2004 CLK number plate lamp fault

[E240estate]

Hi, got a problem I'm struggling to sort on the Mrs CLK. Number plate lamp warning came up on the dash, right hand. So I replaced both right and left bulbs. Right hand was a 10w and had caused some melting to the bulb holder and the left hand was the correct 5w but badly silvered.

Unfortunately did not fix the fault with the rh. Next I traced wires, have continuity from the rear SAM to the corresponding side of the bulb holder, and from ground to the other side of the bulb holder.

So with a new bulb in, I put the meter on either end of the bulb and noticed a momentary 12v on turning on the lighting switch, then back to 0v, or again when opening the drivers door it flashes 12v but does not light the lamp.

I mean it goes 0v to 12v and back to 0v within a couple of seconds.

To my mind I've eliminated the bulb and a physical wiring fault, bit stumped as to what to do next really

Any suggestions gratefully received!!

 

[Uky]

I had a similar experience with one of my rear lights when owning a CL500.
The SAM senses the current drawn and if it is in excess of a preprogrammed
value, it will shut down the supply voltage to the specific bulb.
Replacing the bulb shall be done with engine/ignition turned OFF
since this check is run during crank. (A new bulb will not turn on
once the overcurrent function has tripped.) Also the balance seems
to be checked if there are separate wires for left and right. Then both
bulbs must be replaced coming from the same supplier/batch.

You must make sure that the bulb used is within MB's specifications
as the overcurrent sensor otherwise will trip.

Another "ouch" which may not apply to you is water ingress from
the 3rd brake light, if it is located on the boot lid above the
numberplate area where the number plate lamps are located.
An aged gasket will allow water to leak in and then drip down
the interior and when reaching the number plate lamps cause
galvanic corrosion and rust in general. Since the rear lock
is rarely used (mfd by "Huf") the lock may very well be heavily
corroded. It can be repaired by c a r e f u l l y removing and
taken apart/cleaned/lubricated. Just remember the position
of each lock element.

 

[E240estate]

Cheers for this, needed to understand a bit more about what the SAM is up to. Think narrowed down to the damaged bulb holder - managed to find another one for £17 I'll post back if that does the trick