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LED/HID lighting upgrades confusing 7" VGA car LCD illumination

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  • LED/HID lighting upgrades confusing 7" VGA car LCD illumination

    Hi all,

    I installed a car PC in my Mazda 3 MPS (2006 model), with the +12 illumination wired up and working well dimming the screen when the headlights get switched on. The setup is a Mini-ITX AOpen setup with CARNETIX PSU and a 12V 7" HDMI/VGA LCD.

    Recently I replaced all the internal globes with LEDs, and then the headlights with HIDS and some LED lights externally (eg. number plate, parker, reverse lights). They all work perfectly without issue, but I note the dashboard displays a "globe blown" icon suggesting some sort of CANBUS/low voltage issue. I do not believe the Mazda 3 is picky with CANBUS issues as much as some other cars, in fact there is next to nothing online about this fault I am having.

    I can live with the error on the dash, but the 7" LCD illumination is strange. It'll now flicker to bright when I switch on the lights, then back to dim again. When I turn on the foglights, the LCD goes bright (it should dim). Obviously the car is getting confused about the lower voltage of the LEDs, as is the illumination of the LCD.

    Would wiring up a load resistor on the 7" LCD illumination wire help things, or any other suggestions? I could try wiring up the LCD to it's own 12V dimming switch but have loved the current setup



  • #2
    Did you use an HID conversion kit and ballast?


    • #3
      Yes - a 35W Phillips (so not cheap) kit + ballast. The same issue occurred without this installed however. I'm assuming I need some sort of load resistor etc. as when I turn on on the fog lights and error on the dash disappears. Just not sure what do put in, or if it should be around the lights or the LCD illumination wire itself.


      • #4
        The "blown globe" is because they use current sensing - a certain drop in current means a blown globe, and LEDs consume fare less current than globes (typically at least 20x less).

        I'd expect the LCD dimming circuit to be a traditional "if 12V is present, dim the LCD" - ie, the light switch's on position (parkers or beam) means 12V at the LCD "lights on dimming" terminal.
        That should work ok, but maybe it is a different type of circuit, or maybe noise from the HID ballast is confusing the circuit (especially with less globe current to help "swamp" the noise).
        Maybe a filter cap would solve the problem - eg, a diode or resistor from the parker/beam circuit to a cap across the LCD "dimming" input.


        • #5
          Re using dummy loads, I'd be tempted to retune the sensing circuit (if possible) else remove the "error" light (if that's applicable; and assuming it's now effectively useless).

          But otherwise the dummy resistor(s) or globe(s) in parallel with the new LED etc lights anywhere in their circuit.
          Usually because of the heat involved, the dummies are placed close to where the original bulb(s) were.


          • #6
            Hi OldSpark,

            thanks for your assistance. And I seem to be off moderator post approval, hooray!

            I'd love to retune the sensing circuit but doubt that's possible. Regarding placement of the dummy load resistor, I assume I need to wire up most of the larger lights with them? For example, reverse lights, low/high beams. I've found them on ebay:






            • #7

              Whatever sensed bulb has been replaced by a LED must have its original load - eg, a 21W blub with a ~21W resistor or two 21W reverse or stop or (same side) flashers with a ~50W or 2x20W resistors etc.
              Note the warning - "the temperature of resistor can get up to 150-170C".

              I prefer to use the original bulbs since they exist and are cheaper (unless they blow too often) and may be cooler (except for halogens).

              But since the respective blown bulb circuit then becomes useless, I want to disable that circuit entirely (else retune it).

              Obviously those that change to LEDs etc to reduce power consumption would retune to disable.