UPDATE 25 Aug 2021.
I have just replaced the starter motor on my C270cdi w203 because it kept blowing the 20A fuse... Then I could not get it to crank over.. just completely dead!!! I traced the the control Purple cable to the fuse/relay box and checked continuity.. no issues, checked all the relays with my relay tester (apart from one relay that had 2 extra large pins, I manually checked this one by putting it across 12v and metering the pins) all of the relays were ok.
I next removed the starter relay (half size black one near the 20A relay fuse ( one closest to the headlamp on the side of the relay box is) and connected the two contact terminals using a couple of flattened pin nails.. this cranked the motor over... confirming the cables to the starter were all ok.
Next I probed the relay coil connector socket and found that there was just 0.334v there when trying to start the car! I suspected dirty terminals where the relay plugs in to... but the car is 2005 and always been in central Spain... very dry and the relay terminals were very bright and clean!
I then checked my battery... thinking that maybe the voltage is low and the car is refusing to even attempt to start? The battery voltage was 11.6V... very low... but it still cranked the engine at what sounded like a normal speed earlier when I bypassed the relay terminals. I charged the battery over night and tried it again... Still no cranking and no sounds of the relay switching!
Just now I tried again by putting the gear selector in N (using the emergency remove from park switch that is under the coin holder with a screwdriver) AND it cranked over and started... YES.. I tried this several times and it always started.
If I tried to start the car with the gear selector in P then it would crank for a fraction of a second and refuse to any more..... Put the selector back to N and it starts!!!!
The gear shifter P position sensor switch is faulty!
DOES anyone know the part number for the switch/sensor or have an exploded diagram of the selector assembly that shows the switch/sensor or even a photo? I can not find anything on the internet.
Thank you in advance.

I have posted this diagnostics as there are a few threads on this, but I could not find any successful conclusions.... Please bear in mind, I have not yet replaced the switch as I don't have one, and I am assuming it is just the switch that has failed or worn out as the car has done over 300,000Km.

UPDATE

I managed to get a used gear shifter from the same model car (C270 CDI), installed it, and it still DOES NOT crank when in PARK. The status indication on the dash indicatews it is in P or D with either S or W. When I was removing and re installing the old shifter and new replacement shifter and trying different things, I accidentally tried to start it WITHOUT the data cable connected... the car acted exactly the same.. no crank in P, it would only start in N! The only thing difference was the status indicator was not working on the dash.

I also tried removing the Park to Ignition mechanical cable from the shifter and operating it with my finger... Still exactly the same, no crank when in Park, Fine in N!

So... the problem MUST be within the 722.6 transmission. IS there a separate Park sensor or switch on the gearbox? I have already replaced the conductor plate about a Month ago and metered the shifter solenoids... their coil resistances were within specification.

PLEASE can someone enlighten me on this issue?

There was a reply earlier regarding the camshaft position sensor??? If this was the issue, it wouldn´t start at all? Or even the Crankshaft position sensor... would prevent it from starting?

When the car is running... it drives no problem.

PLEASE HELP ME.
Thanks.

 

It’s is ***** to get to it’s in back of motor half way Down to thr right once you change that you’re good

 

There is a known issue with certain gear selectors. The cheap way to repair is to open it up and reflow the solder on the components using something like a toaster oven.


As for your starter motor blowing the 20A line, I remember reading there was some TSB where the fuse (52) is replaced with a 25A one for diesel cars. Look at this thread for reference.

 

If it was the camshaft or crank sensor, then surly it wouldn't start at all or run?

 

Thanks, Yes, I am aware of that issue... The starter motor was knackered... I metered it out and stripped it.

 

I have seen the videos on re flowing the solder in an oven at 250 degs. I have updated my post.. I got hold of another shifter, and exactly the same thing was happening.. But I also found the same thing happened when the data cable was not plugged into the shifter... the only thing that didn´t work was the dash gear status indication... so the fault must be in the 722.6 transmission.... Is there a sensor or switch in the tranny that detects the P position? I replaced the conductor plate about a month ago without any problems, also the O rings on the connector, even though there was no ATF leakage. The reason for replacing the conductor plate was that when the tranny was hot, it kept jumping out of gear or not changing up. I also metered the solenoids, they were within specification.