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Thread: Driving Lights

  1. #1
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    Driving Lights

    I am sure I know the answer but I will put it out there anyway on the off chance someone knows. Does anyone know of a car with LED headlights that has had driving lights fitted and they work. We are attempting to fit driving lights to my Tiguan and are having trouble getting a suitable signal voltage to the relay from the high beam circuit. My son works as a day job doing diagnosis and repair on new cars at a very high level though not on VW unfortunately and he can't find a way to access the signal that will work. If we pick up the signal from the wiring loom going into the light the lights stay on and won't return to low beam. He thinks there is a controller in the light itself and we are interfering with that, could be of course and maybe this won't work at all. A single 12v rail seems to be supplying current to the headlight and the PCM or controller in the headlight decides what function in the light needs to see that 12v rail and needs power. The headlight switch has a 0.2v signal wire that happens when the switch goes to high beam which would make the above scenario work.

    I am guessing in my naive way that if we had an external controller/relay that could use the 0.2v reference signal from the switch then the problem goes away but that part is beyond our knowledge, anyone got any ideas?

  2. #2
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    The headlights in your car have 2 sets of controllers...
    The instrument stalk sends a CAN signal to the FCM (fernleicht control module) at address 4B which in turn is the master controller for 2 LCM (one for each LED headlamp assembly, addresses D6 and D7).
    I'm assuming these are just a LIN connection from the FCM to the LCM, which runs over a single wire, I'd have to check the wiring diagram.

    So the high beam is not simply a 12v circuit being switched on from the BCM like it would have been in a simpler setup.
    I think ideally you would need a relay that understands the commands being sent over the LIN bus from the FCM to the LCM so it knows whether the command is to engage high beam, or to turn on the indicator etc.
    Given that is probably just a theoretical device, not an actual product, you are probably better off just installing your own manual switch...
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  3. #3
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    A friend of mine had an idea about this - find an unused pin on the BCM and configure it via adaptation channels to be triggered when high beam is engaged...

    This would require some in depth research:
    Check the BCM and wiring diagrams to find a pin that is not used but could be controlled via leuchte channel programming
    Gather measuring block data on the BCM whilst high beam is engaged/disengaged
    Configure the BCM via adaptation channels to now turn on this pin with high beam
    Wire in your relay to this pin for your driving lights.

    This would result in a factory-like solution.
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  4. #4
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    We have given it some thought and the current thinking is to get the front bar off and have a look at the controller, find the high beam out and get between it and the headlight. I will show my son your suggestion and see what he says, as familiar as he is with this stuff VW is new to him. As for your suggestion I will get back to you but work has stopped, codes will be cleared tomorrow and some research needs doing.

  5. #5
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    I'll email you the repair guide.
    Once you crack open the LCM which is bolted to the headlamp, you should be able to probe the guts and hopefully find something that you can solder on a wire to run a relay off...given that LEDs are voltage rather than current devices, hopefully pulling a small current off a circuit to run a relay won't upset the LCM to the point where is logs a fault....maybe??
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  6. #6
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    Quote Originally Posted by kamold View Post
    I'll email you the repair guide.
    Once you crack open the LCM which is bolted to the headlamp, you should be able to probe the guts and hopefully find something that you can solder on a wire to run a relay off...given that LEDs are voltage rather than current devices, hopefully pulling a small current off a circuit to run a relay won't upset the LCM to the point where is logs a fault....maybe??
    Thanks, he suspects it will be potted so what is possible is yet to be seen.

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