In modern automotive electrical applications, relays play the role of discreet but critical gatekeepers, controlling high currents with weak signals. Understanding exactly how the contacts on the relay are indicated, is a basic skill for any car enthusiast who wants to independently diagnose faults or install additional equipment. An incorrect connection can lead to a short circuit, ECU failure, or even a wiring fire, so labeling standards cannot be ignored.
There are several marking systems that often confuse novice craftsmen: these are the classic DIN numbering and the letter designations adopted in some old schemes. ISO 12V - This is the standard that you will encounter in 99% of cases when working on passenger cars of recent decades. Knowing these codes allows you to quickly identify the control and power circuits without resorting to testing with a multimeter each time.
In this article, we will analyze each pin in detail, explain the logic behind their numbering, and provide proven algorithms for checking the functionality of the component. You will learn to read circuits of any complexity and understand why such numbers were chosen to designate certain circuits.
Labeling standards and numbering systems
The basis for understanding the electrical circuits of cars is the standard DIN 72552, which regulates the assignment of numbers to the terminals of electrical components. It is this standard that dictates that control contacts (winding) and power contacts (switching contacts) must have strictly defined numbers. This is done for unification: relays from Bosch, Hella or Denso will have the same pinout, even if their cases are painted in different colors.
In old cars or specific equipment you can find letter markings where Latin letters are used instead of numbers. For example, the winding could be designated as Coil, and the power circuit is like Main. However, the modern automotive industry is dominated by a digital system, where each number has its own physical meaning and location on the body.
⚠️ Attention: Never rely on wire color alone to make a diagnosis. The previous owner or unqualified service may have changed the color scheme of the wiring, leaving the numbers on the relay terminals as the only true guide.
There are two main types of relays based on the number of pins: four-pin and five-pin. Four-pin circuits usually implement simple opening or closing of a circuit, while five-pin circuits allow you to switch current between two different consumers. Understanding the difference between normally open and normally closed state is critical for proper installation of an alarm or additional light.
When purchasing a new relay, always check not only the number of contacts, but also their location on the body, since the geometric pinout pattern may differ slightly from different manufacturers, despite the same numbering.
Decoding of control terminals (Winding)
The control circuit is the "brains" of the relay that receives a weak signal from a button, sensor or control unit and causes the mechanism to switch. In the DIN standard these pins are designated by numbers 85 and 86. It is between them that there is an electromagnetic coil that creates a magnetic field when voltage is applied.
For most DC relays used in automobiles, the polarity of the winding connection does not matter. You can apply “plus” to 85, and “minus” to 86, or vice versa - the magnetic field will arise in any case, and the armature will be attracted. However, in modern circuits with diode protection or built-in resistors, polarity becomes critical.
- 🔌 85 - usually connected to the negative pole (ground) or to the minus control signal.
- ⚡ 86 — most often a positive voltage (+12V) is supplied from the control device.
- 🔄 In circuits with a diode inside the relay, current flows only in one direction, so if you mix up 85 and 86, you will not activate the relay.
If you are installing a relay for high-power accessories, it is important to understand that the current in the control circuit is extremely small (usually less than 0.2 Amps). This allows you to use thin wires and weak buttons without worrying about them overheating. The main load falls on the power contacts, which will be discussed below.
Power contacts and their purpose
The power part of the relay is responsible for switching the high current required to operate the starter, headlights, fans or fuel pump. Conclusions are used here 30 and 87 (and also 87a in five-pin versions). These contacts are made of more refractory materials, often coated with silver, to withstand sparking when opened.
Contact 30 is the central power input. The wire should always come to it directly from the battery through the fuse. It is a source of energy for the consumer. Unlike control contacts, here the current flows only in one direction: from 30 to 87.
Contact 87 - this is an exit to the consumer. When the relay is turned on, pin 30 connects to 87 and current flows to the device (for example, a headlight bulb). If the relay is off, circuit 30-87 is open and the load is de-energized. This is the basic diagram of the operation of any normally open relay.
Five-pin relays have an additional output 87a. This is a normally closed contact. When the relay is turned off, pin 30 is connected specifically to 87a. As soon as the control signal is applied, connection 30-87a is broken and 30-87 is closed. This allows you to implement switching logic, for example, between high and low beam or between standard and additional audio systems.
