The only way you will see voltgage from the dist is using a very LOW A/C scale. Unplug the green wire from the Ignition control unit. Conect one meter lead to the center connector in the wire, and the other lead to the outer metal shell of the connector. Crank the engine and you will problably see less than 1 volt A/C. If you see anything on the meter, the pickup coil in the dist is problably good. If it turns out that the Ignition moduel is bad, get an MSD 6AL Ign moduel from a speed shop. That is what I did and saved myself much money. It works great, has a tach output that is compatable with your 500SEL, mileage increased , Power increased (not much, but some), works with the MB coil,etc.
84 500 SEL (307,xxx miles)