Mercedes Benz repair: 500sec no spark, correct diagnosis, fuel injection

Question
Hi,there is no spark from the plugs or directly from the coil on my 1983 500sec Mercedes. There is +12v onto the coil but no switchable earth I guess comes from the distributor or does the ignition control box control the spark. Any ideas will be very appreciated. Thanks

Answer
Answer:
You must understand the system for correct diagnosis.
Here are the links you need to study.

Fuel Injection
http://www.mercedesshop.com/shopforum/showthread.php?t=142406

No-start help for beginners.          
http://www.mercedesshop.com/shopforum/showthread.php?p=1076969