I am replacing a single Xenon HID headlight with a pair of LED head lights on each side of my car. Putting in LEDs normally means adding a resister across the wires to trick the CAN BUS into not reporting a blown bulb because of the change in draw. The idea seams to be that the resistor negates the effect of using the lower power LEDS. This much is fine but you obviously need to fit the correct resistance and some cars, probably mine, are quite fussy so need the end result to be quite close to the original.
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I am thinking that I will join the wires from both lights together before the point the resistor is connected, This way I will only need 4 resistors on each side - high beam, low beam, DLR and Indicators - rather than 8.
Any reason not to just connect the 2 earths, the 2 low beam, the two high beam, the 2 DLRs together (there is only and indicator in the 7" lamp) and then connect the resulting 4 wires (plus indicator) into the main car loom?
Any guesses or advice on calculating the resulting resistance on each? The results shown are from pressing a multimeter against the lamps, holding the black on the earth and then the red onto each of the other terminals in turn. For some reason the DLR wire gives weird readings even though it lights up fine when I stick 12v through it.
So 13.5v ish squared divided by watts of the original bulbs = what the car is looking for?
It’s been a while
Joining the sides together isn't likely to be a good idea, as each bulb will very likely be monitored individually, so joining them is likely to cause issues with the monitoring.
Also measuring the resistance of bulbs won't give an accurate figure, as the filament resistance increases with heat.
And jamesoz has the correct formula. R = V²/P
I am replacing a single Xenon HID headlight with a pair of LED head lights on each side of my car
Is that possible, legally? I recall some changes, and could it be a MOT failure if the correct lamps are not used?
It is now ok if you replace the entire lamp assembly, rather than just the bulbs.
alanl - These are purpose made LED units, not just LED bulbs chucked in the old casings.
mc - on the Porsche there was originally just one headlight and so one set of monitoring. Now there are two headlights but still just the single wires returning the signal to the CANBus
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That is four separate LED lamp units replacing the original 2 HID units. I think the HIDs ran at 35W which was less than the Halogen at 55W but I might be wrong. There is a load of gubbins inside the Xenon HID unit with a ballast etc so I am noit sure if this affects things.
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Just found this on a website which might help "Xenon lamps are only 35 watts. The ballasts only draw 1.2 amps of power!"
Now, I am thinking that 2x15W LEDs is pretty close to 1x35W Xenon. Can I daisy chain the LED units together so they combine to replicate the original?
Is there no way of resetting the CANBus to the new light voltage?
See that danger/warning sticker ?
You need some proper research on this..
hid has a voltage of 18-30 k volts on startup
high-energy-discharge.
most folks are going from halogen to led or hid .
Least complex may be to go with whatever the lights units are designed to use so presuming you are bypassing the hid system?
Why on gods earth are you trying to replace a xenon with an LED?
A xenon lamp is the pinnacle in terms of brightness and spectrum. But they’re hard to design into an optic. You have a Porsche xenon optic and you’re trying to undo that…………..?
goldfish - start here and it will all become clear :
Also, I am just going to find someone with a PIWIS2 and get them to code out the error
If you can’t get it coded out and have space, maybe you could leave the old lamps wired in and power the new ones though transistorised relays.
on the Porsche there was originally just one headlight and so one set of monitoring. Now there are two headlights but still just the single wires returning the signal to the CANBus
I did think that after I posted, as I had been thinking that you were wanting to join both sides together.
I'd try wiring everything in temporarily without resistors, and see what warnings ping up.
My guess is indicators, and sides will most likely need resistors. Probably DLR as well, but with it being HID, the dips/mains might not detect an issue.
And given it'll run on typical VAG modules, changing the coding is likely feasible, and will eliminate the need for mounting resistors (which itself comes with the issue of where to mount them so that they can dissipate 30+W of heat without melting anything).
BTW have you considered buying a Durametric diag tool?
Aren’t headlights on separate left and right circuits to prevent a dead circuit from killing both lights?
afaik vcds can read and in some cases code Porsche systems. It’s not comprehensive (you need piwi for that), but you may be able to code out the circuit monitoring. Possibly worth a shot
The Durametric is similar to a PIWI but does less, correct? I looked at the PIWI but even the Russian rip-offs are £500 which is more that I want to clear a code, although it might be useful in the future.
I will just find a local who will do it for me I think. That way I can carry on with the build.
If you study the AD diagram, below, carefully then you will see that they is some room for wiring in resistors within the area boxed inside the brown lines but I want to minimise the space used there and maximise the dotted orange area for max cooling.
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