HomeTips & GuidesAdding a Relay to Headlights: Why is it Essential for Automotive Headlights?

Adding a Relay to Headlights: Why is it Essential for Automotive Headlights?

About Adding a Relay to Headlights The headlight switches controlled the headlights in almost all vehicles until the 90s and even after that. 

In this mechanism, a thin wire runs from the switch to the headlights. All these wires had tiny contacts to build considerable voltage drop along the path. 

As a result, headlights did not receive a complete power supply, affecting their output.

Thus, manufacturers considered giving full power from the alternator or the battery to the headlamps. 

For this, they had to shorten the length of wires between the power source and receiver and maximize the current carrying capacity by changing the wire gauge. 

However, at the same time, it was required that they could control the headlamps from a remote location. 

So, the best solution they came up with was “installing relays.”

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How do relays work with the headlight system?

A relay is an electrically operated switch that either completes or breaks a circuit, thus sending or hindering the current flow to the intended device. 

A relay is attached to the path of wires that connect the main headlight switch to the headlights. 

When you send power to this relay, it completes the circuit between the power source positive terminal and the headlights. 

Relays need only a small amount of power, and thus, you can add thin wires for their operation. You can use headlamp wires to switch ON and OFF the relays. 

As relays come with high current carrying capacity, it is possible to use heavy-gauge wiring. 

Thus, you can bring total current to the headlamps without any voltage drop with these heavy gauge wires.

For relay operation, you only need one or two watts of power. On the contrary, even low-beam headlights use a total power of around 100 watts. 

It means they need more than ten amps of current. As:

Power (in watts)= current2 (in amps) * resistance (in ohms)

P= I2*R

Thus, if the headlights have a resistance of one ohm only (due to age or underspecified contacts) and the current is 10 amps, you need 100-watt power to switch on the headlamp.

Relay load control switch example diagram

Relay load control switch example diagram

How to add a relay to your vehicle’s headlights?

In this part, we will read how to add relays to the headlights.

Safety first:

First, disconnect the battery to cut off the power supply.

Preparing a headlight relay kit:

A headlight relay kit has a 12-volt relay and a four-wire plug connector (butt and splice connectors).

As you need two relays, one for the high and the other for the low beam, you must use relay mounts as they incorporate all relay wires in one socket, resulting in neat relay banks. 

The relay installation packages come with relay mounts with a snap-lock feature. 

Locating the relay

As relays are small and compacted devices (about 1-1.5 inches), it’s easier to mount them at the desired location. 

However, as you use relays to reduce the length of the power circuit between the power source and the consumer (headlights), you must choose a suitable location at the car’s front near the battery and the headlamps, even if you need to extend a few extra inches for that.

Adding relay to the headlights:

First, locate the wires that connect the dimmer switch to the headlight with the help of a wiring diagram. 

Cut this wire so that you can butt-splice the relay wires. 

Splice the wire to the dimmer switch with the relay’s 86-wire.

The headlight wire goes to the relay’s 87 wire. 

The relay’s 30 wires receive power from either the fuse panel or the ignition switch. 

The 85 relay wire goes to the ground. 

Once you complete the wiring, reconnect the battery and check if you can operate the headlights with the switch and the dimmer.

Headlight relay wiring diagram 

Headlight relay wiring diagram 

A properly configured relay harness at Cloom:

At Cloom, you can get appropriately configured relay harnesses for installing the relays.

Protecting the wires:

Our relay harness has fuses towards the power supply side of the headlights’ power circuit. We keep the fuses close to the battery’s positive and negative terminals.

 Fuses allow a proper connection in a newly added device that was not present in the vehicle. 

Apart from the fuses, you can also use manual reset circuit breakers, but don’t go for auto-reset ones. 

This is because some loads, like power window motors, can draw a higher current than usual due to some unusual conditions, but it happens for a short time. 

For such cases, it is okay to use auto-reset circuit breakers as they will open in case of a high current, and by the time they reset, the normal current condition comes back.

However, in headlights and their circuits, the case is different. Under normal conditions, they will draw an average intended current.

 However, in short circuit conditions, a circuit breaker draws a higher current than usual. In such cases, keeping the circuit off until you fix the problem is essential. 

An auto-reset circuit breaker will continue sending power to the circuit even in high current conditions, leading to damage and danger.

 Thus, Cloom prefers manual reset circuit breakers in properly configured headlamp harnesses.

Connect the headlamp relay harness to the vehicle’s primary electrical source, battery, or alternator.

 If you do not use a fuse, any high current situation in old or new wires may lead to massive fires. 

When a short circuit happens, the alternator can draw a current up to 60 amps, and the battery can contribute a further 80-100 amps of current. 

This current can blow the fuse or fusible link. Imagine if there is no fuse; your entire wiring harness will fry in just a few seconds.

Choosing wires:

Cloom always uses stranded copper wire instead of solid wire in its automotive products, and that’s what reputed American brands, including Deka, Ancor, Whitaker, etc., use in their automotive services. 

Further, most local manufacturers prefer PVC insulation. However, we prefer cross-linked insulation (higher grade) as this is more resistant to repeated flexing, oil, heat, and other conditions that a vehicle goes through. Also, you can find them in multi-color packets.

It is much better to use marine-grade tinned wire as they are more corrosion-resistant and come in different colors.

As a result, the installation becomes easy and looks neat. If you don’t get these cables, get some wire looms for your wire protection.

It is also essential to choose the correct gauge wire for the conductor and insulation. 

A too-small wire can lead to more voltage drop, while too-large wires become stiff, resulting in connections prone to damage. 

Mostly, 14-gauge wire is suitable, but 12-gauge is much better than that.

Further, ensure that you use the right gauge wire in the ground legs and on the feed of the circuit, as inadequate grounding can also lead to voltage drop. 

For proper grounding, it would be better to run ground wires near the battery’s negative terminal or towards the alternator metal housing. 

Working with wire:

Though a good-quality crimper may cost a bit, it will also help in other projects. Apart from crimping, the Posi system is another good technique to tap and connect wires.


So, you see how important it is to add a relay to the vehicle’s headlights. 

However, for the smooth functioning of the relays, you need to get suitable relay harnesses that can fit according to your vehicle’s wiring system. 

At Cloom, you can get high-quality relay harnesses for automotive headlight relays. Contact our professionals for detailed information. 

Hi I am Christa, sales manager of Cloom.

I have extensive expertise and experience in wiring harnesses and I believe I can help you.

And we have a very professional technical team who can clearly understand the customer’s needs and give professional suggestions and solutions after receiving the drawings.

If you also have wiring harness needs, please send me the drawing so that we can give you our quote and start our business.

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