Rear bulb failure warning but the bulb is still working

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MB Enthusiast
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Mar 4, 2015
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Location
Cheshire
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2009 W204 C180K
I've been getting an intermittent error for a rear light bulb failure but the bulb still works.

On inspecting the bulb, it's somewhat blackened so I will replace it with a new one, in fact I'll replace all the bulbs with signs of blackening as they are no doubt the originals from 2009.

My question:

is the bulb monitoring circuit sensitive enough to detect a blub is close to failure. I presume the circuit resistance or current draw is monitored and a bulb close to failure will have a higher resistance than a new bulb. I was able to confirm that by comparing the resistance of a blacken bulb with a good one and there was a couple of tenths of an ohm difference.
 
Just change it for a good quality bulb and I think you will have it sorted. Yes the system will pick up the change in resistance and throw the warning.
 
Just to close the loop, a new bulb cured the problem even though the old bulb was still working. So the bulb monitoring really can detect a bulb that is close to failure.

For information most of the rear bulbs on the phase 1 W204 are the same type P21W. In a pinch you could swap out one of the little used bulbs such as the reversing light or fog light and use it as short term replacement for a failed rear/stop light bulb. No tools needed and it's a very easy 2 min job.
 
I've been getting an intermittent error for a rear light bulb failure but the bulb still works.

On inspecting the bulb, it's somewhat blackened so I will replace it with a new one, in fact I'll replace all the bulbs with signs of blackening as they are no doubt the originals from 2009.

My question:

is the bulb monitoring circuit sensitive enough to detect a blub is close to failure. I presume the circuit resistance or current draw is monitored and a bulb close to failure will have a higher resistance than a new bulb. I was able to confirm that by comparing the resistance of a blacken bulb with a good one and there was a couple of tenths of an ohm difference.
Check the connector plug at the bulb holder, earth wire usually burns out
 
Check the connector plug at the bulb holder, earth wire usually burns out

I did thanks and there was no sign of any problem at all otherwise I would have added a supplementary earth wire.

I've no doubt MB undersized the earth wire and I read about it failing here but only rarely. I suspect the problem of a burnt out earth connection is more prevalent in the US where they habitually stand on the brakes for minutes on end at the traffic lights. That's something I never do, both because I'm aware of the weakness and out of sympathy for the driver behind me. The W204 rear light cluster has five P21W bulbs. It would be something of a challenge but if you managed to get them all illuminated at the same time, I've no doubt the earth wire would fail in short order.
 
I did thanks and there was no sign of any problem at all otherwise I would have added a supplementary earth wire.

I've no doubt MB undersized the earth wire and I read about it failing here but only rarely. I suspect the problem of a burnt out earth connection is more prevalent in the US where they habitually stand on the brakes for minutes on end at the traffic lights. That's something I never do, both because I'm aware of the weakness and out of sympathy for the driver behind me. The W204 rear light cluster has five P21W bulbs. It would be something of a challenge but if you managed to get them all illuminated at the same time, I've no doubt the earth wire would fail in short order.
I've done tons of those earth wire repairs during my time with Mercedes so it's a lot more common than you think !
 
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