Aftermarket tail lights start up fault

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Gfromleeds

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Joined
Mar 13, 2024
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13
Location
Uk
Car
C220
Morning guys so this weekend I fitted a pair of aftermarket tail lights to my 2009 w204 c220 and everything went well . Untill I started the engine.
The right hand tail light continually goes through its "boot up" sequence, but only the right hand and only when the engine is running.with the engine off ignition on everything fine!
Anyone any ideas?
I would have thought if the light was faulty it would do it all the time, and if the bcm wasn't happy with it, both lights would do it, and or I'd get a dash warning?
Just want to be sure it's not something I've missed or done before I send them back. Cheers
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Just a guess.
It sounds like the new light is marginal on the current draw, and the bulb failure monitor system thinks a bulb is out.
You could probably test this by putting a five watt bulb into the sidelight circuit (possibly grey/red wire)
Also check that the ground/earth wire is good and there is a good connection at the plug and where it bolts to the chassis- brown wire.
W204 is known for poor earth connection on the harness plug.
 
Cheers ill check these
I just find it weird that it only does it when the engine is running. Does the bulb fail monitor not start untill after the engine starts?
 
It starts with ignition on and lights on. If it detects a blown bulb then it ‘pulses’ the circuit which may be why it is doing what it is.
Can’t explain why it only does it with engine running, but the voltage will be a couple of volts higher than without engine running - just noting that as a difference not a cause.
I’d deffo be checking your earths 👍🏻👍🏻👍🏻
 
It starts with ignition on and lights on. If it detects a blown bulb then it ‘pulses’ the circuit which may be why it is doing what it is.
Can’t explain why it only does it with engine running, but the voltage will be a couple of volts higher than without engine running - just noting that as a difference not a cause.
I’d deffo be checking your earths 👍🏻👍🏻👍🏻
I'll try it tonight with a temporary earth direct to the battery see if that helps cheers
 
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So ive had a play about with the multimeter and found there's a little bit of resistance between the earth at the plug and the battery but really its minimal so not sure if that's the issue.
I've also found that it's the car that's pulsing power to it so I'm a little relieved the lights arnt faulty (still not sure why there's no bulb out warning or why it continues pulsing it?)
Anyway I've temporarily run an earth directly from the battery to the plug, and in the 5 minutes I stood watching it, it started pouring down with rain, so I abandoned it lol.
But it so far didn't reset, but sometimes it doesn't do it! so I'm just gona keep an eye on it for now and look into it again at the weekend.
Fingers crossed 🤞
 
The earth pin on the plug for the rear lights is known for failing and causing the housing to melt. I've had to do mine recently, new bulb holder, plug housing and new pin for the earth wire.1000009780.jpg
 
The earth pin on the plug for the rear lights is known for failing and causing the housing to melt. I've had to do mine recently, new bulb holder, plug housing and new pin for the earth wire.View attachment 158000
Yea the conection looks a little weak considering how many bulbs are in the tail lights. it's not as bad as yours but there are signs it's been hot!
If it turns out that is the problem I'll make the temporary patch permanent with thicker gauge of the wire
 
No need to run a new wire back to the battery. Make sure the wire from the aftermarket light goes to a good clean point on the body in the boot.
The pulsing is just the way that the bulb and circuit is tested for continuity. Once it sees the correct resistance it should stay live all of the time that the lights are on.
 
Oh, and don’t be fooled by a low resistance.
1/2 ohm resistance at 3 amp current flow (sidelight, brake light, indicator) sat at the traffic lights - I’ve not included reverse lights, if you’re waiting to reverse into a parking spot, will result in a voltage drop across the plug of 1.5 volts - so 4.5 watts - as hot as a sidelight bulb. It would burn your fingers.
Any sign of getting hot needs to be dealt with as it will oly get worse.
 
The battery is in the boot on the c220 so the wire was only 18inches long!
It seams the negative/earth was the problem because it seamed a lot happier. so i took the earth direct from the light and bypassed the plug (where it looks like it's been hot before)and run it direct to the earth point by the rear bcm !
When I first powered it up it cycled through the start up twice and I thought "no not fixed" then it stayed on and it seams OK now! Been driving it 2 days now and everytime I checked it, it seamed to be working, but of course I don't know what's going on whilst driving so fingers crossed for now and I'll just keep an eye on it untill I can be assured its working all the time.
Thanks for all the advice
 
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