Automotive Relays

The vehicle electrical system supplies all components of a vehicle with voltage supplied by the car battery or alternator. Electrical or electronic components are located between the power source and the respective consumer, which switch the desired component on or off

This is done either manually by the car user via switch, but can also be linked to pre-defined conditions, such as outdoor temperature or rain. This also includes the detection of a passenger seat occupied in order to trigger (or not) the airbag in the event of a fault.

Although many innovative components in the car have a high degree of efficiency with a significantly reduced power consumption, as is the case with LED lights, a lot of consumers with high power consumption must be supplied. In addition to the starter, these include glow plugs for diesel engines, window heaters, hi-fi systems, seat heaters and heated exterior mirrors. So-called flashing relays are installed for the flashing, also for trailer operation. When using several on-board car batteries, isolating relays ensure their coordinated operation.

Due to the relatively low on-board voltage of usually 12 volts, high currents occur in the motor vehicle. This requires cables with larger cross-sections in order to supply the corresponding consumers as low-loss as possible. In order to keep the length of cables with a large cross-section from the power supply of the car to the consumer as low as possible, the consumers are controlled with relays – i.e. without a spatial "detour" via the switch. These remote switches are particularly robust in order to meet the special operating conditions in vehicles

High-power relays are designed for the 12 V/DC on-board power supply in the car

Open automotive relay with visible coil through which control current flows

In terms of design and function, automotive relays do not differ from "normal" relays: A coil fed through from the activated control current magnetizes the coil core . The in turn attracts a magnetically effective metal plate, which closes, opens or changes a contact via a mechanism.

These are normally monostable relays whose solenoid coil must be permanently powered to maintain contact status. This creates functional reliability with vibrations occurring in the car. Bistable relays require only one switching pulse to maintain the captured state, which is provided by a permanent magnet. Modern automotive relays of this type are now in no way inferior to the monostable relays and relieve the on-board power supply of a car

The relay contacts are designed for use in vehicles thanks to highly refined surfaces so that they can easily switch high currents without wear-out due to contact erosion. High-current automotive relays can handle up to 500 ampere switch-on current.

Relays for vehicles are very compact in design and are encapsulated in such a way that they are adequately protected against vibrations and environmental influences such as heat, moisture or aggressive media such as oils or fuels. Accordingly, the working temperature range is also large. Glass-fiber reinforced polyamides are often used for the relay housing, so that manufacturers can secure working temperatures from approx. -40 to 85 °C.

For a secure installation, many car relays are simply plugged into suitable relay sockets. They are protected against accidental release by retaining lugs or spring force. Thanks to this base mounting, it is hardly possible to insert defective relays incorrectly.

The electrical connections are often designed differently to prevent incorrect connection of the control and power circuit. In addition, the wiring and a numbering of the connections are usually printed on the housing according to a key common in the automotive industry.

30 A relay with integrated fuse holder and special mounting

Our practical tip: Mount automotive relays

When attaching relays in the car with plastic or metal clips, always use new (disposable) clips. When using metal drive screws, ensure that the screws are screwed in place to maintain intact rust protection. This can be of interest, for example, when retrofitting high-performance hi-fi components.

A separate voltage source with the on-board voltage should be used to test a removed relay. 12 volt relays can be tested with a 9 volt block. A multimeter should be used to test the function of switch contacts of a relay.

Plugged relays can be very tight due to contamination or wear on the plug contacts. Therefore, care should be taken when changing. Use of force can destroy the assembly unit or relay base , which leads to a greater repair effort.

Connecting or disconnecting plug connections under load causes wear of the contacts, increased contact resistances and can also lead to the destruction of devices. Therefore, the vehicle battery should always be disconnected before working on the car's on-board power supply.

Make sure that any codes required to restart car radios are available.

Car relay with 6.3 mm flat plug for easy installation