Faced with the need to replace a burnt-out element in the headlight or fuel pump power circuit, a car enthusiast often discovers that the numbers on the body of the new relay are different from those on the old device. Understanding the logic behind terminal markings is a basic skill for any do-it-yourselfer. auto electricians. Connection errors can lead to a short circuit or failure of an expensive electronic control unit.
Modern industry adheres to uniform standards, but visual perception can be confusing due to different pin locations on the housing. Knowing which contact is responsible for what will allow you not only to replace the part, but also to correctly diagnose a circuit malfunction without contacting service. In this material we will analyze in detail the international numbering system.
The basis for marking is the German standard DIN 72552, which has been generally accepted in the automotive industry for several decades. According to this document, each type of terminal is assigned its own unique digital code, which is applied directly to the plastic housing or indicated on the electrical diagram. It is these numbers that allow us to universalize the repair process, making it independent of the brand of the car manufacturer.
DIN 72552 standard and numbering logic
The development of a single standard was dictated by the need to unify components for different automakers. The numbers on the relay body are not a random set of symbols, but strictly regulated designations of the functional purpose of the terminals. For example, the group of numbers 80-89 is reserved specifically for switching devices, which simplifies the search for the necessary information in manuals.
It is important to understand that contact numbering works on the principle of logical groups. Control circuits (winding) have one meaning, and power circuits (switched contacts) have another. This allows, even in the absence of a diagram at hand, knowing the basic principle, to guess the purpose of an unknown terminal, relying on adjacent markings.
There is also a relationship between the type of relay and the set of numbers. A simple four-pin device will have one set of designations, while a five-pin device will receive an additional pin with its own index. Knowledge of this system helps to quickly identify the type of device and its switching capabilities.
When working with electrical circuits of foreign cars, you may come across Latin letter designations, which are analogous to DIN digital codes. For example, the winding may be designated as Coil, and the power contacts as Switch. However, in 95% of cases, you will see numbers on the case itself, so you need to focus on them first.
Group 30, 87 and 87a: Power contacts and switching
The central element of any power circuit is the power line through which the current flows to the consumer. In the DIN system, the contact is responsible for this 30. It is always connected directly to the positive terminal of the battery through a fuse. This is the high current "input" to the relay, and its voltage is constant, regardless of the position of the ignition key (unless the relay itself is in the ignition circuit).
The output contact, which supplies voltage to the consumer (headlights, fan, pump) when the relay is activated, is the terminal 87. In a normal, de-energized state of the circuit, there is a gap between contacts 30 and 87. When a control signal is applied to the winding, a “patch” is closed inside, and the current flows from 30 to 87. It is this pair that most often burns out at high current loads.
⚠️ Caution: Never use pins 30 and 87 to connect low current signal circuits without load. Although the circuit will physically close, the lack of minimum current may cause the contacts to oxidize and become “sticky” in the future.
Five-pin relays have an additional output 87a. This is a normally closed contact. In the rest state (when the relay is turned off), it is connected to pin 30. When a control signal is applied, connection 30-87a is broken and circuit 30-87 is closed. This circuit is often used to switch operating modes, for example, low/high beam or main/emergency fan operation mode.
Contact 30 is always a constant “plus” from the battery, 87 is the output to the consumer when turned on, and 87a is an alternative output that is active when turned off.
Understanding the difference between normally open (87) and normally closed (87a) contacts is critical when installing accessories. If you confuse them when installing an alarm or an autonomous heater, the device may work the other way around: turning on when it should be silent, and turning off when activated.
Control winding: pins 85 and 86
A low-current control circuit represented by an electromagnetic coil is responsible for turning on a powerful power group. It is marked with numbers on the body 85 and 86. A current is supplied through these contacts, creating a magnetic field that draws in the relay armature. The current here usually does not exceed 0.2 Amperes, which allows you to control powerful consumers through thin wires and weak buttons.
A common question for novice auto electricians: is there a difference where to apply the “plus” and where the “minus” to the coil? For standard DC relays, polarity 85 and 86 it doesn't matter - the coil will work anyway. However, modern relays often have a built-in diode or transistor to protect against voltage surges (inductive surges).
If the relay has a protective diode built into it, polarity becomes critical. Usually in such cases, a small diode diagram or marking is drawn on the case next to the numbers 85 and 86. In this case, the “plus” of the control should be applied to the contact where the diode triangle (cathode) faces, and the “minus” - to the acute angle (anode). By reversing the polarity of the relay with the diode, you will create a short circuit in the control circuit.
- 🔌 Contact 85 - most often used as the negative terminal of the winding (connected to ground or controlled by “minus” from the ECU).
- ⚡ Contact 86 - most often it is a positive control input (comes from a button, ignition switch or controller).
- 🔄 Diode protection - if the relay is equipped with a diode, the current through the winding flows in only one direction, extinguishing the reverse pulse.
When diagnosing faults, winding resistance between pins 85 and 86 is often checked. It should be between 50 and 120 ohms for 12-volt systems. If the multimeter shows infinity (open) or zero (short circuit), the relay must be replaced.
