Symptoms:

1) Insert key and the EIS makes it's usual Kla-click sound
2) Turn key to position #1 - Stereo Operates, Clock & Interface Illuminate, per normal
3) Turn key to position #2 (Run) - All dash warning lights illuminate and chimes normally
4) Turn key to spring loaded position #3 (Start) - Nothing happens. No additional sounds of any type.

Sub symptoms:
a) The shifter can be moved freely to any gear with or without key
b) When the key is in the ignition in the #2 (Run) position, the Gear Indicator on the dash correctly registers the proper gear as it's selected (P,R,N,D)
c) When the key is in the ignition in the #2 (Run) position, when shifting to R or D, the exterior parking lights and dash backlighting illuminate.

Battery is 6 months old, but I removed and tested it anyway. Confirmed good and fully charged.

No stored codes present.

Prior to this No start issue, occasionally the vehicle would not shift from neutral back into either Reverse or Park, but if you let the vehicle sit off for about 30 minutes, it would then finally let you shift it back into Reverse or Park. The key could be removed while the car was in neutral.

Secondly, the power windows and rear view mirrors ceased operating, but every time I disconnected and reconnected the battery, both the windows and rear view mirror would both function correctly again, but only for about a day - then back to their non responsive state. For the past 8 months, I've only been driving this vehicle once a week for 30 minutes for battery maintenance, and to let it stretch it legs. I took it on a 1800 mile roadtrip to pick up a new rear bumper cover last September, and she operated flawlessly. The issue with the "No Reverse/Park access, & windows and mirrors, started the week after that roadtrip.

with out a proper scan its hard to say. The brake lamp switch might be a good place to start. Cold be cps, bad key, ignition switch among other things.

A bit of an update for future searchers attempting to fix a similar issue:

1) Since the EIS would not let the key turn if a successful mating wasn't made, I've eliminated the Key/EIS as the culprit.
2) Since the Dash correctly identifies the correct gear, I don't thinks it's a TCU issue.
3) I replaced the brake light switch about a year ago, but if #4 doesn't solve the issue, I'll install another one just to eliminate it as a possibility since it's such a cheap potential solution.
4) This led me to the K40 Module in the right hand side fuse panel in the engine bay. I removed and inspected the relay and then jumpered circuit 87 & the car will start and drive.

So I'll first pick up a new starter relay on monday, and if that doesn't solve the issue, I'll remove the K40 Module for furthur inspection and potential replacement. I'll post the results to assist future searchers...

Thank you for posting and as usual, another great diagnosis and troubleshooting effort.

Fortunately, the solution was 'old school', involving only the power supply to the solenoid / starting motor.

The subject relay is the same multi-purpose relay, as used for the W208 fuel pump, the cabriolet hydraulic pump etc.and only a $6 part.

When switching high currents, the relay's 'make, break' contacts produce a spark, that in time, results in contact pitting, which then becomes a resistance to providing the required current to operate the motor.

Note: The engine immobilizer, shift control module and brake light switch, could have been checked as OK, by just measuring the ECU signal voltage at the relay's actuating coil terminals.

Had similar issue with BMW 328, Sometimes it was even hard to pull the key out and shifter was freely traveling PRDN. End up replacing shifter assembly and problem went away. Might be the same with Benz.

Apologies to all for not posting the final solution as I said I would. I don't get to drive the W208 much these days, and most of my automotive troubleshooting has been devoted to the R230 over the past couple years. "Noud" was correct. Ultimately the issue was the floor shifter. After replacing the shifter, all the issues above have been eliminated.

My 2000 CLK320 had a no crank no start issue. I would spry CRC electrical cleaner in the key hole and then it would work fine. But now the issue has become worse: the srs light if flashes very dim, the turn signals and wipers do not work, and the cigarettes lighter moves between 14 amps and 0 amps!! I can see the voltage change while i wiggle the key. Is this my EIS? or should I start checking my SAM modules?

having the very same problem with mine right now, going to hope it's that cheap $6 part tho

i too am having the same problem. my key broke, lost the spare and 6 months later i had a key made finally and it wont start the car. same symtoms. the key unlocks the steering and everything turns on/lights up. the battery is fully charged, starter is good. however the car wont turn over or start. after the initial clicks and such after turning the key to position #3 theres nothing. when i then turn the key to position #4 theres no clicks no sounds, no nada. ive been searching in vain to find a starter relay/fuse or something that exists between the eis and the starter.. finally ive come across this thread after about 3-4 days of trying to figure this out. did anyone else replace the shifter and fix the issue? i recently had to bypass the shift lock to move the vehicle, could i have inadvertantly damaged something inside? i too was really hoping it was just a $6 relay i was having a hard time finding. can anyone tell me exactly where it is cause im having a difficult time finding it. maybe im just missing it. any input would be greatly appreciated
thanks!

Hey did you end up fixing it?

When I jumped the wires in where the relay was now it won't do anything any ideas

You will probably need to type in more than 1 sentence to get a response.

Check to see that your break lights come on when you step on the pedal.