Led headlight question

DuKeNoMaD

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Ok so I have read most of the current led threads...but I am having an issue I have not seen. My drl for some reason only work on one side...I know they work by putting the high beam on a low voltage...and the high beams both work ..and I just put in resistors today to stop the headlight code. But same issue. An
 

joegr

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...I know they work by putting the high beam on a low voltage...

Actually, it PWM's the high-beams. This is they are turned on at full power and then back off, very rapidly. If the percentage is 50% (I don't think that it is), then for example they may be at full power for one millisecond, and off for one millisecond, then repeat over and over. With an incandescent bulb (what it was designed for), this would have the same effect as lowering the voltage from 12V to 6V. With an LED, the result is unpredictable, without knowing the exact design of the electronics in the LED bulb assembly.
 

DuKeNoMaD

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No kidding...huh. well that's a neat trick but kinda dumb in retrospect. Thanks for the insight Joe. Now question is....why do one of the lights work in drl mode and not the other? The plugs are flipable so I'm going to try flipping the polarity on the one not working and see what that does. Keep posted
 

joegr

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Like I said, I can't say why without knowing the design of the current limiters in the LED assemblies. Most likely, that working one has a slightly lower voltage drop over the LEDs than the non-working one.

They do PWM because it is much more efficient and easier. The bulbs are 50 watt, so they each take 4.17 amps of current. To reduce them to half power, without PWM, you would need a 1.2 ohm resistor.* That would dissipate (waste) 10.4 watts per light (21 watts total). So, you have to get rid of 21 watts of heat somewhere, and the alternator would need to generate 21 more watts (gas mileage hit). Or, you could do PWM and only waste a watt or two (losses in the switching FETs).
*: Half power = 25 watts, so about 2.95 amps at about 8.47 volts.
 

DuKeNoMaD

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Ahh. I get it. And I flipped the polarity on the one and now they both work! But then I just got hit with a vehicle inspection
 

LunaEros

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DRLs? Thank god mine seems to have not had that option ordered. It has pretty much every other option but thankfully not that one. ;)
 

joegr

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DRLs? Thank god mine seems to have not had that option ordered. It has pretty much every other option but thankfully not that one. ;)

No LSes sold in the US had DRLs. That said, it is a FEM programming option, not a hardware change.
 

LunaEros

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So I could program it in if I wanted them? What do you do it with, FORScan or something?
 

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