Mystery ! Bulb OK but non-working numberplate light, Mot in 3 days

martin_sv

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Hello everyone as per subject line plus a bit more after my investigations. Got the N/s numberplate light failure message on my R230 SL 2003. Bulb not alight but bulb tests out fine and lights up with 12volts. Checked the voltage appearing at the terminal when switched on. reads 0.4 volts, doesn't sound good. Checked resistance on assumed live terminal (LH) and earth terminal, 12ohm and 0.3ohm respectively. Certainly the earth seemed fine. ( its only a £5 multimeter) Checked out other working N.P. light same values. I thought I'd then check the voltage at the working light when switched on read The same 0.4volts. What?. It gets worse replacing the working side bulb and you've guessed it---it's now not lighting up.
Any ideas? Is the second bulb now not working caused by something annoying like the CAN bus system shutting off voltage to the terminals? Oviously I need to fix these major items absolutely vital to road safety so I can pass my MOT :lol:
 
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television

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As these bulb holders and contacts are subject to humidity get some non drying switch cleaner (Maplins) and spray the contacts of the bulbs, and this usually fixes the problem.
 
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martin_sv

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Thanks Malcolm, I've already given the contacts a good clean up. What do you think about the voltage readings? With other cars I would expect to find 12 volts at the live terminal when the lighting circuit is switched on. I have juat a miserly 0.4volts--can't be right can it??
 

television

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If only one lamp faulty then the connections on the faulty on, if both lamps then could be broken wire in the hinge area.

The bulbs will only light correctly with 12volt on them
 
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martin_sv

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Solved---well maybe for now

Well, I went back to re-address the problem and fiddle some more and lo and behold, I put both bulbs back and they're now both working!. I checked the voltage on the live terminal whilst the bulbs were alight and it was now showing 12 volts as expected. I then removed one bulb and agian checked the live terminal and guess what-- only now shows 0.4volt between live terminal and a (good) earth. So it seems when the circuits go open for any reason ie bulb failure/removal of bulb then the system shuts off the voltage to it. It really does seem a bit like overkill to me but no doubt mercedes had their reasons.
I've no idea how long this state will now last but won't be suprised if one bulb goes out and I get the warning message again:( I just hope it last till thirsday and my MOT!
 

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Sounds simply like any/every series circuit with a hole in it!
 

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I notice that my S320 does the same when a bulb fails, you have to turn the lights off (and the ignition) then back on when you replace the bulb, maybe mercedes did it to stop a bad connection causing the bulb to flicker or flash!
 

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I notice that my S320 does the same when a bulb fails, you have to turn the lights off (and the ignition) then back on when you replace the bulb, maybe mercedes did it to stop a bad connection causing the bulb to flicker or flash!
The more cynical amongst us might say they did it to ensure we took it to the dealers :Oops:
 
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martin_sv

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>>Sounds simply like any/every series circuit with a hole in it! <<

But the lighting cicuit would never be wired in series -----imagine it, tail light fails and all the rest of the lights go out!!:lol:
 

television

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I have just woken up to what has been happening I think, when a bulb fails the car can switch to a substitute lamp
 

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I think maybe the voltage is shut down to prevent fire if the circuit goes open circuit due to a broken wire.

As Malcolm said the wire loom at the hinge area is prone to broken wires on many models, this can cause intermittent faults with the brake and number plate lights.
 
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martin_sv

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I think maybe the voltage is shut down to prevent fire if the circuit goes open circuit due to a broken wire.

As Malcolm said the wire loom at the hinge area is prone to broken wires on many models, this can cause intermittent faults with the brake and number plate lights.

I now think that's more than likely to be the case. I gave the main loom that runs to the boot lid, I assume its thick covered batch of wires actually running up the rhs boot hinge-- a wiggle and one of the nplate bulbs did flicker just a little but clearly that shouldn't be the case. I seem to be3borderline for full time functionality at the moment so out of interest what approaches can be used to fix this part of the loom or is only full replacement the option? Anyone know where any multiconnectors reside for this section?
thanks
 

television

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Soldering and sleeving are the best options, also try and ease any tension on the bunch of wires, the trouble is they will be set in their ways by now.

If you do solder and sleeve, do fit a splint over the damaged area or it will soon fail again
 

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Replacing the loom is not necessary, simply remove the boot side lining to expose the wiring near the hinge, then flex each wire in turn until you find the one that causes the lamps to flicker.

Once the break in the wire is located you can bare the ends and either solder or crimp the ends together or even use a small connection block.
After testing your repair out, insulate the joint either with sleeving or insulating tape.

After repairing mine I wound some spirabound around the loom near the hinge and tried to improve the routing to reduce the bending.
 
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martin_sv

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Thanks to you both. Just so I don't end up breaking anything else---what is the best way to remove the (rhs I assume)side panel. Wil I need to remove the cloth sheathing to expose individual wires?
thanks
 

television

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Yes you will, best to cut it, then when you put it back tape it up at the cut, do be sure though that is where the fault is
 

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On my W211 it was the left-hand side panel I had to remove (looking into the boot).
First remove the carpet in there then with a flat blade screwdriver prise the centre out of the round retaining press studs/clips. You will have to bend the top of the panel down a little to free it and remove it.
 


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