I've been searching this forum for almost half and hour, and still couldn't find the answer to my question, so here it is.

I recently bought an LED H4 headlight from ebay and wanted to do a side by side comparison of brightness with stock halogen bulbs. However, when I turned on my headlights, the LED bulb did not light up, whereas the stock bulb did. I took out the LED bulb and replaced it back with the stock, and both work. Switching sides (driver/passenger) or unplugging the halogen bulb makes no difference - the LED bulb doesn't turn on. Orginally, I thought the bulb was just damaged during shipping, but after hooking it up to an external power supply today, the LED bulb lit up. What's causing the LED bulb to not turn on when hooked up to my car (2012 Fit base)?

PS - I know that LEDs aren't bright enough for driving at night. I'm just playing with them for fun.

 

This sounds like the light socket is wired for reverse polarity.. You might unplug the lamp and probe the lamp socket with a meter and to verify the polarity us correct. Normal bulbs do not care about polarity. LEDs do care. I know of one Japanese designed GM vehicle with 2 element bulbs that is wired that way intentionally. They put +12 volts on the lamp all the time and select the bright / dim on the ground side.

I do not have a Honda Fit wiring diagram to look it to see how they do it.

 

Just do a stock H4 connector pinout and match it up. Clearly it's polarity specific and you have a ground on the stock connector plugged to a positive terminal on the bulb.

 

Thanks for the responses. I tested it out this morning, and the polarity is indeed reversed.

 

So how is the brightness compared to a normal bulb?

 

Horrible. It looks cool, but definitely do NOT drive with them at night.

 

Kind of a given. Even if you happen to find a bulb that is bright enough, it will burn up due to lack of heatsinking, AND your headlights are NOT meant for anything besides a filament bulb. You'd have to invest in some high power LEDs and optics to achieve this.