Can't start the engine - need advice

[Hankiss]

Hello, greetings to all Members.
I am struggling with my C class W204 220CDI (2009). I can’t start the engine. When I turn the key to start the cluster goes black, can hear some clicks and nothing. Fully charged the battery. Removed the starter motor, checked it on a bench, solenoid and motor works OK. Put it back. Jump lead connected another (extra) battery to increase cranking power. Same problem. I short circuited connection 12V directly to the starter and it turned the engine. But when I do it with ignition key, same problem. The Relay is OK. I observed, after few attempts it drained the batter down to 10V. I tried to diagnose the car with Star Diagnosis but it cannot initialize the multiplexer ( terminal 30 has open circuit)??? I observed that there is no Ground “-“ in OBD socket pin 4 and 5. Someone suggest poor negative connection. Please advice if you have any suggestions. Thanks for reading

 

[ajlsl600]

So you took 12 v direct to starter and it turned the motor but possibly, lack of power elsewhere prevented the ecm from recognising a start attempt and therefore not energising the injectors check all ur earth connections, u say batt dropped to 10v not much going to operate on mb with 10 v. If u had it on star you prob have a bundle of low voltage related faults to clear as well

 

[Craiglxviii]

Check all Earth straps…

 

[M80]

The battery doesn't turn the engine, but it goes flat pretty fast.
Battery is goosed,
not enough oomph to spin the motor up,
but still goes flat trying on an engaged starter.

 

[mioba]

How old is your battery.
Take it out, place on a bench, give it a charge, measure the V and again after 24H.
If the voltage stays same its ok, if it drops somewhat its goosed