Man this headlight system on my truck does not make sense to me. On my 2002 K3500, my headlights cancel out with voltage applied to each terminal.

I was told to add a ground wire to the yellow and I would be able to get both the hi and lo beams on.

Now this does not make sense to me. Wouldn't this cause a direct short on these really small wires? And wouldn't this cause a fire?

I have seen quite a few "all headlight" mods for headlights, but the 2002 does not have hi AND lo beam relays. It only has the HDLP relay and that will not make any difference.

I also see a lot about diodes, but no one states which direction they go and exactly where they go on the 2002 models.

Would someone explain the logic behind Chevrolet's thinking of the headlight circuit? I sure would appreciate not blowing out my BCM.

There is a real dead spot with just the hi beams on and I am getting ready to go across the PA pike next week and would like a little more light in front of me.

"Doc"

The headlights work like this:

There are three main componenets. The headlights, the High current relays and the BCM. The headlights are actually powered by the high current relays contacts. The coils of the hi current relays are powered by 12V on one side of the coil. The other side of the coil then goes to the BCM. The BCM then uses the equivelent of an open collector transistor to "sink" the relay control voltage to ground. When the transistor is tuned on current is allowed to flow and energize the headlight relay.

If you were to measure the voltage at the BCM connection when it is not on you would see +12V. This is why you need to apply a ground to make them come on.

Problem is you do not just want to add a wire. If you add a wire and tie the lows to the highs you will also get highs when on Low. That is why the diode is added. The diode allows current to only flow in one direction. So to make other light circuit control relays to operate on high also you need to put in a diode between the Low relay BCM wire and the high relay BCM wire. The diode cathode always connects to the high beal relay BCM wire.

You can see this in the schematic of my 2003 mod. http://www.dieselplace.com/forum/showthread.php?t=146575
The diode can be added at either the bottom of the relays or under the dash near the BCM. My mod does them at the BCM. Others will show you pictures of the diodes under the relays. Both work because diodes are effectively on the same wire.

Your comment that no high or low beam relays I can not confirm and would need to see a set of schematics. There has to be some relays somewhere for each light circuit because there is no way the BCM can control the high current that light require directly.

The "03 is different than the "02 models. I don't know if I can post a scan of the schematic from the manual, even though I have one.

From the schematics, I just do not see why I get battery voltage on both sides of the headlight connectors when the light is supposed to be off and it is unplugged from the bulb. When I the light is supposed to be on and the lightbulb is disconnected, I have one hot and one grounded terminal at the connector.

I do see that to get the headlamp to light, the dimmer switch goes to ground.

I was thinking that I could install a 30A relay, with "T" taps, and connect terminal 85 and 86 for the relay to the purple and orange wires off of the high beam to trigger the relay. Then connect terminal 30 to the yellow wire on the lo beam and terminal 87 to a 1 ohm resistor and then to ground. The resistor would simulate the load of a lo beam headlamp element, rather than a dead short to ground.

Since I don't have seperate hi and lo beam relays, I have to come up with a different hook up. I am tired of holding the TS lever in the flash to pass to get rid of the dead spot in front when I need hi beam. (the headlights are adjusted as per specs)

Next project will be better bulbs, probably ULTRA's for even better light up front. My only complaint about the whole truck.

"Doc"

From the schematics, I just do not see why I get battery voltage on both sides of the headlight connectors when the light is supposed to be off and it is unplugged from the bulb. When I the light is supposed to be on and the lightbulb is disconnected, I have one hot and one grounded terminal at the connector.

Since I don't have seperate hi and lo beam relays, I have to come up with a different hook up. I am tired of holding the TS lever in the flash to pass to get rid of the dead spot in front when I need hi beam. (the headlights are adjusted as per specs)


Doc

When light relay is off you will get 12V on both leads of the light because there is no current flow so the resistance of the bulb does not show any voltage drop. Also the same is true when you have the lights on and bulb removed one lead will have 12 V and the other ground. The relay supplies the ground. If you were to manually apply a ground connection on that lead when lights are off you will then light up the bulb. Again that is all normal.

I don't have 2002 schematics but still find it hard to believe there is no seperate relays. You could actually confirm the relay condition. In the fuse/relay box under the hood look at the legend and see if there is any indication of relays for lights.
- turn your lights on high and then go and pull a relay out. If the lights go out that should be the relay for that circuit. Do the same thing for your low beams. If you find you have two relays you can do the diode trick under the relays to get your lows when your high beams are on.

I found the long mother mod on the headlight mod.
http://dieselplace.com/forum/showthr...=headlight+mod

If you go to page 17 there is some pics of what has been done on 2002 truck. There might be some more information later on.

I found the long mother mod on the headlight mod.
http://dieselplace.com/forum/showthr...=headlight+mod

If you go to page 17 there is some pics of what has been done on 2002 truck. There might be some more information later on.
I could not get the link to work.

BTW, got your PM and I am finally starting to feel comfortable with the ground wire off of the yellow wire.

Thanx:

"Doc"

Well, I used 3 "T" taps, 4 pieces of wire, 1 30 Amp relay and one loop connector along with one self drilling screw.

I removed the left headlamp and the front valance to get access to the front of the radiator support. I installed red "T" taps (I found that the blue "T" taps were to loose) on the Orange and Purple wires at the left Hi Beam headlight and a red "T" tap on the Yellow wire on the left Lo Beam. I them mounted the 30 Amp Relay on the front of the radiator support. Made up a harness for the three wires from the relay to the headlight wires and attached a loop connector to the ground wire to the #30 terminal of the relay. Installation took less than 5 minutes. The research to make sure that this did not create any problems on my 2002 K3500, took weeks just to be certain.

Now I have all 4 lamps burning while on Hi Beam, I still have all the other functions that I had before the MOD.

If there is any type of problem with this MOD, all I have to do is pull the left headlight and disconnect the wires from the relay to the three headlight wires. Should take about 60 seconds.

I know this works well on my 2002 K3500 Silverado, but I can will not advise this for any other year of series of Chevrolet or other GM.

I hope this helps others with 2002 models that would like to have the All-On Mod. Maybe I could have just grounded out the yellow wire and let it go at that, but since it was not designed in, I wanted to open the MOD when not needed.

HTH:

"Doc"

That is way more than you needed to do. The yellow wire referenced in the all-on mod thread for the 99-02 trucks is the ground circuit for the low beams. Our trucks have a relay to switch the ground between the low and high beams. There is another relay for headlight power. If you add a dedicated ground wire to the yellow wire in the fuse box, the lows will stay on when the headlights are on. In low beam mode, the bulbs have 2 ground paths (which doesn't cause any problems.) In High beam mode, the highs have one ground path through the relay as designed and the low beams stay on because of their dedicated ground.
I did some experimenting last week while driving on very dark rural roads and have decided that I will rewire the fog lights to come on with the high beams. In my Chevy, using the flash to pass circuit, I noticed that the extra light right in front of the truck actually decreased the distance I could see. It got rid of the dark spot in front of the truck, but caused way too much glare from road signs and shortened the distance I could see. I had read about that on Daniel Stern's site and so I was paying close attention to what I could see after having all the lights on for a couple minutes at a time. I found that I need more light to the sides and not as much right in front of the truck. The roads were wet, and I know that made the glare worse than it would be on dry roads, but the point of modifying the headlights is to make it better in all driving conditions. I think that adding the fog lights to the high beams would be the perfect combination for optimum visibility.