LED light weirdness

[Jamie KC]

I fitted an auxiliary fog lamp to our Panda 312, as I converted the existing one to another reverse lamp.

This a surface mount LED panel light, mounted centrally in the rear bumper, and it is bright enough to do fog duty. I connected it by chopping into the loom and connecting both + & - to a lead to run to the new light. Works fine, and, as expected, it triggers a check bulb warning, but I can live with that for its very occasional use.

The weird part - it seems to pick up residual voltage from elsewhere. When I unlock the car or open a door it comes on faintly, then goes out 10secs after the interior lamp. With ignition on it comes on slightly brighter as seems to be picking up voltage from the DRL circuit. It’s like a DRL for the rear of the car. Switching on rear fog gives it full brightness, and off is immediate return to a low level.

Not a big deal, but any ideas why?

 

[Scoober82]

Hi, I had a similar problem with cheaper leds. I drive a Grande Punto and it has a canbus system from what I gather. The led bulbs I chose stayed on after turning the vehicle off but only dimly…
So the only way I got around it was to buy branded leds! They are expensive but come with a built in resistor which creates enough resistance to tell the canbus system that the bulb is working. No resistance means bulb is out.

I have tried with multiple leds and even pairs where one was ok but the second brought the diagnostic light up. Frustrating but down to build quality I assume.

I threw all the eBay ones in the bin

 

[vexorg]

Some cars with bulb fault detector do this, or pulsed currents, to see if the bulb is ok or not. A standard filament bulb wouldn't light, but the voltage/current is too low for it to come on. LEDs can turn on at much lower voltages.

 

[Scoober82]

You explained it better than I could @vexorg