Hey,
So I put a new fog light bulb. But I thought about using an LED one.
It works great, turns on normally as expected, except that it starts rapidily blinking when it's off. It's unrelated with other bulbs being used in the rear lights.


For what I understand the Body Control Unit behind the fuse box power the light bulb directly, and there's no separate fuse or relay for it.
What I am observing is that with the ignition on, the light blinks all the time. With ignition off, light stays blinking for a minute, and then goes off. It starts blinking again for a few seconds, and then goes back off again -- this pattern repeats itself forever. If I open the car, it stays blinking for a minute or so. And then the pattern starts again.

So what I am guessing is that when the Body Control Unit is awake the light blinks, when it's sleeping the light is off. Since the LED needs less power to turn on this is visible with an LED bulb, but not a normal halogen bulb.

Does anyone have any ideas if this is normal? or if something in the Body Control Unit needs a path to ground that doesn't have some other way?
Something like a FET is acting up?
I could just out a resistor and solve this, or use a normal bulb, but I was wondering if someone has seen this behavior, and if it "normal", or an indication that there's something less right with my Control Unit.

EDIT: Can anyone assist me in knowing where is connect in the Body Control Unit the cable that goes to the rear fog light?

Hi,
This is not a fault. It's a consequence of the particular LED bulb you have used and the electronic drive from the body computer. The body computer supplys a tiany current to the bulb when ether the ignition is on. With a filament lamp this makes no difference and the voltage across the bulb is small. It appears that the LED bulb you fitted has allows the voltage to build across a capacitor (in body computer or possibly lamp) until the LED turns on, flashes discharging the capacitor which then start charging again (called a relaxation oscillator). I suggest going back to a normal bulb or connecting a resistor (start with 100 ohm) across the lamp holder.

Robert G8RPI.

What I have noticed is that when the ignition is ON the LED is always flashing but not very brightly.
It comes on and off, only when the ignition if OFF.
And the time the LED flashes changes, for example if I press the unlock button in my key, it will start flashing immediately and for longer. If nothing "interferes" with the car it will come flash at regular intervals. So I think it only happens when the body computer is awake performing some sort of task. And not when in sleeping mode.

I will look into placing a resistor. I will see how what is the resistor of the LED bulb I have too.

Do you happen to know in the body computer what is the cabler connection and pin number that goes out to the rear fog light?

Leaving this here, might be useful for someone in the future.

I found out what is the cable, and what PIN it connects to in the body computer.

It conects to the socket on the left of the OBD 2 port. It's the first PIN from left to right, and up to down.
In the cable plug/cable harness is the hole number 31. Grey cable as expected.

The best solution here is to use the designed filament bulb which is super bright and easily sufficient as a good fog light that is going to last many hundreds of hours and most likely outlive your already aging car.

I am also saying this as a 63 year old who finds these awful LED lights that are now fitted on audis and mercedes as standard to make for incredibly distracting and difficult night time driving.

As you get older and the eye is no longer as clear these super bright lights spread across a very large amount of your vision and block out feinter lights either side of the light.


I think the car lighting rules need to be tightened up and enforced

Or maybe i need a cataract operation and just stop being some old fool complaining about the young uns???

But how do we explain the fact that this old guy with old eyes drives around on feint dipped headlights and can see just totally fine and some complete tosser is overtaking him with a billion lumens of headlight???

g8rpi said:

Hi,
It appears that the LED bulb you fitted has allows the voltage to build across a capacitor (in body computer or possibly lamp) until the LED turns on, flashes discharging the capacitor which then start charging again (called a relaxation oscillator).
Robert G8RPI.

Click to expand...

I think blinking is not the correct term for what I mean. I was researching relaxation oscillator and for what I have read, that's not what goes on.

I will add a video showing what I mean.
Sadly I don't have an oscilloscope, but using a multi-meter, I can see 3V on the rear fog light, when the ignition is on. And the expected 12V when the rear fog is on.

I suspect in the ECU, maybe by design, there is some current that enables the transistor that switches on the light bulb to leak those 3V.

crankshaft said:

I think blinking is not the correct term for what I mean. I was researching relaxation oscillator and for what I have read, that's not what goes on.

I will add a video showing what I mean.
Sadly I don't have an oscilloscope, but using a multi-meter, I can see 3V on the rear fog light, when the ignition is on. And the expected 12V when the rear fog is on.

I suspect in the ECU, maybe by design, there is some current that enables the transistor that switches on the light bulb to leak those 3V.

Click to expand...

On my MK2B, the ECU tests some of the bulbs and shows a bulb fault light for a failed or faulty bulb.

crankshaft said:

I think blinking is not the correct term for what I mean. I was researching relaxation oscillator and for what I have read, that's not what goes on.

I will add a video showing what I mean.
Sadly I don't have an oscilloscope, but using a multi-meter, I can see 3V on the rear fog light, when the ignition is on. And the expected 12V when the rear fog is on.

I suspect in the ECU, maybe by design, there is some current that enables the transistor that switches on the light bulb to leak those 3V.

Click to expand...

On most modern cars the ECU checks the lamp filaments by passing a small current through them even when off. This current won't light a filament lamp but can cause issues with LEDs

Robert G8RPI.

I didn't thought that it would check the bulbs even when it's off, maybe that's it.