consumer.
The main rule of replacement is that geometric compatibility is important, but electrical performance (current and voltage) are a priority for vehicle safety.
Can I repair the burnt relay?
Theoretically, you can disassemble the body, clean the contacts with a cover and collect them back. However, the tightness will be broken, and the resource of the restored contact is extremely small. Given the low cost of new parts, repair is not advisable.
Why does the relay warm up when it's working?
A small heating to 40-50 degrees is permissible. Strong heating indicates poor contact in the connector (oxidation), overloading current or internal resistance of contacts due to coar.
What is the difference between a 4-contact relay and a 5-contact relay?
In the 5-contact relay there is an additional normally closed contact (87a). At rest, the chain is closed through it, and when power is supplied, it switches to contact 87. In the 4-contact versions, the 87a chain is missing.
How often should I change the relay in the car?
There is no regulatory replacement. The details change only when the refusal is made. The average life of a quality relay is 100,000 on-off cycles, which in a car can be 10 years or more.