I recently installed some Spyder brand Projector HID Headlights in my 1998 GMC K1500 SLE that I bought from Napa Auto Parts, and figured out a new way to run the Daytime Running Lights (DRL's), and I figure since it worked so well for me, that I should share my results, and how I did the modifications to the truck.

For many years I have been dissatisfied with my trucks factory lighting, as I am sure many others have been as well. I always struggled with feeling that they where just too dim. I had tried putting some higher end bulbs in last year, but had minimal improvement, and I could argue that it may have even been worse. I do a fair deal of driving at night, and in combination with also driving into other peoples bright headlights, I just couldn't see, so it was time for a change. I didn't dare put LED bulbs in my headlights because I worried that they would be too bright, and blind other drivers, so I began to research options that replaced the entire original headlight. I decided to go with a set of HID projector units, which as many of y'all may know, are supposed to be nice and bright, without blinding other drivers. And based off of the driving I have done at night so far, it has made a obvious difference, and is certainly a worth while change.

And for those of you just looking for brighter headlights, the install is supper easy, 5 screws, 2 marker lights, and 3 clips and you can have the whole grill off of your truck in a matter of minutes, and then just 4 10 mm bolts holding each headlight on, and you can change the headlights in your truck, the new ones bolt right up where the originals did, and fit pretty good. Then all you have to do is change over the high beam bulbs from your old headlight, and plug in the low beams, and begin reassembly. And if that is all you want, that is all you need to do, but I wanted to make a functionality change that I quite enjoy.

If you look at them, you will notice that the new headlights don't just have a low beam and a high beam bulb, their is a 3rd one in between them, and it is just their to be a sort of Daytime Running Light I believe, or maybe it is supposed to be a marker light. Stock, the truck does not have anywhere to wire up this bulb too, so if you would like to utilize it, you have to do some sort of wiring yourself. I decided that I wanted to wire up these bulbs to my DRL circuit. The problem with this though, is the DRL's on these particular trucks, are just the normal headlight bulbs. So, whenever the ignition is in the on position, and the parking brake is off, your headlights are on. To me this is greatly annoying because their are sometimes you want your headlights off, and personally, I don't like using my parking brake to do that. So I broke out my Haynes manual, and flipped to the back of the book and found the wiring diagram for the headlight circuit. And my goal was, as I read the diagram, to find a way so that whenever the truck was running, the new DRL's in the new headlights, would function just as the originals had, except, they would now use a different bulb, the new 3rd one in the fixture, instead of the main headlights, and that the low beam and high beam lights, would work completely independently of the DRL's, and would only come on when I used the switch in the cab. And also, I wanted to make sure that nothing else would be effected, and I believe I figured it all out.

Hopefully you can zoom in on this picture, but I redrew what the new wiring would look like.
I know this is not a full picture of the wiring diagram, but what is shown is the Daytime Running Lights (DRL) Relay (Top) The Dimmer Switch (Lower Left) and the Headlights (Lower Right).


This is the best picture I could get of the wiring diagram for the headlight system.


The DRL Relay is located underneath the left side of the drivers kick panel, not too far up from the floor, here is a picture of what it looks like.

Note: This picture was taken with the fuse panel partially removed from the truck. It is a two part panel, and by removing two, very hard to access screws under the hood, you can pull the front portion off of the firewall, their are two clips inside the truck you have to fiddle with next to the screw holes to remove it. Don't pay attention to the small black box hanging to the left, that is something I had installed aftermarket that has nothing to do with the headlights.

Here is the DRL relay on the above panel.

This is the DRL Relay, to do this modification, you will need access to the wires going into the back of this relay.

If you look back at the wiring diagram, you will notice that it list a Tan wire, and a Purple wire. The Purple wire, in the factory configuration is the power wire for the low beam bulbs that runs through the dimmer switch, and the Tan wire, runs to the low beam plug on the front of the vehicle.

Here are these two wires cut, the Tan and the Purple. DO NOT CONNECT THE ONES COMEING OUT OF THE RELAY IN THIS PICTURE. You want to connect the ones going up behind the dash to complete the circuit. Connecting this half of these wires will do nothing. But one of these pictured wires coming out of the relay will be used, but in a latter step.

What I did was connect the Purple and Tan wire together, not the one in the picture above, the ones that go up into the main harness, which you can see in the next picture...

Here is the Purple and the Tan wire connected (Top of Photo), in this configuration, these wires are in no way connected to the DRL relay anymore. I just want to be clear which half of the newly severed wires get connected to what. Note, you can see a Purple wire and a Tan wire in the bottom of the photo, these ARE coming out of the DRL relay, and you will see what happens with those wires next.

By connecting the Purple and Tan wires in this configuration, what happens is the DRL relay is taken out of play in the normal operation of the headlights, and will no longer turn on the headlights as the DRL's. The truck will now act as if it has normal headlights. To still have daytime running lights though, we can take the Tan wire that comes out of the back of the relay, and run a power wire to wherever we please, in this case, the new 3rd bulb in the new headlight assembly.

New connection between Tan wire, and new power wire that will go the the new DRL's. This is the tan wire that is coming out of the relay.

