• General
  • Relay connections
  • Switching with a relay (consumer switched off)
  • Circuits with a relay (consumer switched on)
  • Contact making relay
  • Contact breaker relay
  • Changeover relay

The relay is widely used in automotive electronics. Without using relays, wiring harnesses in the car would be at least 3 to 6 times as thick. A relay's job is to turn a small control current into a large main current. For example, a large electronic component can be switched on and off with a small switch. The electronic components that are switched by relays include the interior and exterior lighting, the electronic fuel pump, the engine cooling fan and the windshield wiper system. The current flowing through, for example, the wiper motor is fairly high. Thick wires and a well-insulated (ie large) switch are needed here. This is difficult, however, because the space at the steering column is too small to feed thick wires. The relay offers the solution for this: Very thin wires run to the wiper switch (for the small control current) and thick wires go to the wiper motor (for the large main current). The relay is mounted between the small and the large wires.

There is often one across the relay coil resistance connected in parallel. This resistor absorbs the peak voltages during the switching off of the relay. In this way, the peak voltages that arise when switching on and off cannot influence the operation of the electronic switch (this is often a control device).

There are different types of relays; the make relay, the break relay and the changeover relay. These are described in the chapter “Different types of relays”.

Relay connections:
A relay has four connection points as standard, namely:

  • Control current input (86)
  • Control current output (85)
  • Main power input (30)
  • Main current output (87)

These entrances and exits always have these fixed numbers. These numbers can also be found on the page terminal codings.
These numbers are also displayed on the relay itself at the contacts (see image).

Circuit with a relay (consumer switched off):
The schematic shows a contact make relay in a circuit. This type of schedule is called a waterfall schedule. The positive battery cable is shown at the top and the ground at the bottom. The relay is controlled by the ECU. The ECU is an electronic switch. In fact, the ECU could also be replaced by a manual switch. As soon as the ECU switches through, terminal 85 of the relay is grounded. At that moment, the control current circuit is closed. In that case, the leads from the plus of the battery, via the terminals 86 and 85 through the electromagnet, to the ground of the battery.
Because a control current now flows, the electromagnet is made magnetic. The mechanical switch between terminal 30 and terminal 87 is closed by means of the magnetism. At that moment, a main current starts to flow. This main flow goes to the consumer. In this diagram the consumer is a lamp, but all consumers or. are actuators that require a high current to operate. Think of electric motors such as the window motor, the horn, the rear window heating, etc.

So when the relay is switched off, no control and main current flow. The on-board voltage is measured at three of the four connections.

  • Terminals 30 and 86 are connected to the plus of the battery in this diagram. This is the board voltage. This will vary between 12 and 14,4 volts, depending on the state of charge of the battery and the engine running;
  • Terminal 85 also has a voltage of 12 volts (measured in relation to ground). There is no current flowing yet, so the electromagnet is not yet energized and the voltage is not used to make the electromagnet magnetic. The voltage of 12 volts thus arrives at the ECU;
  • Terminal 87 shows 0 volts, because the consumer is not yet supplied with voltage and current.
Terminal 30:12 v
Terminal 85:12 v
Terminal 86:12 v
Terminal 87:0 v

Circuits with a relay (consumer switched on):
The relay is operated in the diagram below. The control current is indicated in red. This runs through the ECU to ground. The main current (indicated in blue) flows through the lamp to ground via terminals 30 and 87. The light is on.
The moment the ECU switches through, the voltage values ​​on the contacts of the relay change. These can be read in the table below.

  • Both terminals 30 and 86 do not change anything compared to the relay when it is switched off, because they are connected directly to the battery;
  • The voltage on terminal 85 is now 0 volts, while it was still 12 volts when the relay is switched off. At this point, the 12 volts is used by the electromagnet to become magnetic. The electromagnet is now also a consumer;
  • There is now a voltage of 85 volts on terminal 0, i.e. on the output of the control current;
  • Because the relay has now switched a main current, there is now a voltage of 87 volts on terminal 12. The consumer, in this case the lamp, is now supplied with a voltage and current.
Terminal 30:12 v
Terminal 85:0 v
Terminal 86:12 v
Terminal 87:12 v


In the diagram below, the relay (component code A1) is used to supply voltage and current to the fuel pump (A2). Terminal 86 is connected to terminal 25 of the control unit. It reads 12 volts when the relay is on. Terminal 85 of the relay is connected to terminal 26 of the control unit. Terminal 25 is connected to ground to switch on the relay.
The main current flows through fuse F8 to terminals 30 and 87 of the relay. The main current flows from terminal 87 to the fuel pump (component code A2). The fuel pump will start running.

On the page parasitic drain describes a fault caused by the relay. Due to the fault, the consumer, in this case the fuel pump, remains switched on. 

Contact making relay:
This type of relay is switched off at rest. The previous section was about this type of relay.

Contact breaker relay:
With this type of relay, the consumer is always controlled. In this case, this is again the lamp. There is always a connection between terminal 30 and 87a. Only when switch “S” is actuated, the coil is energized and the connection between 30 and 87a is broken. The consumer is therefore switched off when the switch is actuated. As soon as the switch is released again, the relay will return to the rest position. Terminal 87a will be energized again, so that the consumer is switched on again.

Changeover relay:
With the changeover relay, two consumers are switched on and off separately from each other. In the rest position (as shown in the figure), the lamp L2 is switched on. The moment switch “S” is energized, the coil in the relay is energized and a connection to terminal 87 will be made. The connection to terminal 87a is then broken. As soon as the switch S is released again, the terminal 87a will be energized again. With this type of relay, one lamp is always controlled; either L1, or L2.

Locations of relays:
Relays are often mounted in one place in the car. This can be in the fuse box (as in the picture below) or on a separate relay board. Relays may also be mounted under the hood, such as the engine cooling fan relay. The relay positions can be found in the owner's manual and/or workshop documentation of the car.

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