Headlight Relay Harness Upgrade

Written by: Rob Zulian
Additional Information: Russell Ziegler
Additional Information: Marcus, aka Boxrodz

If you have ever driven a 73-87 GM truck at night you know one thing, the stock lighting leaves something to be desired.  Dull, dim yellow light just doesn’t cut it in today’s world with the latest advancements in lighting.  Halogen bulbs that were the best you could get 10 or 20 years ago, but they cannot match the latest stuff coming out of Detroit on the newer cars and trucks.  To improve your trucks lighting, options include upgrading to Silverstar sealed beams, or the latest trend would be replacing the sealed beams with an H4 conversion bulb.  Both of the options will increase the light output, but not as much as possible due to the limits in the stock GM headlight wiring.  

The stock headlight circuit is very inefficient for delivering voltage to the headlights.  Power flows from the battery, through the firewall junction block, to the headlight switch, then to the hi/lo switch and then back out to the lights.  Add up the total resistance of this circuit and you end up with less than the available system voltage.  In my case on my 75 Blazer, the available voltage at the lights was over 1 volt less than the available system voltage measured at the alternator.  This is why most lights on these trucks are dim, add any corrosion or other wiring issues and yours could be less. 

Think about this.  Assuming that 12.6 volts where you receive 100% of your light output, look at this quick comparison chart showing light output percentage as compared to the voltage the lights are receiving:

12.6 V   =    100%
11.5 V   =    75%
10.3 V   =    50%
8.7 V     =    25%

According to the chart, you’d only be getting approx 50% of your possible light output at 10.3 Volts!  

To correct this problem and provide more voltage to the headlamps, a simple change can be made to the wiring.  By adding relays to the system you can use the stock lighting circuits (hi/lo) to remotely turn on relays that are connected to direct battery power for the simplest circuit possible.  And, by using a larger wire gauge (14 AWG or greater) than that which is used in the stock wiring (18 AWG/16 AWG), the voltage loss at the headlamps may be further reduced.  Follow along and I’ll show you how I did this on my ’75 K5 Blazer. 

LMC Truck now sells a kit to do this, but when I wanted to upgrade mine it was not out yet.   

Keep in mind that my setup is for a ’75 model year and should work for all ’73 to ’80 single headlamp setups.  It will work for single headlamp setups from ’81 and up also, but I have not verified the wiring colors on these trucks.  Dual headlamp (4 total) operate similar to the single headlamp setup with 2 lamps lighting up under the low beam mode while all 4 lamps
lighting up under the high beam mode.

Items you will need:
2 30 amp relays
2 30 amp inline fuse assemblies
2 rolls of 14 GA wire (2 different colors)
1 package of shrink tubing (for 14 GA wire)
1 package of female spade connectors (I got the weather proof kind with heat shrinkable ends) get at least enough for 4 connections at each relay!
1 package of ground eyelets

Tools you will need:
Wire cutters/strippers
Soldering iron and solder
Heat gun or lighter for shrink tubing/spade connectors


Single Headlight Schematic (click image for larger view)

Dual Headlight Schematic (click image for larger view)

Here's what I did.  I planned on putting the relays directly above and behind the d/s headlamp on the backside of the radiator support.  I did this for two reasons, ease of access (not hidden by the battery) and it’s close to where the headlight wiring comes from the fender/cab.  Knowing where I wanted to put the relays, I could set the length of wire I needed to power up each relay as well as the ground wires for the relays.  I decided to pull the power from the junction block on the firewall (just above and to the left of the brake booster), as I hate having a billion wires tapped off of the battery.  I routed the wires in the same split loom that the stock headlight wiring ran in on the inside of the d/s fender.  This way it's all hidden except where I put the inline fuses nearest the junction block. To add the inline fuse holder to the circuit I spliced the wires, soldered the connection and covered the connection with the shrink tubing.  I could then hook up the power wires to the 30 terminal on the relays (keep the fuses out so you don't have a hot circuit to play with).  I then hooked up the ground wires I made up with the female spade on one end and the ring terminal on the other to the relays and ground on the radiator support.

Now we need to get the switched on signal to the relays and output to the lights.  I cut the brown and green wires that led to the d/s headlamp (don't make the cut in the wires to the p/s bulb, see the important note below). Each wire is still used; just the relays will be in between the where the circuit was cut.

****VERY IMPORTANT:  The stock headlight wiring is set up in a parallel circuit.  If you notice at the d/s headlamp the lamp connector will have two of each wire (brown and green) going to it.  If you peel back the conduit you will see that one set of the brown and green wires actually goes to the p/s headlamp.  This is a clever little wiring trick the GM engineers did to eliminate having 4 wires from the dimmer switch to the lights.  Still before and after this mod, if the d/s headlamp bulb fails the p/s light will stay lit. (Not like a cheap set of Christmas  tree lights!)

With the brown and green wires cut, add the female spade connecters on both sides of the cut, on the switch side and the side going to the d/s light.  Depending on where you made the cut, you might have to add a few inches to each side of the brown and green wires to allow enough slack in the wires for easy connection to the relays (try and use the same colors if you can to limit confusion later!).  Following the schematic, you then hook up the brown wire from the switch side to terminal 86 on one relay and then the other brown wire to 87 terminal of the same relay.  Do the same for the green wires to the same terminals on the other relay and you are done with the wiring.  Secure the relays to the radiator support, making sure you got all connections tight and recheck your connections to the relays.  You should have both brown wires to one and green to the other. If you have a brown and a green wire to each relay, your dimmer will work backwards.  Stuff the wires back into conduit/split loom, put the fuses back in the inline holders and hit the switch.  LET THERE BE LIGHT!

One thing I would like to change on mine after seeing the LMC harness is the connections to the relays.  Having 1 connector with 4 terminals to the relay would look a lot cleaner than 4 individual spade terminals.  Plus you would be less likely to reconnect it wrong if you ever had to replace a burnt out relay.

You can see the one major difference in my setup to the LMC kit is that you cut the wires to the d/s headlamp and use the existing headlamp wiring/sockets whereas the LMC kit does not have any cutting and supplies new wiring/sockets for the headlamps. This is your call if you want to cut your stock harness. The LMC kit is quickly reversible, take the stock headlamp socket from the LMC kit and plug it back to your headlamp to go back to stock.  This is a bonus if you are out in the boonies and a relay quits.  My setup can be changed back to stock, but you have to splice the two wires back together.  I'll carry an extra relay with me if one was to fail.

One other thing, since my setup uses the stock wiring at the bulbs, you might be inclined to ask why with the smaller stock wiring?  I used mine, because the stock wiring was free of any corrosion (benefits of Colorado not using salt in the winter) and the resistance was just as low as the same length of 14 gauge wires.  That and the fact that the wiring for the p/s lamp was tucked well under the top of the radiator support and I was too lazy to fish it out.  Basically, this is a judgment call for you to make.  If your stock wiring is thrashed and corroded, rewire the lights with fresh wire and new connectors.  Rewire it just like it was factory so you don't have two wires crammed into one terminal at the relay.  

Overall, the modification is fairly simple to do and is inexpensive as well.  You will be amazed at the increase in light output…even if you don’t upgrade to H4 bulbs.   If you do plan on adding H4 conversion bulbs it is a worthwhile upgrade to get all the output possible from your new bulbs.

USE THIS AS A GUIDE.  Your wiring colors may vary by year of truck.  Use the GM service manuals for the stock schematics on the later trucks (Chilton’s or Haynes are not perfect in this area).  If you don't have access to a GM manual, break out the test light and verify the color/purpose of each wire yourself.