Eddie123

Hello.

I have a really strange problem with my rear fog ligth, on my 2001 A6.

It doesnt work, so I stripped down the lining, to get access to the bulb. I measured the bulb, and it looked ok. I also connected it straigth to a battery, and the bulb works.

I then measured the connections in the socket, and there was approx 13. V there. So that seems ok.

But when I insert the bulb, all voltage disappear. There is absolutely no voltage at the socket when the bulb is inserted. As soon as I remove the bulb, the voltage appear again.

Now, I am a trained electrician, and understand electricity, but this gets me baffled. Is there something in the circuit that can disconenct the voltage to the fog ligth ? And why does it happen ? I have tried with a new bulb as well, samw result.

All answers appreciated.

THank you

Vegard

Jerry L

Sounds strange, maybe change bulbs, could be some odd short or open created with bulb on the holder??
In checking Voltage with bulb in are you piercing the wire insulation or using another connection upstream to see if you have voltage?
Will stay tuned mine is out right now, 2001 A6 Avant

Regards

N_Jay

You have a high resistance in the source.
If the light sis switched electronically, I would suspect the module, if a switch or relay, I would suspect that.

Eddie123

Hello.

Thanks for answers.

I will try to find out if it is electroncally or relay controlled.

To Jerry: I measured on the contacts that goes to the bulb. I have tried with a new bulb as well, same result.

SloopJohnB@mac.com

V=IR….with high Mohm R to ground on your DVOM you'll see 13V. But if you put anything with low resistance in series ahead of the load (the bulb) you'll see voltage drop (IR) and concomitant no light. It's a lot like a bad negative starter cable to the starter…looks like 13v to the starter but the voltage drop across the corroded cable drops to 9v and you have a no start.

another poster is right, there is a large voltage (if not an IR) drop before the light, probably caused by a high resistance in the source or in the wiring from the source to the bulb. You can check IR drop by measuring the voltage drop from the source to the bulb with the bulb on (even though it's not lit). Anything over essentially zero is too much.