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#1 |
Registered User
Join Date: Dec 2010
Location: Apison Tn
Posts: 490
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Please someone explain the headlight relay mod
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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http://67-72chevytrucks.com/vboard/s...d.php?t=438550 ![]() ![]() 1994 Buick Roadmaster 1994 GMC Sonoma |
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#2 |
Registered User
Join Date: Jun 2011
Location: GA
Posts: 281
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Re: Please someone explain the headlight relay mod
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. |
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#3 |
Registered User
Join Date: Aug 2006
Location: Redmond, WA
Posts: 6,332
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Re: Please someone explain the headlight relay mod
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. |
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#4 |
Junior Member
Join Date: Dec 2006
Location: Southeast Missouri
Posts: 2,436
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Re: Please someone explain the headlight relay mod
Here is a picture.
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'67 Chevy C-20 short stepper - build complete, 454/SM-465. '75 C-30 Single Cab DRW-350 small block/NP-435. '77 GMC-6500 Dump Truck, 427 Tall Deck. '92 GMC K-3500 Duallie, 454/4L80E. |
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#5 |
Registered User
Join Date: Dec 2010
Location: Apison Tn
Posts: 490
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Re: Please someone explain the headlight relay mod
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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http://67-72chevytrucks.com/vboard/s...d.php?t=438550 ![]() ![]() 1994 Buick Roadmaster 1994 GMC Sonoma |
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#6 |
Registered User
![]() Join Date: Jan 2007
Location: Central Coast California
Posts: 1,026
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Re: Please someone explain the headlight relay mod
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.
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#7 |
Registered User
Join Date: Nov 2007
Location: Sask Canada
Posts: 1,666
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Re: Please someone explain the headlight relay mod
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.
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Trueman |
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#8 |
Registered User
Join Date: Dec 2010
Location: Apison Tn
Posts: 490
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Re: Please someone explain the headlight relay mod
With a few minutes of study I figured it out but thank you 463, I appreciate the clarity
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http://67-72chevytrucks.com/vboard/s...d.php?t=438550 ![]() ![]() 1994 Buick Roadmaster 1994 GMC Sonoma |
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#9 |
Registered User
Join Date: Dec 2010
Location: Apison Tn
Posts: 490
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Re: Please someone explain the headlight relay mod
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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#10 |
Registered User
Join Date: Nov 2007
Location: Sask Canada
Posts: 1,666
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Re: Please someone explain the headlight relay mod
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
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Trueman |
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#11 |
Registered User
Join Date: Nov 2007
Location: Sask Canada
Posts: 1,666
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Re: Please someone explain the headlight relay mod
http://www.6066gmcguy.org/HeadLights.htm this link will help
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Trueman |
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#12 |
Registered User
Join Date: Dec 2010
Location: Apison Tn
Posts: 490
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Re: Please someone explain the headlight relay mod
Already did that :-), I have a 140 amp alternative laying around from a 95 Buick roadmaster station wagon that I'd like to wire up, I'm not sure what amprage rating is on the current alternator, it charges but I'd like to swap the pulley onto that Buick alternator and do some wiring to have 140 amps
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#13 |
Old member
Join Date: Jan 2002
Location: Liberty, & Garden City S.C. , U.S.
Posts: 19,937
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![]() ![]() #86 will be either low beam are high beam which ever one you are working on. #87 will be going to both headlights either L/H beam. #85 is grounded to frame near relay Make it a good ground. #30 is a fused source straight off the battery are unfused on the fuse block. ![]() ![]()
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1971 LWB Custom, 6.0LS & 4L80E, Speedhut.com GPS speedometer & gauges with A/C. 20" Boss 338's Grey wheels 4 wheel disc brakes. My Driver Seeing the USA in a 71 ![]() Upstate SC GM Truck Club 2013,14 and 2016 Hot Rod Pour Tour http://upstategmtrucks.com/ Get out and drive the truck this summer and have some fun! It sucks not being able to hear! LWB trucks rule, if you don't think so measure your SWB! After talking to tech support at Air Lift I have found out that the kit I need is 60811. Per the measurements I gave them. Ride height of truck inside spring and inside diameter of springs. |
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fuse, headlight, mod, relay |
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