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Brake light

1K views 5 replies 3 participants last post by  503865 
#1 · (Edited)
Replaced a faulty right brake lamp bulb. Tested it and it came on when brake pedal was pressed. And then..... two malfunction messages came on 'Tail light, right' & 'Tail light right. Substitute lamp on!' Everything seems to be working but these messages wont go away. Will all of my customised adjustments eg seat memory, lighting cluster etc disappear if I reset settings?
 
#2 ·
It's easy to do. I've done it a few times myself. You may have damaged or mislodged the offending tail light bulb when you fixed the brake light. (Can't remember if brake and tail lights are separate bulbs or one of those dual filament jobs). Getting the taillight inside assembly back in place can be really awkward and it's easy to damage / mislodge a good bulb when you do. Worth checking out before you do any resets and AFAIK doing a reset won't clear a bulb warning anyway.
 
#3 ·
Bulb operation is controlled by the rear SAM, which can also detect malfunctioning bulbs based on their current draw. As bulbs age, their electrical properties change. I am not sure of exactly why this happens, but quite often when installing a new bulb, the older bulbs will then register as out. I think it's the electronic circuitry within the SAM doing a comparison of the draw of the new bulbs vs. the old one. Or, maybe it's some electrical feedback from the new bulb. Generally, when I have one tail light bulb go out, I just replace them all. Yes, it costs me a few extra dollars, but if I don't, I get these messages or, within a few weeks, other bulbs go out, too.
 
#4 ·
^ Tungsten filaments break down as they slowly get eaten away by the amps going through them. Which is why incandescent bulbs ultimately fail. Consequently the impedance rises. The detector probably trips when the impedance rises above ? Ohms. Think about it - a blown bulb has theoretically infinite impedance.

I too have the BTDT for replacing all bulbs in a short space of time.

FWIW Autozone et al have many of the bulb listings for the CLKs wrong. Best to jot down the old bulb numbers and go in with a shopping list. And before anyone says it you do not need to use kosher MB bulbs any more than you need kosher MB air in your tires!
 
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