W209 CLK rear foglight on when not switched on

[Adam3313]

Hi all,

I have just bought a 2004 CLK 200 Kompressor.
Love the car, such a great drive. One thing that I have noticed though and it really bugs me is that my left rear foglight comes on as soon as I switch the headlights on.
If I swich the fog lights on, the left one stays on and the right one comes on too. As soon as I swich them off, the right one goes off and the left one just stays on.
It's on even with the "follow me home" lights

I really want to sort it out, as it's not pleasant to the other drivers behind me.
I have attached the picture of it (it's obvious what I mean though anyway)

Does anyone have any suggestions, please?
Thank you

 

[bigasotonuk]

Hi,
Have a look at the bulb, and compare it with the side that is working ok, I think the bulb will be a dual filament bulb, an the bulb has been fitted the wrong way round so the wrong filament is coming on at the side light stage, or the wrong bulb has been fitted at some time.

 

[teddyk]

Hi,

Are you sure your car is supposed to have both left and right rear foglights?
My (albeit older) W210 only has a right rear one.
It may be the case that there was an intentionally unused bulb holder in the left hand side rear bulb cluster that someone has mistakenly put a bulb into.
Maybe someone with a W209 can confirm if you should have rear foglights on both sides?

teddyk.

 

[Alex Crow]

Wrong bulb or wrongly fitted is a really good guess.

Does the dash display any bulb warnings?

 

[Adam3313]

Thank you!

I will check the bulbs tomorrow during daytime.
The dash computer does not display any errors or warnings. I am not sure if this W209 is meant to have both fog lights actually. When I turn them on, the one on the right side comes on and is at the same intensity as the left one.

 

[whitenemesis]

Pretty sure there should only be a fog light on the driver's side but there may be a bulb fitted to the nearside, to be used as a substitute for a failure on that side?

 

[Alex Crow]

Pretty sure there should only be a fog light on the driver's side but there may be a bulb fitted to the nearside, to be used as a substitute for a failure on that side?

Quite correct, with the bulb possibly forced in the wrong way around.

 

[television]

Only the off side bulb fitted and most that I know of are not wired for the passenger side, it does sound like a man made fault wit the wrong bulb fitted to the near side

 

[EmilysDad]

...... with the bulb possibly forced in the wrong way around.

All the bulbs fitted on my car are single filament 21W even the stop/tail lamps. As far as I understand it, the CAN bus does its thing re voltage/current to simulate a 5W side lamp, so you can't fit them wrong like you can a twin filament 21/5W stop/tail lamp :???:

 

[television]

All the bulbs fitted on my car are single filament 21W even the stop/tail lamps. As far as I understand it, the CAN bus does its thing re voltage/current to simulate a 5W side lamp, so you can't fit them wrong like you can a twin filament 21/5W stop/tail lamp :???:

The bulb monitoring does the checking and sends the relevant data over the CAN bus, just a minor detail

 

[Alex Crow]

The bulb monitoring does the checking and sends the relevant data over the CAN bus, just a minor detail

As above, quite correct.

The brightness is controlled with pulse width modulation, which explains how the replacement light function can work.
A duty cycle of, say, 30% will give the brightness of a 5w lamp from a 21w lamp.
Much is controlled with PWM on cars these days, like turbos and EGR valves, sometimes carrying some current and sometimes just used as a signal.

For a while from the late eighties we used to have 'dim dip' front lamps coming on with the front side on some cars, lighting the dip beam through a big ballast resistor - not good as they would generate a lot of heat and prone to burning contacts etc.
I think the later W126 was the first I saw using PWM to modulate the brightness, and much better, but the heat sink on the controller still got fairly warm.

 

[EmilysDad]

.....
For a while from the late eighties we used to have 'dim dip' front lamps coming on with the front side on some cars, .....

I thought the idea behind it was good ie it prevented people from driving on just side lights

EmilysMum had a Hyundai with dim dip, it failed by leaving the headlights on when everything else was off. :???: I fixed it by unplugging the box of electrickery driving them

 

[Alex Crow]

I thought the idea behind it was good ie it prevented people from driving on just side lights...

Yes, seems to be something like that, this from Wikipedia...

"The dim-dip lights were not intended for use as daytime running lights. Rather, they operated when the engine was running and the driver switched on the front position (parking) lamps. Dim-dip was intended to provide a nighttime "town beam" with intensity between that of the parking lamps commonly used at the time by British drivers in city traffic after dark, and low beam headlamps; the former were considered insufficiently intense to provide improved conspicuity in conditions requiring it, while the latter were considered too glaring for safe use in built-up areas."

...but I never really understood it, and it was dropped from regulation pretty quickly...

"UK regulations briefly required vehicles first used on or after 1 April 1987 to be equipped with a dim-dip device"

LED DRLs do make some sense to me though, and Volvo were not so stupid way back when!