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Relaying Your Headlamps Simplified.

Brighter stock headlights.

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Upgrading your stock headlamps to brighter headlights

We are not responsible for damages caused while following the directions in these pages. These pages are for informational use only.
Terms: AWG = gauge in this writeup

US headlights on our cars have extremely low wattage somewhere around 35-50 watts. Euro headlights with H4 bulbs start out at 55 watts and can go above 100 watts per bulb. Our stock wiring cannot handle this extra wattage. The way the headlight system works in our cars is for the power to go from the Battery to the fuse panel, then up to the headlight switch, then all the way back out to the headlights. And then is grounded. This is called a grounded system, meaning that at a point (or various points) the system is grounded to the frame or chassis by a wire.

A relayed system (the one being proposed) acts as a remote controlled switch system. Instead of running the 16-20 foot inside inferior grade wiring which is around 18 gauge, the power now runs from the battery to the relay to the headlight in a distance of 5.5 ft on wire that is better suited to handle the load without reduction of amperage. The relay is told to turn on by the stock wiring. A relay uses far less current than the headlights do, taking a load off of your electrical and charging system, making your switch, as well as alternator last longer.

Before you begin

The information offered in this writeup should be used for information purposes only, you should only attempt this modification if you have experience with wiring and electricity. You need/must understand the hazards involved with modifying an electrical system, you could seriously mess something up if you connect something wrong. Continue at your own risk.

Items you will need.

14 AWG primary stranded wire in 3 colors - about 6 feet of each (try to avoid trailer wire) it is quite cheap and will do the job, but it is cheap for a reason, trailers are often wired with this and it is subject to splits and the casing tends to deteriorate in high temps and is found in 3packs of white, brown and green sometimes with yellow in place of the brown or green. (I won't be using it for this writeup)

18 - 20 awg primary stranded wire in two colors - about 2 feet of each (this is used as signal wire from your stock switch

two Bosch-type relays (these have 5 prongs, later I will describe what and how to hook them up)

A weather-tight enclosure for the relays, But I use heatshrink.
Quick Disconnect 1/4 inch terminals for the relays in Yellow, Blue, and Red sizes (You need a total of 8)
3-4 large loop crimp terminal ends

Headlight connectors that match your bulbs (This is tricky, because autozone sells both the H4 and the 9004 in both regular (which has 16 awg wire) and High output (which uses 14 awg wire) IIRC they are around 5-7 bucks

Fuse holders with the correct fuses for your wire (For this write up I will be using 30 amp fuses)

(styling) A loom of some style to hold the wires together and to protect them (premade casing, can be found in a variety of colors, for this example I will just be using heatshrink)

Tools you will need:

Use the proper tool for the job, you don't want to go at this with a steak knife and a tack hammer.

Wire Cutter, Stripper and Crimper
Wrench for your battery terminals to install power and ground wires
Heat gun if you use heat-shrink tubing

The first thing is to plan your system. You need to figure out how many amps your headlight system is going to draw, you will need this equation to do it: Watts / Volts = Amps (Math, as promised)

So, if I had two 60 watt bulbs running at 12v in my car, that would mean that I have 120 watts/12 volts, which makes 10 amps. (you want to take the higher end rating of the bulb, so if you have a 35/60 bulb, 60 is the higher allowance, so this is the number you want to use.
Lemme simplify it a little more
2 60 watt bulbs is 2 x 60 = 120 watts
now divide 120 watts by your voltage, which is 12
120 divided by 12 = 10 amps

So I can easily power the bulbs with only one relay. This would be no biggie for it. But what if I had a REALLY high powered system? You can get 90/100W h4 bulbs or even 9004 bulbs. That's 20 amps of power running through a relay which still isn't a lot. You could do this with one 30 amp relay, I would stress a 2 relay setup, so that if one relay fails (like the low beam relay) you can easily swap the connection feed on the relays to allow the low beams to still work. Its like having a spare relay. And if the entire system fails on you, you can always still run it back to the original connections (which I stress that you keep them) Just in case. Not to mention, having the 30-40 amp relays mean that you could run aftermarket driving and/or fog lights as well all you would need to do is add a little extra wire and a switch.

It is important to use the correct type of wire for your harness. Do not use THHN or solid core wires for your harness. These are inflexible and will break in an automotive environment due to general vibration. Use Automotive Primary wire (called GPT) which is commonly available in auto parts stores.

It is important to select the correct wire gauge for your harness. Gauge means the size of the copper conductor in the wire, and a smaller number means a larger wire. It is useless to 'overkill' here and use a larger wire than necessary. It will just be harder to build, more costly and less flexible.

Use this chart as a guide(this is where the math we did earlier helps out a lot in determining):

16 gauge: 22 amps
14 gauge: 32 amps
12 gauge: 41 amps
10 gauge: 55 amps
8 gauge: 73 amps

Wire Colors

Our cars use red/yellow and red/green for low-beams, red/white and red/blue for high beams, and black for the common/grounds. You can use any colors you want on your harness, but be sure not to use the same color for all of them (just because you got a really good deal on 100ft spool of wire doesn't mean your cost went down, you will pay for it later if you need to troubleshoot problems.

A relay is a remote switch. Terminals 85 and 86 are the trigger to that switch. (I would use one for high and one for low, just in the event that I needed to troubleshoot later.)The polarity of 85 and 86 do not matter.
Terminal 30 always goes to the battery via a fuse. The load (your headlights) go on terminal 87. That's where the power goes out when the relay is triggered. (87a is not used in these, as this terminal is on when the lights are not on(no, no power drain is happening at this time))

The fuse is very important, and makes sure you don't set your new harness on fire if there's a problem. Choose a fuse based on the capacity of the wire, not the load. A fuse just makes sure that the amount of current does not exceed the ratings of that wire. Bulbs can use more current as they age, so don't fuse based on the wattage. Use the Wire gauge guide above, and pick the closest fuse below that max current rating.

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Special thanks to Timob & the guys at vintagewatercooleds.org for permission to use their diagrams. I should have put this up 4 years ago and forgot about it.
Honda, because it's hard to look baller in your mom's Volvo.Posted Image 
The entire tool selection at Sears or the contents of a Snap-On truck will do you no good if you do not know how to use them.