Is W212 Xenon ballast compatible with W211? What do ballasts actually do?

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Active Member
Jul 4, 2009

Looks like ballasts for W212 are much more abundant than for W211. Some (though not most) Ebay sellers list them as inter-compartible. (I.e. that the same ballast is listed as compatible with part number 5DC009060-00 (W211 ballast), 5DC009060-20 (W212 ballast) and a few others.)

What does ballast actually do? If it is just a voltage converter shouldn't they be compatible if sockets / shape look identical as they convert 12DC to whatever D1S bulb requires? Or do they have other functions in addition to voltage conversion?
I would personally think they are all the same if its for the same size lamp, their main aim is to limit the current to an acceptable level required to optimise the life and lumination of the lamp after it has ignited.
Unfortunately it looks like ballast in W211 is much more than ballast. On a working W211 it looks like with OBDII - like tool I am actually able to get some information from the ballast like voltages, rear axle feed, even whether the light is in the tourist mode.

On W211 with non-working light I can not get any information from it via the tool. Also the working light displays error messages about not being able to communicate with the non-working light...

Based on this it looks like it is really a control unit and not just ballast....

I now do wonder whether chinese copies that are advertised even with exactly the same part number will actually work properly and no even thinking of getting W212 "ballast" for W211.
Did you get anywhere with this and finding out if they're interchangeable?

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