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Hi, my name is***** does not provide testing for the ignition module itself. Basically you just test for spark from the coils when the car won't start. If you have no spark then you make sure that the crank sensor signal to the module is good. Since you already replaced the crank sensor I would condemn the ignition module if you have no spark when it dies.
Sorry for the confusion. The first thing you need to do is test for spark from the coils when the car won't start. If you have no spark I'd condemn the module.
If you find that you have spark from the coils when it won't start I would inspect the caps and rotors for moisture and corrosion.
Do you have the part number of the one from the S500?
I do specialize in Mercedes but without the part number I can't tell you whether it will interchange or not.