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All About Relays

If you have the German Relay Number, starting with either 4RD or 4RA, please click here for a cross reference list.

Please Click Here to choose Relays by Type and Voltage.

Hella manufactures many relays and not all versions are available individually; some can only be purchased in bulk.   In addition, not all relays Hella makes are available to the after-market.

What is a relay and why use one?

A Relay is a remote-control switch. An electro-magnete operates the contacts through which the main current  flows. With the use of a relay, the circuit carrying power to a device can be as short and direct as possible. This reduces the losses through the stock wiring due to their circuitous path.

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The importance of optimum operating current and the negative effect voltage drop can have, on the luminous intensity of a bulb, can be seen from the table above.

Not Good

Current must travel from the battery to the headlamps  via the dashboard switch.                                              

Result: a long run of wire with a voltage drop causing a reduction in headlamp performance and possible overloading of the switch.

The Best Solution

The  current is routed over the shortest distance between the battery and the headlamps via the relay. A control very small control wire is required between the dashboard switch and the relay.

What sort of relays are there?

SPST Normally-Open
SPST   Normally-Closed
SPDT Change-Over
SPST Relay normally open
SPST Relay normally Closed
SPDT Relay

Almost all SPST Relays are this type.    A normally-open relay completes an electrical circuit when current is passed through it's coil.

Normally open relays are required for all types of vehicle lighting, horns, fanfares, heating circuits, fan motors and air-conditioning equipment.

A   normally-closed relay opens an electrical circuit, i.e. it interrupts   the flow of current.

This type of relay is used, for example, to automatically disconnect headlights and radio when the engine is being started since for these few seconds the starter motor needs all the power it can get from the battery.  

Hella does not offer this type of Relay.   An SPDT Relay can be used to perform this function by using the 87a as the 87 pin and not using the 87 pin at all.

A change-over relay switches current from one user to another.

Change-over relays are used for switching from forward to backward movement of the sliding roof, from opening to closing electrically operated windows and for two-stage users such as fans and some rear window heating systems.

Eliminating voltage spikes: Relays with a Resistor or a Diode.

Voltage spikes from 300V to 500V can occur momentarily when a relay is switched.   Sensitive electronic equipment can be damaged or malfunctions can occur if these spikes reach the vehicle electrical network without suppression.   A relay with a resistor reduces these voltage spikes to less than 100 Volts.   A relay with a diode eliminates them completely.    The correct polarity of the connections is marked on the diode protected relay.

Dual 87 Relays

This relay has two High Current output terminals  so you don't have to splice the wires from two lamps together before connecting to the relay (Configuration B2 below).          

Integral Fuse

Some Relays have a Fuse built into them so you don't have to have a separate fuse holder.

The most common pin layouts are shown below

Relay Diagram A
Relay Diagram B
Relay Diagram B1
Relay Diagram B2
Relay Diagram B3
Relay Diagram C 280 QC
Relay Diagram D HL87416
Relay Diagram L Dual Relay HL87121
C 280QC
D Micro Relay
L Dual Relay