In this configuration, when the DRL's are commanded (by the Daytime Running Lamps Module, found on the full wiring diagram, lower left of page), the Tan wire, and hence this red wire connected to it, will/can provide power to the new DRL's. They will be getting their power from the always hot Yellow wire that feeds the DRL relay.
From here all you need to do is run this power wire to your new DRL's, and it will power them, here is how I ran it.

Here is that same Red wire, from inside the truck, above and to the left I believe their is a cable that goes through the firewall, you can pop it out and run the wire next to it. Also pictured, on the left, you can see a little bit of the back side of the fuse panel where the wiring was done inside the truck, the screws are in the area around the white block.

This is a picture of the back side of the drivers side headlight, my tools are sitting in the battery tray. Once you have your wire into the engine compartment, it is very easy to find the group of wires that run up to the headlights, and you can run your power wire in this loom.

This is that same Red wire, we are currently looking through where the drivers side blinker would normally be (I took these pictures once I had the grill back on, but it is easier to do with the grill off, which you will already have done after installing the new headlights.

One of these white wires goes directly to the drivers side DRL, and the other just connects to another length of Red wire that runs in front of the radiator over to the passenger side DRL. The only reason that it is not red in the picture is because I bought 16 gauge wire to do this job, and it would not fit in the connector with the white wire going to the light. Most of the factory wiring for the headlights is 20 gauge, so that is why their is funny business going on with the colors in this picture, but just imagine that one of those white wires is actually red.

And here is our power wire from the drivers side, coming over from the passenger side, and you can see the connection to the white wire that comes from the DRL in the new headlight. By the way, I extremely highly recommend using these heat shrink butt connectors in exterior automotive applications, because they not only help prevent corrosion, but they also act as a little bit of a strain relief.

Now, we cannot forget about the new ground we need for the DRL's, and I just used the ground wire from the low beam bulbs.

All I did, was cut the ground wire going to the factory low beam bulb connector, and spliced the DRL ground into it. And for those of you who are very observant, do you see the Tan wire by my fingertip going into the connector for the low beam bulb?

Here is the same thing, but on the passenger side.

And that is about it, all you have to do is put the fuse block back on, and reinstall the grill, and now your...


DRL's that look like this

Markers look like this

Low beams look like this

High beams look like this (although their is nothing special about these, they should be about the same as factory.

And for anyone doing this job, I would recommend getting the 20 gauge wire, instead of the 16 that I got (I guessed the wrong size!) and I would get both 16 and 20 gauge connectors, and maybe think of something better that what I did where I had to put 3 wires into one butt connector.

And my review of the headlights over all so far, B+. The beam itself is like a total A+, it is not only brighter out in front of you, it also throws more lights off to the side so you can see the sidewalk and the people who are walking on it, and see if they are going to cross the road. But where I was slightly disappointed was on the adjustability of the headlights, they don't sit 100% level in the hole in the grill (although, it is barley noticeable, as you can see in the pictures), and left to right adjustment is really hard, and doesn't do much, but I can't be too bent out of shape because I was still able to get them adjusted to where I feel comfortable/safe.

So hopefully this helps someone out if they want a solution to not being able to see in their GMT400. Feel free to ask me if their is anything y'all got questions about.

 

Over on the GMT400.com site there's been quite a bit of discussion lately (2019 and later) about the composite headlights that are made in Brazil and were used on GM Brazil's GMT400 platform in South America. They are bona fide E-code headlamps and have the illumination pattern of an "export" headlight (think "RPO T84"). See attached pictures of the L&R reflectors. One can sort-of see from the faceting in the lens that the low-beam pattern will have the classic E-code "lift" on the RH side of the road.




The OE manufacturer still makes the headlights (as of Apr2022 at least) and some members of the forum (GMT400) have managed to make them available to other members (and presumably anybody else who might be interested).

There's some information here https://www.gmt400.com/threads/oem-c...-export.57899/ That page also has links to some relevant threads on GMT400.

Even greater illumination has been tried by adding relays (like in the previous post) to leave the low-beams on all the time and switch only the high beams. See here: https://www.gmt400.com/threads/arteb...6/post-1239633

 

Very interesting. Thanks for sharing.

 

No problem!

I was reading through your post. Did you remove the two diodes marked 203 from the DRL power circuit (see your wiring schematic... orange wire from fused +12V, yellow wire to the DRL relay)? The diode's only function is to add some voltage drop to the DLRs (resulting in ~1.5 - 2.0V less applied voltage to the DRLs). Considering what you're trying to do, I suggest you remove the diodes / jumper around them since, it appears to me, you'll pulling power from that circuit and probably want full voltage applied to those extra bulbs.

I just want to make sure you considered this change because I saw no mention of it in your original post.

 

Interesting you mention that, no I never did. I was told that in the original configuration, that when the headlight bulbs where in the “DRL Mode” they where supposed to be dimmer than when being used as headlights. Before I did the work I tried to see if the brightness changed between headlights on and off, but no noticeable change, so to be honest, I never looked for any diodes or resistors in the wireing diagram when I did my work. Just made sure I wasn’t going to fry anything! My new DRL bulbs are still plenty bright though, so I don’t think it would make a difference in my application 🤷‍♂️