3rd brake light

[Deleted member 138290]

Ok, so I’ve just purchased yet another w211 saloon 3rd brake light, I’ve sussed how to bypass the wiring in the boot so all I need to do is fit it.... so, how do you fit the damn thing without breaking it ???? Yes it’s easy to separate the led bed from the led top half but, how do you fit the top half onto the boot lid ?? I got the impression the led plastic needs to be taken off for the split in the middle ? But I’ve broken every light so far so I must be doing something totally wrong ???? Help please ????

 

[pmcgsmurf]

I'm not entirely sure what you are asking.

From memory once you get the liner etc. off then there are 2 clamps that you slide to the centre of the car then slip the light out and opposite when fitting.
If you are asking about splitting the light unit itself then that is a bit different as I don't think they were designed to be split easily. Sure I repaired some LED's on the old W211 and it was fiddly.

 

[Deleted member 138290]

Hi, yes I have pulled the 2 clamps and got the lens out. After some more research the lens half of the light should just drop in the top of the boot lid but seems the lights I keep ordering don’t seem to drop in and fit correctly ! I would take a photo but it’s currently terriential rain outside and I’ve already got drowned changing much needed front pads lol. Also i have noticed this morning that on full left steering lock fluid is spewing out thru the PS cap ???

 

[Deleted member 138290]

Ok so I’ve found that the thing striped wire from rear SAM to plug for 3rd brake has no power ??? The thick wire to the chassis makes my lighter tester illuminate very dimly ?? I don’t have a multi meter but seems surely that there’s simply no power to the 3rd light ?? Yet I’ve changed the SAM ?? Also I’ve bypassed a bit of wire from the n/s brake light and touched an earth and the led lights my street up lol yet pos wire from n/s light too pos of the connector plug for led and no lighty ???? Totally baffled me ?? Lol I’m not a sparky so I could have missed something very simple ?? Also when I attempt to fit this new led light I will position it and take a photo so hopefully someone can advise me. Cheers

 

[Deleted member 138290]

After yet more research I’m wondering for the sake of £15 wether to buy another wiring loom for the boot ?

 

[Deleted member 138290]

Took 3 photo s of led sat in hole of boot lid as it won’t fit due to the centre of the led being to big....... so either I cut the centre of led part out or there must be another led out there that is deeper in length ? This one is the 2.4 inch of which I’ve now had 3 of..... so what actual size should a 2003 w211 saloon be ?

 

[pmcgsmurf]

Can't you just check the part number?

 

[Deleted member 138290]

I did check the part number mate. And I got it off this forum ‍

 

[Deleted member 138290]

So, I’ve got the led fitted and bypassed via the n/s brake light but still get error on dash so still not fixed as yet ! Atleast all 3 brake lights work for now..... what an abs nightmare there early w211 s are for electrics...... still got the front o/s mirror module to change and a few other gremlins..... my diagnostic kit says open or short circuit regarding the led via the rear SAM but I’ve tried all the common issues and still not fixed so think I’m beat..... the alarm/ horn module comes up as faulty too tho my horn works as papped at a mate today lol..... they do say just disconnect and remove the alarm module ?

 

[Deleted member 138290]

So my multi meter came yesterday and although I’ve bypassed the led brake light it still comes up as a fault on the dash so thought I might as well see what’s occuring with the wiring/ fuses/ relays......

 

[Deleted member 138290]

Multi meter red wire broke today..... and after testing the electrics by turning almost everything on with ignition on I got battery convenience facilities would appear on the dash after approx 3-10 mins and with engine running would top voltage back up...... so I’m beat so booking car into diagnostic in my local town and see what he says...... something is clearly draining the battery , but no idea what.....

