R171 slk200 battery light

[dave2007bc]

I have a really strange problem on my r171 slk200.

The battery light comes on 10 seconds after startup.

Battery is good (12.6v)
On running and idle output is good to battery (13.6v+)
Ancillary belt is good
I've changed the alternator rectifier and regulator
I've checked all the fuses
I've plugged in proper Merc diagnostic equipment and it shows not a single fault

Anyone have any ideas of what it could be?

[Classy6]

Have you fitted an aftermarket/cheaper alternator/regulator?
Only use a genuine alternator/regulator. I cannot stress how important this is.