Correspondence table of contacts and their functions
For quick orientation in diagrams and during installation, it is convenient to use a pivot table. It will help you match the numbers with their physical meaning and typical wire colors, although colors may vary depending on the vehicle manufacturer.
| DIN designation | Function | Chain type | State without power |
|---|---|---|---|
| 30 | Power (Input) | Power (+) | Constant Potential |
| 87 | Consumer (Output) | Power (+) | Open (No current) |
| 87a | Alternative exit | Power (+) | Closed from 30 |
| 85 | Control (Coil) | Low current (-/+) | No current |
| 86 | Control (Coil) | Low current (-/+) | No current |
Using this table, you can easily read any electrical circuit diagram of a car. Please note that in some rare cases (for example, in turn relays or specific comfort units), the numbering may be supplemented with letters or other numbers, but the basic logic 30-87-85-86 is almost always preserved.
What to do if there is no number 87a on the new relay?
If the 5-pin relay is not marked 87a, but has 5 pins, use a multimeter in continuity mode. Find a contact that calls 30 at rest - this will be your 87a.
Practical testing and diagnostics with a multimeter
Knowledge of notations is useless without the ability to apply them in practice. To check the health of the relay, you will need a regular digital multimeter. The first step is to visualize the pins on the body and find the corresponding sockets in the block or on the element itself, if it is transparent or marked.
The testing process begins with measuring the winding resistance. Switch the multimeter to resistance measurement mode (Ohms) and touch the probes to the contacts 85 and 86. The device should show a value in the range of 50–120 Ohms. If the resistance is too high or equal to infinity, then the winding is burned out or broken.
Next we check the power group. At rest (without power supply to 85/86) between the contacts 30 and 87 there must be infinity (gap). Between 30 and 87a (if it exists) - zero Ohm (short circuit). After applying 12 volts to the coil (you can use wires from the battery), a click will indicate operation, and the multimeter will show a short between 30 and 87.
☑️ Relay testing algorithm
⚠️ Attention: When applying 12 Volts to the control contacts to test the relay, keep your fingers away from the power terminals if you are applying voltage directly from the battery. Sparking at the moment of closing can cause burns.
A common mistake is to check the relay for spark without load. Although this is acceptable for high-power relays, for micro relays it is better to use a light bulb as a load to avoid damaging the delicate contacts inside. It is also important to check not only the presence of a short circuit, but also the quality of the contact - the resistance in a closed circuit should not exceed 0.5 Ohms.
Features of markings in different cars
Despite the DIN standard, automakers often make their own adjustments, especially for the German concern VAG (Volkswagen, Audi) and French brands. On Volkswagen relays you can find four-digit numbers, for example, 103 instead of 30, or 104 instead of 87. This is the internal coding of the factory, but the functionality remains the same.
In cars BMW and Mercedes sometimes alphanumeric markings are used on the printed circuit board itself, inside the relay or on the cover. For example, “B+” instead of 30 or “D+” instead of 87. In such cases, it is extremely useful to have the electrical diagram of a specific model on hand, since the arrangement of the legs may be mirrored relative to the standard.
Japanese manufacturers such as Toyota and Nissan, usually strictly follow the DIN standard, but can change the colors of the housings depending on the purpose (black for standard ones, blue or green for special ones). Also, on Japanese relays, a miniature circuit is often applied directly to the side face, which greatly simplifies the diagnostician’s life.
If you are changing the relay on a right-hand drive (JDM) vehicle, carefully check the diagram on the housing. In rare cases, the order of the legs in the last may differ from European counterparts, even if the numbers are the same.
Frequent errors during installation and operation
One of the most common mistakes is installing a relay with a lower current rating. If a standard relay is rated at 30 Amps, and you install an analogue at 20 Amps, the contacts will quickly burn and weld. Always pay attention to the current markings stamped on the housing (e.g. 30A 12VDC).
The second mistake is ignoring the condition of the pad. Often, when replacing a relay, it turns out that the contacts in the mating part (chip) are melted or oxidized. Installing a new relay in such a block will lead to its rapid failure. It is necessary to clean the contacts or change the entire block.
The third problem is incorrect installation orientation. Since the pins are arranged symmetrically in some types of relays (for example, 4-pin squares), there is a risk of turning the relay 90 or 180 degrees if you do not look at the numbers. This is guaranteed to lead to a short circuit in the on-board network.
Can I use a 5-pin relay instead of a 4-pin?
Yes, you can, if you don't need the 5th pin (87a). In this case, it simply remains free (not connected). The main thing is that the coil voltage and the permissible current coincide, and the mounting dimensions allow the relay to be installed in the block.
Why does the relay get very hot during operation?
It is normal for the relay housing to heat up to 50-60 degrees. If it is so hot that the plastic melts, it means that either the load current is exceeded, or there is poor contact in the block (high contact resistance), or the relay is faulty (short circuit in the winding).
What do the “NO” and “NC” markings on imported relays mean?
These are English abbreviations: NO (Normally Open) - corresponds to contact 87 (normally open), and NC (Normally Closed) - contact 87a (normally closed). The common pin is designated COM (Common), which is the same as pin 30.
How to extend the service life of relays in a circuit of powerful consumers?
For high-power loads (for example, additional lights or compressors), it is recommended to use relays with gold-plated contacts or install two relays in parallel to share the current. It is also important to ensure a tight fit in the block to prevent heat from poor contact.