What did you do to your NNBS GMT900 Tahoe/Yukon Today?

[gooffeyguy]

The fogs are on anytime the headlights are on and vice versa. All it’s doing is jumping the fog light relay at the same time the headlight relay is triggered or vice versa. Both relays get triggered from the same signal. The fog switch is momentary, it doesn’t stay pushed in. It just sends a signal to the bcm, which turns on the relay. The headlights will come on if you push the fog button, in which case the fog indicator in the cluster comes on. But if you just turn the headlights on or if they come on automatically the indicator doesn’t come on when the fogs come on.

So since the yukon uses the headlights as daytime running lights, doing this mod would have the foglights come in also during the daytime?

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[gooffeyguy]

So I guess you could say you can control the fogs with the fog light switch but it will turn the headlights on too. But I can’t picture a scenario where I would want the fogs on without the headlights.

When it's really foggy, I've turned my headlight knob to the momentary left position so it turns off headlights, then push the foglight button so just the parking and foglights are on.

With them being lower to the ground the light isn't reflected back into the drivers face as much and makea it easier to see while driving in fog

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[Rocket Man]

So since the yukon uses the headlights as daytime running lights, doing this mod would have the foglights come in also during the daytime?

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There’s a separate DRL relay that’s triggered by a different wire from the BCM than the one that triggers the headlamp relay so that should be no. I’ll check tomorrow. If you look at the schematics you can see the negative signals from the BCM. The fog relay is D-GN/WH going to pin 86 on that relay, the headlight is PK/WH going to pin 85 on that relay and there’s another trigger that goes to the DRL relay. I’m jumping the fog and headlight trigger together.

Edit: now that I look at it, the fuses are after the relays so there’s no way to “piggyback” two lighting circuits onto one fuse. I checked and those fuses only show voltage when that set of lights are on.

 

[Rocket Man]

When it's really foggy, I've turned my headlight knob to the momentary left position so it turns off headlights, then push the foglight button so just the parking and foglights are on.

With them being lower to the ground the light isn't reflected back into the drivers face as much and makea it easier to see while driving in fog

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Then don’t do it the way I am.

 

[PG01]

When it's really foggy, I've turned my headlight knob to the momentary left position so it turns off headlights, then push the foglight button so just the parking and foglights are on.

With them being lower to the ground the light isn't reflected back into the drivers face as much and makea it easier to see while driving in fog

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I do this.... not for fog but to look ‘cool’..... lmfao ......lol ..

 

[gooffeyguy]

I do this.... not for fog but to look ‘cool’..... lmfao ......lol ..

Does it work? Lol

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[George B]

I do this.... not for fog but to look ‘cool’..... lmfao ......lol ..

I do too. I just wasn’t going to admit it first.

 

[Rocket Man]

You could add a diode in the wire if you wanted to run fogs without the headlights on, that way when you turn the headlights on the fog relay would turn on but when you turn the fogs on the diode won’t allow the headlight relay to turn on. I’ll see if I can find a diode and try it today.

 

[PG01]

Does it work? Lol

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Nope.....lol

 

[Doubeleive]

So since the yukon uses the headlights as daytime running lights, doing this mod would have the foglights come in also during the daytime?

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yes or it should anyway I haven't tried it yet I just looked to see if the relays were in the same position or not and they are. when the system switches at dusk to automatic headlights it cycles the relay (I can hear it) not sure what it would do at that point.