Why do the contacts get hot?
Pins 30 and 87 may become hot if connected poorly or oxidized. If you smell burning plastic from the relay, immediately check the tightness of the terminals and the condition of the wires, as this is a direct cause of fires in the engine compartment.
Designation table
Use a pivot table to quickly access information. It will help you instantly determine the function of each terminal when working with a multimeter or when assembling a new electrical circuit.
| Contact number | Contact type | Function | State without power |
|---|---|---|---|
| 30 | Power input | Powered by battery | Always energized |
| 85 | Manager | Minus winding / Signal | No tension |
| 86 | Manager | Plus windings / Signal | No tension |
| 87 | Power output | Normally open | Open (no connection with 30) |
| 87a | Power output | Normally closed | Closed (connected to 30) |
Remember that having a contact 87a immediately indicates that there is a switching relay in front of you. In simple circuits such as turning on a radiator fan, this pin is often omitted and the four-pin version is used. Using a five-pin relay instead of a four-pin one is acceptable if you simply leave pin 87a out, but not vice versa.
The main rule of installation: pin 30 must always be connected to the power source via a fuse, regardless of the type of load installed.
Algorithm for testing and diagnosing relays
Diagnostics begins with a visual inspection and checking the integrity of the winding. To do this, switch the multimeter to resistance measurement mode (Ohm) and connect the probes to pins 85 and 86. A working winding will show a resistance in the range from 50 to 120 Ohms. If the device shows infinity (open) or zero (short circuit), the relay must be replaced.
The next stage is checking the power section in statics. Without power supply to the winding, ring contacts 30 and 87. The device should indicate an open circuit. If you have a five-pin relay, then between 30 and 87a there should be a “beep” or a reading close to zero, indicating a closed contact.
The final test is carried out with power applied. Apply 12 Volts to pins 85 and 86. You should hear a clear click. At this moment, the resistance between 30 and 87 should become zero (short circuit), and between 30 and 87a (if any) - infinite (opening).
☑️ Relay testing checklist
A common problem is burning of the contacts inside the relay. Even if there is a click and the winding is working, contacts 30 and 87 may have high contact resistance due to carbon deposits. This causes the device to work, but the contacts get hot. You can check this by measuring the voltage between pins 30 and 87 with the relay turned on: it should not exceed 0.5 Volts.
Frequent connection errors
One of the most common mistakes is the incorrect determination of “plus” and “minus” in the control circuit in the presence of a diode. If you confuse 85 and 86 in such a relay, it simply will not work. Always pay attention to the diode diagram on the relay body, if present.
Another mistake is using a relay with insufficient current rating. If you install a 30 Amp relay to connect a winch that draws 50 Amp, the contacts will quickly weld or burn out. Always choose a relay with a current reserve of at least 20-30% of the consumed load current.
⚠️ Attention: Never install the relay upside down or in a position where water can flow into the gaps between the cover and base. Moisture inside the case will cause contact corrosion and system failure.
Also, beginners often ignore the need to install a fuse in the circuit of pin 30. The lack of protection leads to the fact that in the event of a short circuit in the wiring, it is not the fuse that burns out, but the car wiring itself melts or the battery fails. The protection should be as close as possible to the power source.
FAQ: Frequently asked questions
Is it possible to replace a 4-pin relay with a 5-pin one?
Yes, it's possible. The fifth pin (87a) in a 5-pin relay will simply remain unused. The main thing is that the current rating and voltage of the winding match, and the seats on the block allow the installation of a relay.
Why does the relay click but the consumer does not work?
Most likely, the power contacts (30 and 87) inside the relay are burnt, or there is no power at input 30. Also check the integrity of the fuse in the consumer circuit.
What does the 12VDC marking on the relay body mean?
This is an indication of the winding supply voltage. 12V means 12 Volts, and DC (Direct Current) means direct current. Applying 24V or alternating current (AC) will burn out the winding.
How to determine where the relay is plus and where the minus is on the winding?
If there is no diode circuit on the case, polarity is not important. If there is a circuit with a diode, then the plus is usually applied to the terminal to which the diode triangle (cathode) is directed, and the minus is applied to the other.