How-To: 4 High Mod For $1.00 2003+ GMT800.

3500in nc · December 4, 2005

I did, and I found a way to do it and any combo of light changes.

Go to Radioshack and get a 12Volt Diode. And then look under your dash and you will see three 24 pin plugs under your steering colum. The 24 pin plug on the left is the one you want. After you remove the access cover to see, you find the pins you need. I look at them like this, (view attachment) the top row starting on the left is T1 and the last pin to the right is T12. The bottom row at the right is B1 and the far right one is B12. Got it? OK so T1 is the High Beam signal (blk wht) the DRLs(daytime running lights) were T4 (grn blk) the Low baems were B4 (pink) and the fogs were B5(grn wht).

The way this works is you trigger the lights with relays using reverse polarity. So if you want to trigger the lows when the highs came on you would put the diode inbetween T1 and B4, with the bar on the diode twards the T1. And so when you turn on your highs the lows come on too. But when you just turn on the lows only the lows come on.

The other mod that I did was to have my DRLs trigger my Fog lights. So i put the diode in between T4 and B5. So when my DRLs came on anytime they would trigger my fogs to come on as well. But I dont like the way the DRLs look so i put in burnt bulbs, so the DRL system only triggers the fogs.

The easiest was to put in the Diodes is just put them in the plug with the pins on the wire side. You dont have to unplug the plug. Just push the diode in next to the wire with some nedlenose pliers or something. Make sure they stay in there good too. I will take some pics tonight and post them to show you what I mean.

This was done on my 2004 Silverado,

A way to find the pin you need is using a meter on the dc voltage setting have one probe to gnd and the other to the desired pin, if the light is off you should read 11-14 volts, and when you turn it on it should read almost 0 volts. Some lights need to have it in gear on key to the run postion to work.

Headlight_mod.bmp

mmmikkke · December 4, 2005

Hey, thanks!

I just might do this.

lrymal · December 4, 2005

There's an article on this very thing in the Articles section. In case anyone is concerned about the longevity of this mod, I have this on both my '03 and '05 GM trucks since 2003 with no problem:

http://gm-trucks.com/home/content/section/...id=84&Itemid=25

3500in nc · December 4, 2005

There's an article on this very thing in the Articles section. In case anyone is concerned about the longevity of this mod, I have this on both my '03 and '05 GM trucks since 2003 with no problem:

http://gm-trucks.com/home/content/section/...id=84&Itemid=25

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ah thanks for pointing that out i didnt even look there b4 i posted

lrymal · December 4, 2005

..
ah thanks for pointing that out i didnt even look there b4 i posted

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That's ok... you included a graphic from the manual which is helpful. Actually, that graphic needs to be included in that article. "Some day..."

blacksab2 · December 14, 2005

Anyone know if this diode mod will work on a 02?

fpkelly · February 19, 2006

I know its been a month or two since this has been replied to but will a 600V diode work?

lrymal · February 19, 2006

I know its been a month or two since this has been replied to but will a 600V diode work?

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Any diode will work.

fpkelly · February 19, 2006

I know its been a month or two since this has been replied to but will a 600V diode work?

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Any diode will work.

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Thanks

lord-dogbert · April 7, 2006

Happiness for my 03 GMC 2500HD, now to tie the high's the fog lamps tomorrow.

cowboy02 · April 12, 2006

1 thing that is worthy of posting here...

This is a great mod and a cheap way to add the 4 high to your system, however if you are going to run any light besides the standard Hallogens I recomend upgrading to a wire harness type setup.

Currently you are pulling all that current to turn on all 4 headlights thru your dash switch, in time this will only cause the dash to get hot and since GM changed there guts in the new ones it won't cause the switch to go out. On older GM vehicles the switch eventually will burn out (It does even without the diode)

However the real reason is that if you measure Volts across the lamp you will notice a huge drop in voltage when you kick on your highs because of all the resistance between that switch and the small 22 guage wire.

I ran 12 Guage wire directly from the batter to my lights, grounded them to the frame and then ran 2 new relays and they are triggered by that crappy 22 guage wire to turn on (Which is the right size for turn on wires), then it's 12 Guage power right to the light. Measured Voltage across the lights with lows only on is almost 14V, (Truck running) and with all 4 on it's 12.5 Volts.

I noticed a huge difference in how my Silverstars illuminated between the 2 setups and to me it's worth the 60 bucks for the light harness.

It's a sweet mod for $1.00 I give it that!!

fossil · December 13, 2006

Here's a link to an easy lamp mod for 03 to 06 models

http://www.painlesswiring.com/InfoSearch/p...archField=30822

fossil · December 13, 2006

1 thing that is worthy of posting here...

This is a great mod and a cheap way to add the 4 high to your system, however if you are going to run any light besides the standard Hallogens I recomend upgrading to a wire harness type setup.

Currently you are pulling all that current to turn on all 4 headlights thru your dash switch, in time this will only cause the dash to get hot and since GM changed there guts in the new ones it won't cause the switch to go out. On older GM vehicles the switch eventually will burn out (It does even without the diode)

However the real reason is that if you measure Volts across the lamp you will notice a huge drop in voltage when you kick on your highs because of all the resistance between that switch and the small 22 guage wire.

I ran 12 Guage wire directly from the batter to my lights, grounded them to the frame and then ran 2 new relays and they are triggered by that crappy 22 guage wire to turn on (Which is the right size for turn on wires), then it's 12 Guage power right to the light. Measured Voltage across the lights with lows only on is almost 14V, (Truck running) and with all 4 on it's 12.5 Volts.

I noticed a huge difference in how my Silverstars illuminated between the 2 setups and to me it's worth the 60 bucks for the light harness.

It's a sweet mod for $1.00 I give it that!!

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Actually you wont hurt the light switch because the switch only controls the light relays in the fuse box under the hood. It's the relays that handle all the electrical load and there are two, one for each circuit. The actual voltage controlling those is actually quite small as all it has to do is turn the relays off and on, not the actual light assembly.

Nova5 · December 30, 2006

image in original post seems to be broken, i get a download prompt for index.php

ridgekid · December 31, 2006

I agree with Cowboy. I think the diode method is great..but I also think your over taxing your electrical system. With the harness kit (Which I purchased through Mr Taillights) you get two relays and a separate feed for a power supply. For an additional $7 you can get the bypass switch. It will turn off your fogs if your doing the 6HI mod or your Low beams if your only doing the 4 HI mod.

With shipping I paid $35 for the 6Hi and bypass switch. Peace of mind-Priceless.