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Thread: LED Bulb Coding - License Plate Light Without Pulse Width Modulation Option

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    Verified VCDS User GaryM's Avatar
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    LED Bulb Coding - License Plate Light Without Pulse Width Modulation Option

    In preparation for replacing the license plate incandescent bulbs in my car with LED bulbs instead, I thought it would probably be a good idea to actually test the switching off of the "Lamp Monitoring (Cold) License Plate Lamps active" option first which on my 5K0937087R BCM is Bit 6 of Byte 23...



    This I have proven to work just fine. If this coding option is active (factory default) and I remove both bulbs, the "bulb out" warning indicator comes on and I get told in my MFD that the offending bulbs are the license plate ones. If I deactivate the coding option and clear the fault codes, with no license plate bulbs fitted I get no recurring "bulb out" warning. Just what I need to happen so this is good news

    So onto my next question regarding the other related license plate bulb coding value. This is Bit 7 of Byte 17 and is labelled "License Plate Light w/o Pulse Width Modulation (PWM) [LED Light]" (see screenshot below). The very end part in brackets suggests that I probably need to activate this option if I am going to be fitting LED bulbs (i.e switch off PWM). The question I have is do I actually need to activate this for LED bulbs, and if so then why exactly?

    I understand that PWM is used to control the brightness (dimming) of incandescent bulbs, but what about controlling the brightness of LEDs? This can also be achieved with PWM so I'm not quite sure I understand why PWM would not be required for LED bulbs. If PWM is left switched on, would there be any detrimental effects of this on an LED bulb or the lighting circuit of the BCM itself?


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    Benevolent Dictator Uwe's Avatar
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    I don't suppose you have an oscilloscope to see exactly what the PWM does?

    While it's certainly possible to control the brightness of LEDs using PWM, it has to be done at a higher frequency than with incandescent bulbs to avoid visible flicker because LEDs turn on and off almost instantly compared to incandescent bulbs (due to the thermal inertia of the filaments in the latter).

    I suspect PWM is done with incandescent bulbs to keep them at close to an effective 12V, even when bus voltage is up around 14V, in order to extend their life.

    -Uwe-
    The engineering problems are likely insurmountable. It would be like proposing to land a rocket booster section on a barge floating in the middle of the ocean.

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    You need to change both options to make sure you do not get a warning in your dash. On some PQ35 the LED-option does not work, until you update the bcm module.

    greatings

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    Verified VCDS User GaryM's Avatar
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    Quote Originally Posted by Uwe View Post
    I don't suppose you have an oscilloscope to see exactly what the PWM does?
    Yes I do have actually have an oscilloscope although it's a mains powered one so it just means I'll need to reel out the extension lead. TBH it didn't even cross my mind to scope the output in each of the two modes so maybe I'll try and find some time do that and then post back the results once I've done that.

    Quote Originally Posted by Uwe View Post
    While it's certainly possible to control the brightness of LEDs using PWM, it has to be done at a higher frequency than with incandescent bulbs to avoid visible flicker because LEDs turn on and off almost instantly compared to incandescent bulbs (due to the thermal inertia of the filaments in the latter).
    Do you happen to know if the CAN BUS specific LED bulbs circumvent against such flickering issues? I've been unable to find a definitive answer on what differs between the "CAN BUS" LEDs and the "non CAN BUS" LEDs. I guess both will contain voltage regulator circuitry to step down the 12v to 5v (or 3.3v) and the "error free" CAN BUS ones will additionally have built in load resistors to fool the bulb check system into thinking incandescent bulbs are still fitted, but what about LEDs that are marketed as "CAN BUS" ones but don't claim to be error free? I wonder what is so special about those over ones that are marketed as "non CAN BUS" LEDs?

