Brake/Rear Light Weirdness

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AndrewOl

Active Member
Joined
Apr 1, 2017
Messages
820
Location
Romsey
Car
E350 A207
Hi,

My 2006 X164 has developed a fault with the brake/rear light monitoring system. I have changed the lamps and cleaned the contacts behind the lamps and on the multi-pin connector to the light cluster. The fault intermittently occurs on the left and right sides and disappears when I take the clusters out and fiddle on with them for a bit. When operational the rear lights work as they should. I don't get any other faults with the other lamps in the rear cluster.

It obviously looks like a bad connection somewhere but I am puzzled as to why the brake/rear light fault detection circuit is so sensitive. Is there a common component that could be causing this issue? The lighting system is obviously fairly complex using PWM to use the same lamp filaments for rear and brake. Does anyone know how the fault monitoring system works? I had assumed that the monitoring system sent a short voltage pulse out and monitored for current flow.

Cheers
 
I had a very similar problem with my JAG. Every now and then I would get a bulb failure warning for the O/S brake lamp, the bulb when tested was good. I tried various ways to find the fault thinking it was an earth fault and so on. Cleaned everything I could, even tried a remote earth direct to the chassis. Finally in temper I threw the bulb in the bin and broke it, replaced it with a new Hella one and the fault never came back. Cheap and nasty Chinese bulb could have been the problem, they seem to have dodgy resistance values and this will trigger the lamp failure even though the bulb appears to be fine.
 
Thanks Tricky,

Have been using Halfords but will try Osram as I can get them at ECP.

Andy
 
The rear lights on the 164 are a known problem. Something to do with the rear SAM.
 
That is exactly my understanding, that they use pwm to drive the bulbs when they are used for multiple functions.
Also (again my conclusion) is that the SAM sends short pulses out - you can see this on a small testlight if the main bulb is removed - the current is measured through a low value resistor, and if the voltage drop is correct, the bulb is assumed to be ok.
This all takes place in the SAM, and I assume that it is transistor driven with a current limiting foldback circuit.
The system on a Mercedes does seem to be particularly sensitive, and has been pointed out, cheap bulbs can give false positives for bulb failure.
 
I seem to remember reading about a service bulletin on these. Something about modifications to the wiring.

I'm not discounting the rear sam tho!
 
Here it is, dunno if it'll help but good luck! :)
 

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Mercedes appear a bit fickle IMHO when it comes to this, various things can cause this:

Brake Switch
High level brake light failure
SAM
Taillight Unit (Circuit board)
Chinese Carp Bulbs

Bosch do a range of Heavy duty bulbs if thats any good.. its designed to eliminate shock from tailgates etc..

One last thing.. I tend to "Hard wire" where I can, I had this on the Vito. When I bought it it has a tow bar installed. I asked if it was professionally installed and "Oh Yes, there's the receipt" He says.. so it came to the MOT and the inevitable happened.. brake light out, side light on, side light of, Fog light on. So, I checked the circuit boards and they were a little corroded.. new ones on and worse still. I look at the wiring and the dumbass had been in at it and butchered it to 3 core house cable. New meter age of cables ordered, shrink wrap for the joints then self amalgamating tape and a new loom was made, better than original and spliced in... it worked for a short time then came back.. and I think it was Sparkydoodle (?) on here (Neil) that said check my high level brake light - sure enough one LED was out throwing all this crap at me..:(
 
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What do you know. I stuck four osram lamps in from ecp and all seems well. Fingers crossed it’s that simple. I assume the Sam is under the sub woofer. Couldn’t feel any water. If it comes back I will take the sub out and check the connections to the Sam. Thank you for all your help.
 
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