Automotive Relay: Diagrams, animations, and more teaching resources
Relays are a fundamental topic in automotive training, yet they can be difficult for students to visualise at first. This illustrated article is a great resource if you are looking for a clear way to explain:
- What a relay is
- How a relay works
- The types of relays
- How to read relay diagrams
- Automotive relays
With simple explanations, clear automotive relay diagrams, and engaging animations, this article helps teachers present a complex yet essential topic in a way that is easy and relevant for automotive students.
Table of Contents
What is a relay?
A relay is an electrically operated switch. It consists of a coil with a soft iron core. When a current flows through the coil, a magnetic field is produced. This current is called the control current (primary circuit).
This magnetic field exerts a force on the lever. A switch is attached to the lever. The force of the magnetic field moves the lever and closes the switch for the main current (secondary circuit).
Relay operation
A relay can switch a large (main) current using a small (control) current.
A small switch can switch the control current without burning out. The relay contact switches the current to the load and can tolerate this.
A standard relay has four connection points.
- A main current input
- A main current output
- A control current input
- A control current output
Types of relays
Relays can be grouped into three types. Relays with a make, a break and a changeover contact.
Make relays: this relay type makes (i.e. connects) the circuit when it is energised.
Break relays: this relay type breaks the circuit when it is energised.
Changeover relays: this relay type makes a circuit when it is energised and makes another circuit when it is de-energised.
A changeover relay can be recognised by the extra main current connection or by the symbol on the housing.
With a changeover relay, a contact is always connected.
Automotive relays
You come across several relays in automotive engineering. They are also used to switch components such as:
- Lighting
- Fuel pump
- Glow plugs
- Blower
- Electric radiator fan
- Fuel pump
- Lambda sensor
- Horn
These relays can be controlled via the engine management or body control systems.
Indicators on a vehicle are switched by a specific relay that switches the turn signal lights on and off at a regular interval, creating the blinking effect.
Bosch Terminal Designations
One of the biggest challenges for students when learning automotive relays is remembering the Bosch terminal designations:
• 30 = battery positive supply
• 85 = ground (coil negative)
• 86 = positive supply for the coil
• 87 = switched output (connected to 30 when the relay is energised)
• 87a = normally closed output (connected to 30 when the relay is not energised)
With traditional teaching methods, students may be able to memorise the theory, but they often struggle when they are handed an actual relay. They frequently forget which pin is connected to the battery supply, ground, the relay coil, or the switched output.
With Electude’s interactive modules and Connect trainers, students don’t just memorise relay diagrams; they actively work with them, identify terminal functions, and apply their knowledge in practical scenarios. As a result, they develop a much deeper understanding and are more likely to remember the Bosch terminal designations when working with real components.
Engine Management Relays
Three relays (K1, K2 and K3) are used for this engine management (100).
These relays ensure that high current consumers are not directly connected to the positive. This way they can be switched on when needed.
In this article, we will discuss only the main relay: K2.
Relay K2
This double make relay is the main relay of the engine management (100).
- The relay is switched on via connection 8 of the engine management.
- Via connections 6 and 7, it can be checked whether both contacts are switched.
- When this relay is switched on, two switching contacts change over.
- Both contacts are connected to the positive of the battery pole (A).
- The second contact (connection 3) is the power supply for the canister purge solenoid valve (2), the EGR valve (5), the four injectors (1) and the throttle valve control unit (10).
Please note: the ignition coils (11) are not switched via this relay.
Questions for students
1. What parts are in a relay?
See answer
Coil, soft iron core, lever, and selector switch.
2. What is the function of a relay?
See answer
A relay can switch a large (main) current with a small (control) current.
3. What are the three types of relays?
See answer
Make relays; break relays; changeover relays.
4. Check the image “Wiring diagram for the engine management system”. What is the main relay?
See answer
K2.
5. Check the same diagram. Which actuators are switched via connection 3 of relay K2?
See answer
Canister purge solenoid valve (2), the EGR valve (5), the four injectors (1) and the throttle valve control unit (10).
Understanding relay operation is an essential step in building students’ confidence with automotive electrical systems. If you found this introduction useful, you may also be interested in exploring more advanced teaching resources.
Electude offers:
- Interactive lessons that help students understand what a relay is, how it works, and why it matters
- E-books with clear explanations and relay diagrams
- Quizzes and tests to support formative and summative assessments of relay knowledge
- How-To videos with step-by-step guidance on checking relays and repairing wiring
- The Electude Engine Management Simulator, with ready-to-use tasks that enable students to work on relay problems in a realistic simulated environment
- Training aids that provide a hands-on learning experience
Electude bridges the gap between understanding a relay diagram on paper and confidently wiring a relay in the workshop.
Get a free trial to access our digital learning resources and help your automotive students build a stronger understanding of relays.