    I suspect PWM is done with incandescent bulbs to keep them at close to an effective 12V, even when bus voltage is up around 14V, in order to extend their life.
    Is that not what Byte 19 is for? My fronts seem to be set to 13.0V and my rear ones seem to have no restriction. These are both factory settings that I've not messed with. I'm not entirely sure why there would be a difference in settings between front and rear unless it's because my rear light clusters contain some LED lights (stop and position lights that are built into the factory cluster) and some are incandescent bulbs (turn signals, reverse, and fog lights).






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    Ross-Tech Employee Sebastian's Avatar
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    There are no CAN-Bus LEDs! What is offered under that term (which is beyond misleading), are LEDs with a series resistor - no more, no less. That said, some of the discrepencies you're seeing with the coding table is based on the fact that the original table applies to Helly (1K0) BCMs and is not 100% identical for Continental (5K0) BCMs - in addition there are variations based on the control module firmware version, which we are currently not taking into account.
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    Verified VCDS User GaryM's Avatar
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    Quote Originally Posted by Blackfrosch View Post
    You need to change both options to make sure you do not get a warning in your dash. On some PQ35 the LED-option does not work, until you update the bcm module.
    I already did a test with the incandescent bulbs removed (i.e. no bulbs fitted at all) and with just the "Lamp Monitoring (Cold) License Plate Lamps active" option switched OFF and the "License Plate Light w/o Pulse Width Modulation (PWM) [LED Light]" left switched OFF (factory default) I was NOT getting a "bulb out" error.

    Or are you saying that if I was to fit LED bulbs, I would then actually see the "bulb out" error reappear unless I was to also switch ON the "License Plate Light w/o Pulse Width Modulation (PWM) [LED Light]" option?

    I guess I'll find out for certain when I've actually got some LED bulbs.

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    Verified VCDS User GaryM's Avatar
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    Quote Originally Posted by Sebastian View Post
    There are no CAN-Bus LEDs! What is offered under that term (which is beyond misleading), are LEDs with a series resistor - no more, no less.
    Thanks Sebastian. That is exactly what I was trying to find out but there seems to be so much misinformation on the internet that it is almost impossible to find a definitive answer.

    So basically the bottom line is that all "CAN-Bus" LEDs should actually be error free whether they explicitly claim to be or not, as long as the load resistor meets the current draw requirements of the bulb check circuitry for the BCM to which the bulb is connected.

    If I switch off cold diagnostics on the license plate bulbs, am I therefore correct in saying that I have no need for "CAN-Bus" LEDs and can just get normal LEDs (non CAN-Bus) designed for automotive applications without the risk of doing any damage to the BCM? I see no point in drawing excessive current and generating unnecessary heat if there is no need to with the cold diagnostics switched off.

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    FWIW, my 2017 Caddy has stop and tail lights via dimming.

    I have just installed some can friendly rear 382 indicator leds and they work fine with no error.

    Now, as it is the same lamp base as the stop/tail, i fitted it to that socket and whilst the brake light function worked, it did not dim down for the tail light function, in fact it didn't work at all. When i turned the lights off and removed the key the CH lights activated and the LED bulb that was still left connected started to exhibit a very faint flicker.

    I was then pointed to this lamp that should work, however, it does state that it is not an error free lamp so ill see what happens when it gets here from across the pond!

    https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

    Also, i'll post up some screen shots of my codes for comparison if you like


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    Ross-Tech Employee Sebastian's Avatar
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    I have yet to find a single LED with series resistor which is certified, meaning all the ones I have seen so far will make your car illegal on the streets. Since the UK is still part of the EU for the time being, I am also assuming this is the case for you guys.
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    Benevolent Dictator Uwe's Avatar
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    Quote Originally Posted by Sebastian View Post
    LEDs with a series resistor - no more, no less.
    Quote Originally Posted by Sebastian View Post
    LED with series resistor
    Hmm, if I were trying to get an LED light module to simulate the load presented by an old-school incandescent bulb, I would put the resistor in parallel with it, not in series.

    -Uwe-
    The engineering problems are likely insurmountable. It would be like proposing to land a rocket booster section on a barge floating in the middle of the ocean.

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