Any modern car is a complex network of electrical circuits, where relays play the role of key switches that control powerful current consumers. When the headlights, fuel pump, or cooling fan stop working, the first thing mechanics look at is the fuse and relay box. Understanding how to read relay designations, allows you to quickly diagnose a malfunction without resorting to complex manuals.
Often on the body of the element you can see strange numbers, symbols and diagrams, which to an untrained person look like a code. However, these markings are subject to international DIN and ISO standards, which makes them universal for most vehicles. Knowing these rules will help you replace a burnt-out element yourself or understand why the necessary equipment does not turn on.
In this article we will analyze all the nuances of marking, from standard five-pin models to complex multi-pole circuits. You will learn to distinguish normally closed contacts from normally open ones and understand why the polarity of the coil connection is sometimes critical.
Standard pin numbering according to DIN and ISO
The basis for reading the marking is the standard pin numbering, which is applied to the plastic body of the device. These numbers are not random; they indicate the functional purpose of each terminal in the electrical circuit. The most common type is the four- and five-pin relay, using the numbers 30, 85, 86, 87 and 87a.
The key element is pin 30, which always indicates the supply voltage input from the battery. This is a powerful output through which current flows to the consumer. The opposite side of the diagram is contact 87 - this is the output to the load, which closes with contact 30 at the moment the device is triggered. It is through this pair that the main current passes, controlling, for example, the headlights or the starter.
A separate group consists of control coil terminals, marked as 85 and 86. A control signal is supplied through them, which creates a magnetic field necessary for the mechanical closure of power contacts. Five-pin versions add a lead 87a, which is a normally closed contact that opens when voltage is applied to the coil.
When replacing a relay, always compare the diagram on the body of the new element with the diagram of the old one, even if they look the same - the location of the pins inside the case may differ from one manufacturer to another.
It is important to understand the difference between normally open and normally closed states. The normally open contact (87) does not conduct current unless a signal is applied to the coil. Normally closed (87a), on the contrary, conducts current in a quiet state and breaks the circuit when activated. This knowledge is critical when installing additional equipment such as alarms or additional lights.
Explanation of the schematic image on the body
In addition to numbers, a schematic representation of the internal structure of the device is often printed on the case. This miniature electrical diagram gives a complete picture of exactly how the pins are connected inside the black plastic cube. Reading this diagram requires an understanding of the basic graphic symbols used in electrical engineering.
The control coil in the diagram is usually depicted as a rectangle or several turns of wire. Pins 85 and 86 are indicated next to it. Sometimes a diode or resistor may be drawn next to the coil. The presence of a diode indicates that the relay has built-in reverse current protection, and when installing it polarity connecting pins 85 and 86 becomes strictly mandatory.
The power part of the circuit shows a moving armature (pins 30 and 87). If the diagram shows that contact 87a is connected to 30 in the original position, and 87 is open, then you have a classic switching relay. The lines on the diagram may be solid or dotted, sometimes indicating a mechanical or logical connection between groups of contacts in multi-terminal devices.
⚠️ Attention: If the diagram shows a diode parallel to the coil, the reversed polarity of the connection (supplying plus to minus of the coil) will lead to a short circuit and instantaneous failure of the device.
For complex multi-contact relays, the circuit may occupy a significant part of the housing. In such cases, manufacturers often put pin markings right next to the legs on the bottom of the case to make installation easier. Careful study of the diagram helps to avoid mistakes when “testing” the device with a multimeter in ohmmeter mode.
Differences between 4 and 5 pin relays
Visually, 4 and 5 pin relays often look identical, differing only in the number of metal legs on the bottom panel. However, their functional purpose is radically different, and simply replacing one type with another without modifying the wiring is impossible. Understanding these differences saves you from many repair mistakes.
A four-pin relay is a simple switch: it either closes a circuit or opens it. It has one input (30) and one output (87), and two coil leads (85, 86). Such devices are used where you simply need to turn on or off a consumer, for example, a sound signal or a stove fan.
A five-pin relay adds switching capability. It has two outputs: one normally open (87) and one normally closed (87a). The power input (30) in a quiet state is connected to 87a, and when voltage is applied to the coil it switches to 87. This allows, for example, to switch headlights from low beam to high beam or change engine operating modes.
Below is a table to help you quickly compare the characteristics of both types of devices:
| Characteristics | 4 pin relay | 5 pin relay |
|---|---|---|
| Number of pins | 4 (30, 85, 86, 87) | 5 (30, 85, 86, 87, 87a) |
| Function | Circuit closure | Switching circuits |
| Contact 87a | Missing | Normally closed |
| Application | Turning on fans and pumps | Switching headlights and operating modes |
Sometimes car enthusiasts try to replace a 4-pin relay with a 5-pin one, ignoring the extra pin 87a. This is only permissible if the circuit allows this contact to be left free and it is not used in the control circuit. Otherwise, installing a 4-pin instead of a 5-pin will lead to the inoperability of one of the system functions.
