Please someone explain the headlight relay mod

[69C-owner]

I wanna know the mechanics of this, how does it change the operation of your lights, how do the low and high beams both burn brighter and also what's the big deal with relays, I've always assumed a 4 blade relay is a 4 blade relay and same go's for the 5 blade ones. I have three relays mounted on one bracket that came off of a 87 S10 Blazer, those should work just fine for this mod right and as far as fuses for both the high and low beams two of those inline fuse holders is sufficient right?
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[VWBeamer]

You just need a 4 blade relay. The five blade relay has power on the fifth leg when power to the relay is off.

The lights are brighter because your are eliminating all the resistance of the stock light switch and associated wiring.

It also makes your stock light switch last longer, because the current it's conducting is a lot less. This benefit may be subjective, but non the less is a benefit.

[davepl]

Normally the power from your battery has to run from the battery junction block to the headlight switch, out the headlight switch and into the dimmer switch, and from the dimmer switch out to the headlight harness and into the headlights.

That's a long run of wire, and they can only use so large of a wire. DC current at low voltages doesn't travel very well to begin with (witness how big your starter cable is vs the power cord of a table saw, which is similarly powered). So, by the time it gets to your headlights what started out as maybe 13.7 volts is now 12.0 volts. That's still enough to power your lights, of course, but not as brightly as if they were connected right to the battery with a short run of cable.

The goal of a relay mod is to get as much of that available voltage (in our example, 13.7) to the lights as possible. It does this by placing a mechanical switch, the relay, right up close to the battery and lights. The 12.0 volts that make it back out of the switch, that used to power the headlights, now do nothing but activate the relay. So, instead of a run of 13 feet (or however long it is from the battery to the cab switch back to the headlights) you now only have a couple of feet of cable, and its usually heavier cable to boot.

Put another way, what used to power the lights now just turns on the relay. The headlight lamps themselves are now directly connected to the battery with just a short run of cable with the relay in between. So they get much more voltage, perhaps 13.5v of that 13.7v instead of a mere 12v.

The relay is just a mechanical switch; you send it power and a little magnet inside closes contacts.

When you have high beams you need two relays. One is for the two low beam headlights and the other is for the high beams. It varies if you have a two lamp (Chev) or four lamp (GMC) setup, but the idea is the same.

FWIW, I purchased a chinese-made "Heavy Duty Headlight" harness from one of the major vendors and it was wired wrong - I had to disassemble it and switch the wires around. Thankfully they are least used green and brown wires so I could match them up to the factory circuits. Ideally you won't have to cross that hurdle. On my other car I made it myself with Radio Shack relays, but if you can get a pre-built harness it's simpler.

Hope this helps. On modern cars MANY circuits are set up like this. The fusebox in my Range Rover has about 20 relays, for everything from fan to fuel pump to even the horn. It's a better setup that not only allows more power throughput, it allows them to save money by using lighter gauge wire throughout the switch side of the circuit that just triggers the relay closer to the actual load.

[Shyguy]

Here is a picture.

[69C-owner]

Shyguy I've seen a wiring diagram very similar if not the same as that, is that specifically for gmc's? I ask that cuz it looks like four lights, the one I remember seeing was two lights like a Chevy which is what I need to study
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[463]

Originally Posted by 69C-owner

Shyguy I've seen a wiring diagram very similar if not the same as that, is that specifically for gmc's? I ask that cuz it looks like four lights, the one I remember seeing was two lights like a Chevy which is what I need to study
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It's the same idea. chev has a two lamp system so in the diagram just remove the two high beam bulbs and you have it.

[69C-owner]

With a few minutes of study I figured it out but thank you 463, I appreciate the clarity
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[69C-owner]

I understand that you can tie into the positive battery cable to begin this mod as far as building the "harness" but is there a spot somewhere on the drivers side to do this? My overall concern is that when I go out in my backyard with relays fuses wire and wire stripped that I don't mess up and only wire one side, I wanna make sure I get it wired correctly for both sides and keep it lookin somewhat presentable
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[463]

then the nexd mod or upgrade is a 70 amp internal regulated alternator from a mid 80's chev car or truck. http://www.6066gmcguy.org/charging.htm

[463]

http://www.6066gmcguy.org/HeadLights.htm this link will help

[OldCreek]

Essentially, it gives your headlights better feed direct from the battery and uses the relay as the on/off switch...as opposed to feeding through the wiring harness/headlight switch.

Therefore, brighter lights and less chance for shortage.