[Tejas Ingle]

Originally Posted by Xehaust

Well, had a random not starting issue in the C (although its the newer 204) and after several complaints & trips to the MB service centre, they finally diagnosed it as a loose contact on the "fuel system" fuse. Its been starting up perfectly ever since.

The problem seemed similar to what you have stated, everything used to look fine except that there used to be absolutely no response on turning the key. The lights used to come on, there would be no warning chime, everything seemed in perfect order. Just that when the key was turned, nothing No click sound, no ignition, no nothing

Well then, I'll check on those lines too. Though the fuse seems to be OK I'll check other stuff related to the fuel system.

[aquavelva]

Tejas please check the wiring harness for the ignition system (starter) a similar problem had occured with my W124 and the wiring was the culprit.

[ajmat]

May be the engine immobiliser. Check if key fobs have been programmed

[jatinpatel]

it can be also a ECU problem or the solenoid actuator which takes the signal from ECU and grounds the starter connection.

[Tejas Ingle]

Originally Posted by aquavelva

Tejas please check the wiring harness for the ignition system (starter) a similar problem had occured with my W124 and the wiring was the culprit.

Have checked all the wiring using a multimeter. Found no open circuit or short circuit.

Originally Posted by ajmat

May be the engine immobiliser. Check if key fobs have been programmed

I will have a look at that. But do all other electricals work if the immobiliser kicks in? I am not sure about that. In this case, everything but the starter motor is working.

Originally Posted by jatinpatel

it can be also a ECU problem or the solenoid actuator which takes the signal from ECU and grounds the starter connection.

That is what I think is going wrong. It is a starter relay whose ground is controlled by the ECU. When this relay is operated by the ECU, the starter cranks the engine. But I have absolutely no idea why wouldn't the ECU operate the relay.

Let me explain this further. 12V is required to operate the relay which is the control voltage, and the relay in turn switches on another circuit which carries a heavy current (in this case, the starter circuit). The control voltage is already supplied to the relay but the ECU plays a trick with the ground connection of the relay. Instead of 0V, it supplies about 5V on the ground terminal. Hence the resultant voltage or the potential difference accross the two terminals of the relay is about 7V [12-5] which is inadequate to operate the relay. When the key is turned to start the car, the ECU reduces the 5V on the ground terminal to 0V, thus making the entire 12V available to operate the relay. Now here also, the 12V and 5V are available at the relay points, but the ECU refuses to control the 5V .

Excuse me for being so technical, but I hope I was able to communicate the situation.

[Tejas Ingle]

Fresh progress. I had removed the starter relay to check the voltages once again. After putting the relay back in its position I tried to start the car again. The starter worked! The engine was cranked for 2-3 seconds. I tried again, then it did not work. Now the same problem continues.

[jatinpatel]

replay can be culprit here.
Try the same with a new Relay.

and if still it persists try with a new ECU.

[Tejas Ingle]

Sorry for a very late update. Was busy with a lot of stuff recently. The starting problem of the C-Class has been solved. It turned out that the CAN connections to the ECU were lose. When the ignition key is turned to start the car, digital signals are sent to the ECU and not electrical signals. Then the ECU activates the starter relay to crank the engine. But since the CAN connections were lose, ECU wasn't receiving those signals and was just waiting for the start signal.
The car shows similar behaviour if the wires of the transmission control are pulled out (as pointed out by Taran) or if the transmission fuse is blown. This happens because the ECU cannot detect the P or N position of the transmission which is a prerequisite for starting in auto-transmission cars.