High and Low Headlight tandem

03GMCinBC · February 9, 2004

hey gang

A question? I want to wire my 03 Sierra to have all four lights on at the same time. a post on the AV website suggests this mod that is inexpensive and does the job. has anybody tried it or have an opinion either way? $2 bucks and change vs. $50 seems a big difference to acheive the same effect I have seen on other discussions about this. Thanks in Advance

one wire mod for 03

lrymal · February 9, 2004

Go here:

http://www.gm-trucks.com/forums/index.php?...topic=26665&hl=

03GMCinBC · February 9, 2004

Thanks alot. A question, I like the idea with the fogs and DRL's too. would the diode be the same and applied in the same (blak band) direction as the one in your pic?

regards

varmint2506 · February 9, 2004

If you have some 14ga wire lying around all you need is this relay from Radio Shack and a few connectors and about 15 minutes and your done. This relay has two outputs so you can run your fog lights too.

relay = $5.99

connectors = $1.69

in line fuse = $2.49

lrymal · February 10, 2004

Thanks alot. A question, I like the idea with the fogs and DRL's too. would the diode be the same and applied in the same (blak band) direction as the one in your pic?

That single diode can feed several things. Anything you want. You don't need to bring in a second diode. Just tap onto the side opposite of the diode's band mark.

If you want to bring in a relay, that's fine also, but it is kind of redundant for this particular project mainly due to the very low current. All you are doing is energizing the relay electro-magnet coil for each relay that you energize. Not much wattage required.

One idea that I'm contemplating is having my amber DRLs and my fog lamps turning on when I select the fog lamp button. We have a LOT of fog here and with the DRLs being very bright, yet being low, and with the fog lamps beaming downwards, I would think this modification would help me in the fog. Just have not gotten "tuit", yet.

I'd like to repeat what others have said when making electrical modifications to a $30,000 truck. DON'T do this unless you know how to work with electricity and with electrical components. You can really create a world of hurt if you aren't observant with what you are doing. Don't assume anything, don't take short-cuts. Don't let any exposed leads or wiring touch metal. This fuse box is one of the main nerve centers for voltage routing in this truck. Grab a high school kid who loves doing this sort of thing as a hobby and is good at it, if you are unsure.

Ok, there...........

durado · February 10, 2004

For the second diode, it should be oriented the same way. Make sure it goes from the low beam relay to the DLR relay - NOT from the HI beam relay to the DLR relay.

lrymal · February 10, 2004

For the second diode, it should be oriented the same way. Make sure it goes from the low beam relay to the DLR relay - NOT from the HI beam relay to the DLR relay.

Adding a second diode really is redundant: "That single diode can feed several things. Anything you want. You don't need to bring in a second diode. Just tap onto the side opposite of the diode's band mark." A better description would be that you do indeed tap onto the low-beam side of the diode as durado correctly pointed out.

Maybe this will help. Hope I'm not messing up. Be aware that each DEVICE you add to the diode adds up the required wattage that the diode can support. The relays in the fuse box require very low wattage so you should be safe if you use at least an 1/8 watt to 1/4 watt. If you add a super big hunker device that requires a bertha relay then yeah, you might need to throw in a much larger diode.

Firetiger(MN) · February 11, 2004

I did it this way using the wires on the BCM so I didn't have to disassemble the underhood electrical center:

Keep in mind this is on a 2003 GMC - other years may be different, even 2003 chevrolet may be differant although I would think it would be the same. For all lights on with high beams:

The bcm is under the steering column, I had to remove the dash panel to get to it. There are 3 larger 24 or so pin connectors on it. The one I am referring to is the 24 pin on the left side.

This connector has 2 rows of 12 pins, one above the other. There are pin numbers associated with them but they were hard to read so I'll explain it like this:

Top row: 1 2 3 4 5 6 7 8 9 10 11 12 -we'll call T

Bottom row: 1 2 3 4 5 6 7 8 9 10 11 12 -we'll call B

T1 is black-white and is High beam control- tap into this and run to a spst (2 terminal) switch, if you are going to use a switch to disable this mod. On the other side of the switch solder 3 diodes with the band ends towards the switch. I used 1n4005 diodes. Now for each of the other ends of the diodes you will solder a short length of wire on and tap into these leads as follows:

T4- is green black and is DRL control

B4- is pink and is low beam control

B5- green white and is fog lamp control

When you're done make sure to insulate each of these ends of the diodes and wires from each other and everything else. Start the truck,turn the headlights on if it's not dark, and with you're new switch off everything should work as before, or as normal. Turn the switch on and with you're high low beam lever in the low position you're low beams and fog lamps should work as normal. Flip to high beam and everything should come on. All we're really doing here is operating the relays under the hood that turn these lamps on normally. There should be no problem with power as they are still getting power the factory way. My only concern is the grounds on the headlamps, I'm hoping they don't run too warm running both lamps at once, and I don't know if both lamps are grounded together or if they have individual grounds. I may have to upgrade the grounds if they give trouble. Hope this is somewhat understandable!

Further note- if you are testing with the ignition switch off the DRLs will not come on at all, that's why I said start you're truck in the testing. If you don't want to install a switch to bypass this mod just connect the band ends of the diodes directly to the T1 black white wire.

stlmaj · February 16, 2004

Firetiger (MN) Awesome information I was getting ready to do the diode mod under the hood and came in to check on the forum to find what pins on relays to tap in to when I ran across your post. Needless to say it changed my direction and I did your mod instead. I like it much better as a matter a fact it was actually easier than finding the wires under the fuse box, under the hood. You gave perfect instructions!! Saved me 50 bucks form the relay mod (prepackaged one anyways). This mod cost me all of $1 for the diode cheapest mod I've done yet!!!!!! And it works great!!!

Thanks

Matt

zshumo · February 16, 2004

Anybody know the wiring for an 2001 sierra? my brother would also like to do this to his headlights. Do I have to wire them from the fuse box under the hood?

Firetiger(MN) · February 16, 2004

Glad to hear it worked out for you.

For pre-'03 trucks I don't know what wires on the BCM control the headlights. I did the procedure under the hood for my '02 using this : Guide

If someone has a set of service manuals it would be very simple to figure out without having to probe around to find the wires needed.

03GMCinBC · February 22, 2004

My only concern is the grounds on the headlamps, I'm hoping they don't run too warm running both lamps at once, and I don't know if both lamps are grounded together or if they have individual grounds. I may have to upgrade the grounds if they give trouble

Firetiger(MN)

Did you ever develop any problems with the grounds you mentioned? I am fixing to do this mod in a couple of days and was courious if you needed to do any adjustments.

thanks

Firetiger(MN) · February 22, 2004

I haven't yet, but the way traffic is around here I don't run on highbeam for any long periods of time either.

iron_786 · May 11, 2004

can this single diode mod be done on the 2002's all the posts ive seen on it says 2003 models, im not sure if the wiring is different or not, anyone with a 2002 try it out?

lrymal · May 11, 2004

can this single diode mod be done on the 2002's all the posts ive seen on it says 2003 models, im not sure if the wiring is different or not, anyone with a 2002 try it out?

Possibly not. As far as I can tell, the didoe modification is for the '03 vehicles and upwards.