Wanted to share this strange experience that I had after changing my Fog Lights to LED's (CREE) from the stock Halogens. Sometime back, I happened to come across these Fog Lights (H8) and liked them as they had quiet a number of individual CREE's in them. Ordered them instantly along with a pair of Load Resistors of 8ohms/50W each to avoid the "CANBUS" error and fitted them immediately on receiving them. The change was tremendous and the lights had outperformed my expectation. All was good for about about 2 weeks when the LHS failed with CANBUS error and MIL on dash for the defective light appeared. Checked and everything seemed fine, there was power at the LED terminals and the Resistor did measure 8ohms still. So I assumed the culplrit to be the Load Resistor, for which I reduced the effective resistance by adding a 50ohms Resistor in Parallel. This did solve the problem and the light started to work again. Put in a similar resistance on the other side too just in case it failed and things looked good there after.

 

I had started to enjoy this new look that the lights brought to my car and all was well for about 2 weeks when the LHS failed again. Was very surprised this time as nothing could be wrong now as I had given very little room for error the last time I fixed the problem by paying utmost attention to all details including wiring, connections and everything in between. Removed the LED from car and checked it at home with a 12v power supply and the light lit up as it should. Went back to the car, swapped the terminals in the H8 holder and fitted the light back only to see it light up again! Very surprising it was to see the polarity change, but then never gave too much thought to it as everything was fine now and left it at that thinking the problem was with the LED.

 

But last week, the LHS stopped working again and then RHS a little while later. This time, when I checked the voltage at the H8 terminals, the polarity had indeed been reversed back!!! To fix this, built a bridge-rectifier with 4 Diodes (IN4007) which would supply a fixed polarity to the LED's irrespective of the input polarity. This has only fixed the problem to an extent as the lights have gone a lot dim now as there is about 1.5v of voltage drop at the terminals due to the bridge in there. Mind that I have not yet implemented the fix (bridge) in the car as I'm currently searching on the Internet for some "very" low-drop diodes to replace them and have found a couple, will be picking them up soon. In the meanwhile, is anyone aware of any setting that can be tweaked with a VCDS that will disable this auto reversing of polarity? I have currently disabled "Cold Diagnostics" in "Central Eletronics." to give it a shot, keeping my fingers crossed hoping this will fix it. If it doesn't, bridge is the only solution I guess and I don't want to revert back to the stock halogens at any cost now as the improvement over them has been tremendous.

 

Why is the system in this car designed this way to change polarity according to its own whims and fancies? Is there any logic/engineering behind this setup or is it there only to dissuade people from retrofitting any such stuff? Anybody else out there who has experinced this?

How do you know the fog lights don't work?  There hasn't been any fog in the last few weeks so why were the fog lights on?

It's unlikely that the polarity changed as most circuits are earthed to the chassis.

How do you know the fog lights don't work?  There hasn't been any fog in the last few weeks so why were the fog lights on?

 

Maybe not in Gloucestershire, but did you check the weather in Bangalore, India, where the OP is from? :rofl: :rock:

Maybe not in Gloucestershire, but did you check the weather in Bangalore, India, where the OP is from? :rofl: :rock:

Oops.  Didn't know that Bangalore had fog in July/August!  Average daytime temp in July was 28C and night time was 20C.  Is fog possible in those temperatures? A quick Google search suggests that Bangalore can have fog from November to February.

How do you know the fog lights don't work?  There hasn't been any fog in the last few weeks so why were the fog lights on?

 

I use the Fog Lights as DRL's, mainly in the twilight.

 

 

It's unlikely that the polarity changed as most circuits are earthed to the chassis.

 

Apparently not in this case. Trust me, It has changed the polarity on more than 2 occasions and continues to do so, that's the whole confusing part, why???

 

 

Oops.  Didn't know that Bangalore had fog in July/August!  Average daytime temp in July was 28C and night time was 20C.  Is fog possible in those temperatures? A quick Google search suggests that Bangalore can have fog from November to February.

 

It's more than a decade I last experienced fog here, but as I said above, I use the Fogs as DRL's only.

I use the Fog Lights as DRL's, mainly in the twilight.

 

 

 

Apparently not in this case. Trust me, It has changed the polarity on more than 2 occasions and continues to do so, that's the whole confusing part, why???

 

 

 

It's more than a decade I last experienced fog here, but as I said above, I use the Fogs as DRL's only.

Using the standard foglights as DRL's is illegal in the UK but probably not in India and also may be considered to make the car look 'cool'.

Using the standard foglights as DRL's is illegal in the UK but probably not in India and also may be considered to make the car look 'cool'.

 

Absolutely true, it's not illegal here and it certainly makes the car look 'cool'!