Mercedes C350 coupe 2011 ashtray bulb not working!

MercC350Coupe2011

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Mercedes C350 coupe AMG sport 2dr 2011
Hi everyone,

I am really struggling to make the ashtray bulb to light. I have no idea what is wrong with this system. The bulb is working ( I have checked it in the 12V socket) even though initially I thought that was the issue and I had ordered new bulbs.

There are 3 cables coming to the ashtray mechanism, two thick ones and a thinner one. The two thicks ones are obviously for the 12v socket and the thinner one is for the bulb. I have removed the ashtray mechanism in order to measure the voltage on the coming cables. I have measured the voltage between the
two thick cables and I got a voltage of 14. I measured the voltage between the thinner cable and one of the thick ones (which is obviously the groung to both of them) and it is around 11.5. The lumb turns on with the thick cables but not with the thin one. What am I missing here?

Can someone let me know when will this bulb light on? When the vehicle is turn on, lights on and the ashtray lid on?


thanks in advance and really looking forward to hearing your feedback because it's so frustrating little issue :)
 

zipdip

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Well I always have trouble sorting car electrics,but could the small wire have a break in it,have you tried moving it to see if it will connect the bulb for a second
 
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MercC350Coupe2011

Active Member
Joined
Sep 4, 2020
Messages
98
Location
United Kingdom
Car
Mercedes C350 coupe AMG sport 2dr 2011
Well I always have trouble sorting car electrics,but could the small wire have a break in it,have you tried moving it to see if it will connect the bulb for a second
I have found the issue by using some trial and error and a multimeter. Someone has messed up the wiring in the factory if not later. As I said there is a common ground and two positive terminals one for the socket and one for the bulb.

With the engine on I have measured the voltage between the common ground and the socket terminal to be 14.4 as expected and between the ground and bulb positive terminal to be 13.6 again as expected.

However, somehow someone swapped the positive and ground terminals for the socket before the ashtray mechanism (the brown and red cables). That means that in the socket the termainals are swapped (so there is no issue of course) but in the bulb there two positive terminals amd not the common ground!

Do you know how can I swap the red and brown cables without cutting them or breaking the plastic white socket?

ashtray.png
PS By the way something similar I have encountered in my desktop computer. They had swapped the cables for the mic and headphones in the front panel.
 
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