Rear sensor turns it self on when in drive gear

Lambo_Pig

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hi,

Recently my W204 rear sensors seem to be activating sometimes while driving forward at any speed. Most commonly while on motorways. I can see it is sometimes turned on in the rear mirror and every now and then it makes a beep as if it is being activated.

Is there a gear selector sensor that seems to be loose that relates to the rear sensor?

I’m guessing it is just a sensor in the gear selector that tells the rear sensor it is in reverse gear when it actually is in a different gear.

Please let me know what you think should is and how I can get at it.

Thanks
 

JBell

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Could be a faulty sensor, you really need to have is diagnosed with STAR or a MBII to find the fault.

I have had to replace 3 of my rear sensors in the past 6 months as they fail but that gives you a solid twin red light in the display.
 
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Lambo_Pig

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Could be a faulty sensor, you really need to have is diagnosed with STAR or a MBII to find the fault.

I have had to replace 3 of my rear sensors in the past 6 months as they fail but that gives you a solid twin red light in the display.

Yeah I don’t think it is a faulty sensor because the system behaves as if it activates like it has gone in to reverse gear when it actually hasn’t. Each sensor is working fine.

Does anyone know how the parktronic knows when it is in reverse and the rear sensors are turned on? That’s where I think the problem is but I don’t know where that starts from.
 

sonic

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Mine work front & rear all the time under I think 10MPH.
 

JBell

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Does anyone know how the parktronic knows when it is in reverse and the rear sensors are turned on? That’s where I think the problem is but I don’t know where that starts from.

The only way to find out is to have it diagnosed on STAR or an iCarsoft MBII
 

LostKiwi

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It will be detected at the gearbox. Do your reverse lights flash on when the parktronic activates?
 

bembo449

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my S class sensors come on when I touch the brakes , not sure if this is a standard feature ? my 216 doesn't do it , have you noticed when they come on or is it completely random ?
 
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Lambo_Pig

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my S class sensors come on when I touch the brakes , not sure if this is a standard feature ? my 216 doesn't do it , have you noticed when they come on or is it completely random ?
It’s completely random. It happens in park, drive and neutral sometimes. When in reverse it’s on and working as normal. Also they shouldn’t be turning on when you push the brakes. That’s not an MB option.
 
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Lambo_Pig

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It will be detected at the gearbox. Do your reverse lights flash on when the parktronic activates?
No, the reverse lights only come on when the car is in reverse gear. They don’t turn on when the reverse sensors activate and the car is in any other gear except reverse.
 
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Lambo_Pig

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The only way to find out is to have it diagnosed on STAR or an iCarsoft MBII
But it’s not a fault, I highly doubt it’s going to trip an error code as it only seems like a loose connection where it reads the gear location for parktronic only. If the reverse lights activated at the same time I would think it would be a faulty gear selector, but it isn’t, it’s purely the parktronic system only
 

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Do you have a towbar?
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sonic

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Rears should only activate when in reverse gear... seems like you’ve got the same issue? Fronts should activate at low speed that’s normal.
My front & rears certainly work when stationary.
 
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Lambo_Pig

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Looks like I have found the culprit. It’s the drivers side rear tail lamp ground wire. I usually drive with my lights on, the other day I’ve noticed one of my rear tail lights is dimmer than the other, had a look at the back of it and sure enough it’s the faulty ground wire issue. Seems to be affecting the rear sensors as it arcs.

Tried driving with my lights off where possible until I can get some time to start soldering a more appropriate ground wire and in the process realised the rear sensors have stopped turning them selves on by themselves while in drive.

Pretty certain this is related.

Why did Mercedes skimp our on a 5 amp ground wire that is clearly not up to the job of the rear lights...
 
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Lambo_Pig

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FYI diagnostics tool never picked up any faults ‍♂️

I hope I’ve got to the ground wire problem without too much damage being caused. What a headache.
 

LostKiwi

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Why did Mercedes skimp our on a 5 amp ground wire that is clearly not up to the job of the rear lights..
A 5A wire should be more than adequate for rear cluster earth current if you have any LEDs fitted. Basic calculation is 5A at 12V = 60W of lighting. If all are bulbs then that's three bulbs at 21W, one for indicator, one for brake, one for reverse. Only sidelights would cause the current rating to get exceeded.
Given the age of your car I'm pretty sure you will have at least some LEDs so current requirement drops.
Also modern thin wall wiring can look very thin and inadequate but be rated far higher then you expect.
 
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Lambo_Pig

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A 5A wire should be more than adequate for rear cluster earth current if you have any LEDs fitted. Basic calculation is 5A at 12V = 60W of lighting. If all are bulbs then that's three bulbs at 21W, one for indicator, one for brake, one for reverse. Only sidelights would cause the current rating to get exceeded.
Given the age of your car I'm pretty sure you will have at least some LEDs so current requirement drops.
Also modern thin wall wiring can look very thin and inadequate but be rated far higher then you expect.
Nope no LED’s, 110w total glOnes, an extremely common fault recognised by Mercedes Benz USA who offered a recall on this. All the wire insulation as you can see was melted away, the corrosion of only the earth wire was due to the extra heat generated over time. When I opened up the light compartment it smelt of burning plastic.

Pretty poor by merc, you can do all the sums you like but seeing as this is extremely common, I saw over 10 c classes and e classes with one or both dim tail lights just now driving around in London.

German engineering yeah? ‍♂️
 

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LostKiwi

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Nope no LED’s, 110w total glOnes, an extremely common fault recognised by Mercedes Benz USA who offered a recall on this. All the wire insulation as you can see was melted away, the corrosion of only the earth wire was due to the extra heat generated over time. When I opened up the light compartment it smelt of burning plastic.

Pretty poor by merc, you can do all the sums you like but seeing as this is extremely common, I saw over 10 c classes and e classes with one or both dim tail lights just now driving around in London.

German engineering yeah? ‍♂️
Ok I don't see a melted wire there so the wire was up to spec. I see a failed connector and typical failure from a high resistance connection arcing and melting the surrounding plastic.
Is the problem the wire? No. The problem is whatever caused the high resistance at the connector.
 

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