 

[Deleted member 138290]

Fixed my multi meter so tried the positive stripped wire that goes to the 3rd brake light plug as I’ve stripped it down with being faulty. On voltage with brake pedal held down the meter shows 0.05 so I’m assuming there s simply not enough power for some reason..... checked continuity from the earth where it is normally connected to all the other rear light etc and tester beeped so not an earth issue I’m sure ?? So why could be stopping the power to that stripped wire ?? Any ideas would be very grateful ! Also I have the radio issue of coming on with the button when the ignition is off so there s battery drain straight away...... also have issue with driver/ passenger seat memory, I’m assuming means they too are not shutting off ?? Disconnected the horn siren today too as that was giving an error code.....

 

[pmcgsmurf]

Bear in mind on some of these cars when the controller detects a blown bulb it stops supplying power to said circuit.
Usually a stop/start via the key is required to get the necessary circuit to try again and so on.

 

[Deleted member 138290]

All the bulbs are fine, the led just randomly stopped working on its original circuit but everything else bulb wise is still ok ?? Tomorrow in daylight I’m going to put a wire from switch on pedal to the live and see if it works.... I’ve read on here somewhere that a member tried this and never had any problems from them on lol....

 

[Deleted member 138290]

When you say stop/ start I’m a little confused as my merc only has key/ ignition and no stop/start button.....

 

[Deleted member 138290]

Well bypassing a wire from the brake switch itself doesn’t resolve the fault yet a member on here cured theirs, unless I misread what they said ?? So still stuck with rear Sam/ open or short circuit to led brake light..... aaaaagh..... wish I could follow the 2 wire s from the switch but they are well tucked away/ wrapped up. Same with the boot from the Sam....... time to spend £36 on an auto car diagnostic for him to hopefully tell me exactly what’s at fault and how easy/ hard it will be to fix....

 

[Deleted member 138290]

Found this diagram via another forum posted by one member for another.... I can’t work it out, can anyone explain it please as the member that needed it cured his problem ! Thanks

 

[Ted]

That suggests to me that the third brake light wire is a black/pink cable coming from the rear sam pin 22
It is not protected by a fuse as such but electronically, almost certainly by a current sensing foldback circuit. That circuit is probably also used to indicate the unit (third brake light) or circuit failure.
If it were me, I would probe pin 22 with a testlamp - around 5 watts with the brake pedal depressed.
If the testlight illuminates then this would suggest the sam (and associated brake light switch circuitry) are ok. I would then trace with the testlight from there to find the break in the circuit - with the wiring around any moving parts e.g. the boot hinge being the most likely culprit.
If there is no output from that pin AND the brake lights illuminate, I would suspect the sam - practically the only way to test this is by substitution.
If you do find current being supplied to the light unit, it is then worth testing that there is a good earth.
Just fyi, my own experience of testing a light circuit on a recent Mercedes is that you will see the circuit ‘pulse’ if there is no load (bulb/led unit) connected. My assumption is that this is how the circuit detects if a load has been re-established. Not easy ro detect on a digital meter, but more obvious on an analogue meter or my favourite - a testlight.

 

[Deleted member 138290]

Hi, all the rear lights run off the same earth all behind the n/s boot arm as you will already know . That thick wire that is common for being broken by the boot arm I have already sorted as it was broken and had been badly previously repaired. I have also tried another SAM which didn’t cure my problem....... I did have a test light, 2 infact lol but found that the bulb blew in both after just 20 mins use and I’ve been unable to find a replacement bulb but for the sake of a £5 I will get another tomorrow although if I remember correctly I did test the stripped wire from the pin 22 and it didn’t light the bulb but the earth with all the rear bulbs connected did light the bulb but very dim...... going back to the spare SAM I tried, I’m actually thinking that although it didn’t seem to need programming to my car as all but the 3rd light worked that maybe like you and other have said, it may still need the fault code clearing and will maybe sure my issue ? As I don’t think I tried clearing it last time so I will give it a go tomorrow, thanks for the info/ advice

 

[Deleted member 138290]

I have bypassed power from the n/s brake light and earthed with a wire from the boot lid catch just so all brakes lights work for now... also to mention that I know the heater booster and audio gateway are throwing up error codes as not shutting down and are causing battery drain but I don’t think the 2 are related....