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LED Headlights- Resistors?

Old 07-27-2015, 09:49 PM
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LED Headlights- Resistors?

Anyone else have issues with switching to LEDs for the highbeams?
I installed a set of H11/H9 from OPT7 and I'm getting some cross feed from the fog lights. With the low beams on and the fog lights switched on the LED high beams will dimly illuminate. If I put the stock halogen bulbs in I don't have any issues and it operates normally.

Looking at the wiring diagram it looks like it's possible that the high beam could be back feeding through the fog light relay coil to ground through the fog light switch. The LEDs are low enough resistance that it's enough for them to dimly illuminate. The halogen bulbs have enough resistance that they don't illuminate. At least that's my theory. I think if I wire a resistor into the LED harness it should fix the problem.

However, the reason I'm asking the question is I've extensively altered my harness for my swap so I wanted to see if anyone else had a similar issue with the stock harness. I'm just hoping I don't have a ground I've overlooked and now I need to spend hours digging back through the harness to find it.
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Old 07-27-2015, 09:54 PM
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I don't recall anyone trying to use LED headlights. But I would assume resistors are needed since in the rear LEDS wreak havoc (traction control lights, ABS, etc) without them.
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Old 07-28-2015, 07:39 PM
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Originally Posted by 9krpmrx8 View Post
I don't recall anyone trying to use LED headlights. But I would assume resistors are needed since in the rear LEDS wreak havoc (traction control lights, ABS, etc) without them.
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That would be stupid!
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Old 07-28-2015, 07:53 PM
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Headlights aren't tail lights. You installed a driver for your new lights and that driver takes care of all the current and voltage regulation for the light. The tail lights need resistors because they are not on a driver stock (since the incandescent lights have their own high internal resistance). The OEM HIDs already have a driver, and there's no external feedback like the tail lights and turning signals have.

Effectively all you did was replace the light+driver combo with another one.

You might be able to fix it by shielding the headlight wires from the fog light wires with some aluminum foil or metal tape, preferably grounded to the frame.
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Old 07-28-2015, 09:23 PM
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I replaced the halogen high beams, not the xenon low beams.

I did some more testing today and with the high beams off and the fog light switch on I'm getting ~10v at the highbeam socket with no bulbs, ~6V with one led installed, and ~.1V with one halogen installed.
Somehow power is back feeding to the headlight through the fog light relay coil. Since 12v is supplied to the fog light relay coil from the headlight relay I suspect that's where the backfeed is happening. So I removed the feed from the headlight relay and ran unswitched 12v to the fog light relay coil and I no longer have interference between the fogs and the high beams.

The bigger question for me since I have a highly modified wiring harness is: is this the normal operating condition of the RX8 headlights. If someone could verify that they get ~10 V at the high beam socket with both of the bulbs disconnected and the fog light switch on that'd help me figure out if I've got an issue I need to sort out in my harness or if this is normal.
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