What is 4 pin Relay ?
Many times modern vehicles compulsory used equipment is 4 pin relay. It is basically a electro magnetic switch which control the current flowing through two circuits. It consists of 4 terminals . 2 coil terminals and 2 switch terminals . When there is a small current flows through coil become energetic and when the current flows through large current the switch get activated. Because of its multifunctional its main used are as below.
Head light :- this will protect the high current flowing through the circuit .
Horn :- it will efficiently provide efficient sound taking current from directly from the battery.
Cooling fan :- engine coolant system , by relays cooling fans get activated. being overheated is stopped .
Starting motor :- by relay switches starting motor draws current and ease starting of the motor.
How to wire relays?
Step 1: -Identify the terminals:
The relay has four terminals labeled as follows:
Terminal 30: Common contact (C1)
Terminal 85: Coil (-)
Terminal 86: Coil (+)
Terminal 87: Normally open contact (N/O)
Step 2: Connect the power source:
Connect the positive terminal of the power source (usually the vehicle’s battery) to terminal 86 and the negative terminal to terminal 85.
Step 3: Connect the load:
Connect the load (e.g., headlights, horn, cooling fans) to terminal 87.
Step 4: Ground the relay:
Establish a ground connection between the relay’s metal case and the vehicle’s chassis to ensure proper operation.
Step 5: Test the circuit:
Before finalizing the installation, test the relay and circuit to verify functionality and address any potential issues.
Relay terminology
The ISO mini relay we have looked at above has 4 pins (or terminals) on the body and is referred to as a make & break relay because there is one high current circuit and a contact that is either open or closed depending upon whether the relay is at rest or energized.
If the contact is broken with the relay at rest then the relay is referred to as Normally Open (NO) and if the contact is closed with the relay at rest then the relay is referred to as Normally Closed (NC). Normally Open relays are the more common type.
ISO mini relays with two circuits, one of which is closed when the relay is at rest and the other which is closed when the relay is energized, have 5 pins on the body and are referred to as changeover relays. These have two contacts connected to a common terminal.
Make & break relays are also known as Single Pole Single Throw (SPST) and changeover relays as Single Pole Double Throw (SPDT). This is based on standard switch terminology. There are other contact configurations discussed below but make & break and changeover relays are the most commonly used.
5 pin relay
Terminal/Pin number
85 Coil
86 Coil
87 Normally Open (NO)
87a Normally Closed (NC) – not present on 4 pin relays
30 Common connection to NO & NC terminals
Relay body markings
Relays can look very similar from the outside so they normally have the circuit schematic, voltage rating, current rating and terminal numbers marked on the body to identify them.
Circuit schematic
This shows the basic internal circuits (including any diodes, resistors etc.) and terminal layout to assist wiring.
Voltage rating
The operating voltage of the coil and high current circuits. Typically 12V for passenger vehicles and small craft but also available in 6V for older vehicles and 24V for commercial applications (both auto and marine).
Current rating
This is the current carrying capacity of the high current circuit(s) and is normally between 25A and 40A, however it is sometimes shown as a dual rating on changeover relays e.g. 30/40A. In the case of dual ratings the normally closed circuit is the lower of the two, so 30A/40A, NC/NO for the example given. The current draw of the coil is not normally shown but is typically 150-200 mA with a corresponding coil resistance of around 80-60 W.
Knowing the coil resistance is useful when testing the relay for a fault with a multi-meter. A very high resistance or open circuit reading can indicate a damaged coil.
Terminal numbering
The numbers 85, 86, 30, 87 & 87a (or other numbers for different relay configurations) are normally moulded into the plastic next to each pin and also shown on the circuit schematic.
Micro relays
ISO micro relays are, as the name suggests, smaller than ISO mini relays and designed for use in applications where space is at a premium. They are rectangular in section and narrower than a mini relay with a slightly different pin layout, and are typically available in ‘make and break’ and ‘changeover’ configurations, with and without suppression diodes.
In addition, the terminal numbering used is sometimes 1, 2, 3, 4 & 5 instead of 30, 85, 86, 87a & 87.
Terminal/Pin number and size
1 :- (86) – 4.8mm Coil
2 :-(85) – 4.8mm Coil
3 :-(30) – 6.3mm Common connection to NO & NC terminals
4 :-(87a) – 4.8mm Normally Closed (NC) – not present on 4 pin relays
5 :-(87) – 6.3mm Normally Open (NO)
More complex relay types
There are other relay designs that are used for some more complex applications in vehicle systems. They are still based upon the principle of switching higher current circuits using smaller current circuits but often combine this with electronics to perform special functions: Some examples are:
Glow plug relays – provide power to the glow plugs in a diesel engine for a set amount of time using an ignition switch position or other input to energise the relay.
Fuel injection relays – provide power to the electrically activated fuel injectors in a petrol engine for varying amounts of time based on signals from the vehicle Engine Control Unit (ECU).
Timer relays – for example in a circuit for a heated rear window, where the relay needs to be energised for a few minutes before turning off.
Flasher relays/units – used for operating indicators and hazard warning lights and employ electronics to control the timing of the contact opening and closing rather than a traditional bi-metallic strip.
These more complex relays can have up to 9 pins of various sizes. This increase in the number of terminals over the standard 4 or 5 in more simple relays is often necessary because additional connections can be required for the in-built electronics (e.g. inputs from sensors or the ECU and outputs to indicator lights or the ECU).
