GMC Headlight Relay Mod

[mrlobster]

I'm installing the headlight relay mod on my '72 GMC. When I look at the main light harness, it has 2 light green wires leading to the high beams. Does it matter which one is connected to the relay? Should both?
It's a new harness from AAW.
Thanks!

[davepl]

Is it wrapped in a yellow sheath? There was a bad batch out there that were, and the harness was miswired. Those came from LMC, but they could be the same as the AAW.

For a dual headlight system you need two relays, right? One triggered by the light green, one triggered by the brown. Or so I remember.

[mrlobster]

No yellow sheath. There's one green wire leading from the dimmer switch, but two wires from the fuse box to the headights. The wires come together at the pig tail.

[davepl]

Are you sure it's intended for the GMC? Or are you just going to do low beams? Because to do both you need both circuits and two relays.

No reason you can't just do low beams, of course.

[mrlobster]

Yes, I have two relays and was intending to do both circuits. The issue is that I have two light green wires going to the headlights along with the tan wire. I need to know if it makes a difference which green one I tap into.

[VetteVet]

Here's the circuit diagram to install the GMC relays.

You'll have a brown wire and a green wire coming from the dimmer to the relays to terminal 86 on each relay and there will be three wires for the dim lights and four green wires for the brights.




It should be simple to wire in the relays. Just cut the brown and green wires from the dimmer before they get to the headlights and wire them to the 86 terminals on the respective relays, then wire the other side of the cut wires to the
87 terminals on the respective relays. That should complete the circuits to the four headlights. Then You just have to run the power wires to the no. 30 terminal on the relays, fused as shown, then connect the grounds to the 85 terminals.

[mrlobster]

Thanks for all who replied. The tan wire is not a problem. The issues is two light green wires that run in the light harness to the headlights and terminate in the same terminal at the pigtail. I want to know if it matters which green one I use for the relay, or do I have to splice them together going into and coming out of the relay.
I am also waiting on a reply from AAW.
Thanks again!

[davepl]

Is green high beam? Then it is fed to both headlights, as what we'd normally call the low beam is powered differently (3 lugs on lamp) depending on whether the green or brown wire is active.

So maybe that's why you have two wires, one per headlight?

[54blackhornet]

Is the relay/harness you bought from AAW all one harness designed for a four headlight system ? or two two headlight harness just being used together ? I am installing Hella H4 / H1 headlights also and having trouble locating a harness to do four lights.....thanks best upgrade you can do on any vehicle

[mrlobster]

I bought the harness direct from AAW. It's designed for the 4 headlight GMC truck. I emailed their tech dept. and they suggested that I test the wire to see which one carries the current from the high beam switch and tap that one for the relay.
Sounds reasonable to me. I'll work on it and update the post.

[gmachinz]

Originally Posted by 54blackhornet

Is the relay/harness you bought from AAW all one harness designed for a four headlight system ? or two two headlight harness just being used together ? I am installing Hella H4 / H1 headlights also and having trouble locating a harness to do four lights.....thanks best upgrade you can do on any vehicle

If you still need a harness, PM me for details-I can supply you with one....I typically do 1973-up but I'm able to build 67-72 any way you need.

[54blackhornet]

Pm going out

[54blackhornet]

Why did this get moved ? Where did it go ?

[mrlobster]

I tested the green wires leading from the firewall and both carry the high beam current when the high beams are turned on. So I'm just going to run a relay for each of them rather than tie the wires in together.
The low beam relay went in without a hitch.
I've replaced the under dash, engine and headlight harnesses with new ones from AAW and they've been awesome. Perfect fit and not a single issue.

[67 Burb]

My headlight harness plugs into one of the original headlight connectors to fire the relays. Very simple and no way to mix things up.

[mrlobster]

I just finished putting a relay on each of the green wires- works perfectly. Probably overkill but I won't have any smoking wires either.