Headlight/Foglight Relay Operation

Hello all,

I'm wondering how the Main Body ECU controls the coil side of the relays used for low and high beams, and for the fog lights.

Looking at this diagram of the headlight relays, the coil side of the low beam relay is always powered (pin 2, green wire), and the other lead (pin 1, white wire) goes to the Main Body ECU.

Am I correct to assume the ECU controls the relay by providing a ground?

 

Correct.

I’m about to release information (in coming days) for the fog lights to be on with the high beams using some very simple components. I assume that’s what you’re trying to do.

 

Yep exactly.

My plan was to add a Cali Raised LED bar, but have it wired so that it auto comes on with high beams, and that the foglights also stay on. I was planning this with a SPDT switch and a DPDT relay. One pole of the relay would provide power to the LED bar, the other pole would provide ground for the foglight relay control wire.

The switch would be 3 position: off; manual on that would simply energize the new relay; and auto on that would splice into the high beam wire past the high beam relay (pink wires in the diagram above). That way the relay energizes when the high beam lights receive power. The extra power draw from the relay coil would be negligible to the high beam fuse.

My only concern is that grounding the fog light relay control wire (white) would somehow mess with the ECU, as I'm not sure what happens inside.

 

I think you’re combining 2 mods and it’s over-complicating.

Wire the light bar with a SPDT relay. NC to high beam. NO to 12V. Common to light bar positive. Energize the coil with your own switch to change it from high beam to “override” constant operation.

Wait for my mod later this week if you want fog lights and high beams, or tackle it yourself if you feel comfortable with the diagram. I’m testing with my ECU taking back feeds into consideration. I’ve been working on it with an electrical engineer from work. I’m a mechanical. It’s gonna be super simple.

 

I might be misreading, but wouldn't that power the LED bar through the high beam fuse before the relay?

 

Yes. What’s the amperage you’re talking? If it’s under a few amps (it should be), you can easily add it to the existing high beam circuit. I’ve done it.

If not or if you’re worried about it, you could do a similar thing to what you suggested - a SPST relay for the light bar, and a SPDT switch - one pole for 12V, and the other from high beam wire. Common obviously out to your relay coil.

This adds a lot of wiring length and complication though, albeit safer. It also requires a special switch, and they don’t make SPDT switches that fit our dash blanks, so you’ll have to do drilling or cutting presumably. I avoid that at all costs.

Do you need it? You be the judge. Many ways to skin the cat.

 

the Cali Raised bar is rated at 300W, so around 25 amps

 

I’d be flabbergasted if it drew that much current. Sounds like some artificial spec boost to me. Measure it with a DMM. Bet you it’s not even close.

Math adds up though if it’s a true 300W. Sounds like your original plan is the way to go.

I have switch-pros SP8100 so I’m loving not having to deal with things like this. Highly recommend this product.

 

sweet, thanks for the info! looking forward to your post about the fogs!