☑️ Check before installing the relay
Specifics of a relay with a diode and resistor
Modern automotive electronics are extremely sensitive to power surges and electromagnetic interference. When the inductor circuit breaks, a powerful reverse current pulse occurs, which can damage control chips or control units (ECUs). To protect against this, additional elements are built into the relay housing.
Relay with built-in diode (often indicated by a diode symbol on the diagram next to the coil) requires strict adherence to polarity. Pin 85 in such devices is usually connected to the “minus”, and 86 to the “plus” (although there are exceptions, so the diagram on the case is the law). If the contacts are reversed, the diode will open in the forward direction, creating a short circuit in the control circuit.
There are also relays with built-in resistor. A resistor performs the same noise suppression function, but does not require polarity since it does not have a current direction. Such devices are more versatile in replacement, but can create a slightly greater load on the control circuit compared to diode analogues.
⚠️ Attention: Never install a relay without a diode in a circuit where a relay with a diode was normally installed, if the control unit does not have external protection. Sparking contacts can damage expensive vehicle electronics.
How to determine the presence of a diode without a circuit? You can use the multimeter in diode testing mode. By connecting the probes to the coil terminals (85 and 86), you should see a voltage drop (about 0.6-0.7 V) in one direction and an open (or very high resistance) in the other. If the device shows resistance in both directions, you have a relay with a resistor or just a coil.
Micro and mini relay markings
With the development of automotive technology, the space under the hood and in the cabin became increasingly limited. This has led to the introduction of compact form factors known as Micro and Mini relay Despite the reduced dimensions, the principles of their marking remain similar to standard ISO relays, but have their own characteristics.
Micro relays are often used in comfort units, power windows and mirrors. Due to the small size of the case, digital markings may appear in microscopic font or be completely absent on the top cover. In such cases, you need to turn the device over and look at the base - there are often pressed contact numbers next to each leg.
Color coding also plays a role in identification. Standard relays are often black or blue, while micro relays can be green, red or clear. Color sometimes indicates the type of contacts (silver, gold) or the presence of built-in protection. However, you can't rely on color alone - always check the wiring diagram.
Features of micro-relay pinout
In micro relays, the legs can be arranged in two rows or have a non-standard pitch. When installing into adapters or homemade blocks, it is important to accurately measure the distance between the contacts, since standard sockets may not fit.
When selecting analogues for micro-relays, it is important to pay attention not only to the electrical parameters, but also to the geometry of the housing. Even if the device fits electrically, it may not physically fit into the socket or snap into the fuse block latches.
Diagnostics and performance check
Knowledge of notations is useless without the ability to apply them in practice for diagnosis. The most reliable way to test a relay is to use a multimeter. Before starting the test, you must disconnect the battery or remove the device being tested from the circuit to avoid false readings or damage to the device.
First, the integrity of the coil is checked. Switch the multimeter to resistance (ohms) measurement mode. Connect the probes to pins 85 and 86. The device should show a resistance in the range from 50 to 150 Ohms (the value depends on the type of relay). If the resistance is infinitely high, the coil is broken; if it is close to zero, there is an interturn short circuit.
Then the contact group is checked. At rest (without applying voltage to the coil), between terminals 30 and 87 the resistance should be infinite (open), and between 30 and 87a (if any) - close to zero. Next, you need to apply 12V voltage to the coil (observing polarity, if there is a diode) and ring contacts 30 and 87 again. The resistance should drop to zero, which indicates the circuit is closed.
An ideal relay, when the contacts are closed, shows a resistance close to 0 ohms. If the device shows 1-5 Ohms or more, the contacts are burnt and the relay requires replacement, even if it clicks.
Often the fault lies not in the relay itself, but in the oxidation of the contacts in the block. A visual inspection of the connectors for carbon deposits or corrosion is mandatory. Sometimes it is enough to clean the contacts for the system to work again, but if the plastic of the pad itself is burnt, it will also have to be replaced.
Frequently asked questions (FAQ)
Is it possible to replace a 4-pin relay with a 5-pin one?
Yes, this is possible, but only if the 5th pin (87a) is not used in the car circuit. In this case, the additional contact remains free. However, reverse replacement (5 to 4) is not possible if circuit 87a is used in system operation.
Why does the relay hum or crackle when operating?
A hum may indicate insufficient coil supply voltage (for example, due to oxidation of the contacts), which does not allow the armature to be fully pressed. This may also be a sign of an interturn short circuit in the coil or mechanical wear of the moving parts.
What is the maximum current that a standard automotive relay can withstand?
Most standard ISO type relays are rated at 30A or 40A continuous current. However, starting currents (for example, when cranking the starter or running high-power headlights) may be higher. For powerful consumers, special power relays rated at 50A or more are used.
What does the "12V 40A" marking on the case mean?
This marking indicates the rated voltage of the control coil (12 Volts) and the maximum switching current of the power contacts (40 Amperes). Exceeding these values will lead to overheating, welding of contacts and fire.
Is it possible to test a relay without a multimeter?
You can roughly evaluate the performance by replacing the suspect relay with a known good similar one (for example, taking a temporary relay from a non-working node, if they are the same). Also, a working relay should make a clear click when 12V is applied to the coil, but the click does not guarantee good contact.