Headlight gremlins Options

[john77]

My headlights have decided to randomly turn themselves on and off as I drive, which is a lot of fun at night on the I405 as drivers think some obnoxious d*ck is flashing them.

I cleaned up the grounds and all the ground wire connectors by both headlights but it’s still happening.

Could this be relay connected, or are they more a once dead completely dead kind of deal?

I thought i’d ask before I drop $20 on a new one.

For the record, I have those sealed GE led headlights, and it’s happening simultaneously to both, so I know it’s not the bulbs.

[john77]

@spoke Am I reading that diagram right, the fogs only come on when the high beams are on?

[lierofox]

@spoke Am I reading that diagram right, the fogs only come on when the high beams are on?

It's one of those wonky to understand, but very useful circuits.

The ground side of the fog lamp relay is connected to the positive side of the high beam circuit. When the high beams are off, the high beam circuit is dead, so the relay is able to find ground by going through the high beam filament (the current going through the relay coil is so low because of the coil's high resistance (not even 1 amp) that it's not nearly enough to make the high beam filament glow.)

When the high beams are turned on, the supply side of the high beam circuit suddenly becomes 12 volts. Since the fog light relay coil has 12v on one side, and 12v on the other side, the difference between 12v and 12v is... 0 volts, so the relay turns off, and kills the fog lights!

Because bulb filaments work differently from LEDs, this circuit can break spectacularly when you replace the headlights with LEDs.

[john77]

@spoke Am I reading that diagram right, the fogs only come on when the high beams are on?

It's one of those wonky to understand, but very useful circuits.

The ground side of the fog lamp relay is connected to the positive side of the high beam circuit. When the high beams are off, the high beam circuit is dead, so the relay is able to find ground by going through the high beam filament (the current going through the relay coil is so low because of the coil's high resistance (not even 1 amp) that it's not nearly enough to make the high beam filament glow.)

When the high beams are turned on, the supply side of the high beam circuit suddenly becomes 12 volts. Since the fog light relay coil has 12v on one side, and 12v on the other side, the difference between 12v and 12v is... 0 volts, so the relay turns off, and kills the fog lights!

Because bulb filaments work differently from LEDs, this circuit can break spectacularly when you replace the headlights with LEDs.

Wow, I didn’t know this... and it just gave me a headache trying to understand it.

So my best bet if I want to run fogs with the LED headlines is to completely separate the two circuits and run the fog switch directly to the fog relay?

[lierofox]

Wow, I didn’t know this... and it just gave me a headache trying to understand it.

So my best bet if I want to run fogs with the LED headlines is to completely separate the two circuits and run the fog switch directly to the fog relay?

You just have to pull the fog relay's ground wire off the high beam terminal, and move it over to a dedicated ground lug, then it'll always be grounded and the high beams won't cancel it.

Edit: Here's a little interactive/animated diagram I made demonstrating the circuit, you can click on the switch to turn it on and off, the wire colors show where the voltage is (green is 12v, gray is ground) and the yellow dots show the current flow.

http://tinyurl.com/tj74k43

[john77]

Wow, I didn’t know this... and it just gave me a headache trying to understand it.

So my best bet if I want to run fogs with the LED headlines is to completely separate the two circuits and run the fog switch directly to the fog relay?

You just have to pull the fog relay's ground wire off the high beam terminal, and move it over to a dedicated ground lug, then it'll always be grounded and the high beams won't cancel it.

Edit: Here's a little interactive/animated diagram I made demonstrating the circuit, you can click on the switch to turn it on and off, the wire colors show where the voltage is (green is 12v, gray is ground) and the yellow dots show the current flow.

http://tinyurl.com/tj74k43

That makes sense.

Moving the fog relay ground wire, the power to the fogs is still dependent on the headlights being switched on?

If I wanted to use them as independent driving lights I'll need to find a new power source to trigger the fog relay directly from the fog light switch, right?

[lierofox]

That makes sense.

Moving the fog relay ground wire, the power to the fogs is still dependent on the headlights being switched on?

If I wanted to use them as independent driving lights I'll need to find a new power source to trigger the fog relay directly from the fog light switch, right?

Yep! I forget which one but I think I pull power for my switch from either Fuse 8 or Fuse 9 on the under dash panel, so it'll still turn off with the key.