Postby Zaphod » Thu Feb 06, 2014 10:17 pm
It's not voltage, it's load - when normal lamps get hot enough to glow, they also resist current flow - the ECU sees this as a 'load' on the electrical system; LEDs aren't resistive, so when they're turned on, the ECU thinks there's a short-circuit and shuts them down.
That's also where the flicker comes from - the LEDs turn on, there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on, sees there's no load, so the ECU turns the circuit off, then realises the main light switch is set to 'on', turns the LEDs back on.....
And it does this really quickly, so we see LEDs flicker!
To get around it, you need to put a load resistor in line with the LEDs, so that when the main switch is turned to 'on', the LEDs illuminate, but this time the ECU sees a load and knows there's a lamp and lets it run undisturbed!
"The Human species has now evolved to the point where we only have two natural predators; ourselves and peanuts..."