S320 W221 2006 starting issue
 

Hi everyone,
I'm having a really annoying issue with my S320 W221 basically the car does not start, When I try to start it, it cranks a little and stops, I connected power directly to the starter motor and it cranks fine and both front and rear batteries have enough power in them I checked all the fueses they're all fine and someone out there help me out please who might have come across this or similar issue.
I'm going to attach the link of the video that I have taken for better understanding.

Kany thanks

Regards

K.A

Welcome. to me it sounds like a ground issue

I went underneath the car and I attached a cable from engine to chassi still nothing. I even tested it with a multi meter the earth seems ok. Thanks

Doesn't sound like it is cranking at all. You're losing power somewhere. How many amps you getting at the starter when you crank?

​​​​​​It was 12+ amps, I checked that from above with the meter. You're right, it's like it wants to crank then looses power.

you should be getting way more amps than that under load. Switch your multimeter to V and measure battery. Fully charged battery should be around 12.6v. If it reads in low 12s battery is not charged.

Then measure at starter under load. Should not drop down past 9.25v(ish). If it can't maintain above that then battery is to blame. If it does then there is an issue with your starter or it's ground. Also make sure you checked starter wires and ground bolts. You could always be a swapotron and change starter to see if that fixes the problem.

This issue's still not fixed...
 

This issue is still not being solved yet. A friend of mine who is a mechanic diagnosed it and said apparently signal goes to the gearbox but don't give back from the gearbox so it is the ECU so I sent the ecu to (ecu testing) for a check and they could not test this type of ecu so they returned it back to me.
I took the car to "Broadley and holmes independent mercedes specialists" in Huddersfield, they've had it for few months now still nothing but they have tried diagnosing it and they're saying there is some coding issue in the gearbox also they've tried cranking it and it doesn't build fuel pressure. Does anyone know if I could buy another ecu of another S class but different part number and get it coded or reprogram it?
Thanks in advance.

This issue is fixed car's on the road
 

The car is fixed now by an ex mercedes dealer mechanic who is independent now. The problem was gear box had to be programed. When trying to start the car gear box ecu didn't know what gear it was in hence it wouldn't allow the car to start.
Car is on the road now... Thanks to all those who tried to help.
Kind regards,
Kam