[benha]

Hello!

I’m hoping to get a bit of help with an electrical project.

I’m planning to install some driving lights to augment my headlights on rural roads. I’d like to set this up so that when the Fog Light switch is ON, the Driving Lights come ON with the High Beams. In other words, the Fog Light switch becomes the “Additional Light” switch where when it’s ON and the Low Beams are ON the Fog Lights are illuminated, and when it’s on and the High Beams are ON the Driving Lights are illuminated.

What I assumed would be a simple task has, upon inspecting the wiring diagram, started looking much more complicated. Damned computers are a pain in the butt.

If I understand correctly, the BCM polls the Combination Switch at a 20ms interval, inspecting the status of the controls on that combination switch and sending state messages over the CAN bus to the IPDM E/R, which then controls the various relays to activate things.

What I’m afraid this means is that there’s no traditional (ie: non-digital) way to detect that the Fog Light switch is ON when the High Beam switch is ON, since the only system that detects the position of the Fog Light switch is digital.

What I had wanted to do was to wire the coil circuit of a power relay for the Driving Lights by pulling a shunt from the High Beam feed wire and routing it through a control relay toggled by the state of the Fog Lamp switch. Thus, when the High Beams are on, the circuit to the coil of the Driving Lights would be energized, but that circuit would only be closed if the Fog Light switch is ON. But because I’m not seeing a non-digital way of knowing if the Fog Light switch is ON when the High Beams are ON (because the computer turns the Fog Lights OFF when the High Beams are ON), I can’t make this work.

Anyone know an easy way to get a constant 12v (or ground) feed from the electrical system at all times that the lighting system is active and the Fog Lights are ON?

Thanks,
-Ben

 

[The other Sean]

I'm no electrical engineer, but, can you just use two relays? The power needs to run through both relays and both relays need to be triggered to let the power through the circuit?

1st relay trigger wire pulls power off the fog light switch wiring before the BCM and the second relay pulls the trigger wire off the high beam power wire?

 

[benha]

The crux of the question is what you can pull from the switch before the BCM.

If there's a way to just see that switch is ON you're in clover. But if it's powered by a clock-driven inspection of the state of the switch like the wiring diagram says, then there wouldn't be a constant 12V signal through the switch. It would be a 50hz square wave signal. Presumably you could build a circuit with a capacitor to even that out, but I'd be afraid that you'd confuse the BCM if you started monkeying around in there too much.

-Ben

 

[Wintermute]

I'm no electrical engineer, but, can you just use two relays? The power needs to run through both relays and both relays need to be triggered to let the power through the circuit?

1st relay trigger wire pulls power off the fog light switch wiring before the BCM and the second relay pulls the trigger wire off the high beam power wire?

That works in principle, but now you need to actually find those wires. Given that the fog lights are controlled by the computer, not a direct relay, that could be tough. I'm not sure how the high-beam wiring works, but it could be tough to find that wire too.

 

[benha]

That works in principle, but now you need to actually find those wires. Given that the fog lights are controlled by the computer, not a direct relay, that could be tough. I'm not sure how the high-beam wiring works, but it could be tough to find that wire too.

The High Beam is relatively easy since you can splice into the harness where it plugs into the lamp itself. That's inherently a 12V+ signal that's on when the circuit is on. The problem is detecting that the Fog Light switch is ON when the Fog Light lamps aren't energized. In other words, when the High Beams are ON you have no easy way of knowing if the Fog Light switch is ON or OFF.

 

[thenzero]

Interesting problem.

One possible solution from over at club titan: Foglight switch BCM By-Pass - Nissan Titan Forum

Long thread short- you can access the wires from the factory switch directly, bypassing the BCM, add a separate circuit with a relay and regain control of the fog light switch. Then you can use it however you like.

 

[benha]

Oh AWESOME!

This is exactly what I needed. Really appreciate the link.