Apologies as I've searched a fair bit...didn't find much...though I'm guessing its a common issue.

I only have 1 key...it's the original for my 1999 E300TD.

Occasionally I will insert the key and sometimes the immobilizer is lazy...but even when I hear it clickover..occasionally I will turn key to position 2 and then to start...and get a no crank.

Remove the key, reinsert, and it will crank and then start.

Before I parts cannon this with EIS, Key, Gear Selector Switch, K40...wanted to see what folks think...or if there is a diagnostic way to approach testing pieces. Car runs flawlessly other than this little issue that while never leaving me stranded...makes it so the wife is nervous to drive the car if needed.

 

I would recommend your first action to get a spare key. You will have a spare in the event you lose your current key, and you can also rule out whether your original key is the cause of this problem.

 

Your "original" key is 26 years old!
Obtain a spare key now. If it works virtually flawlessly, the problem is the end of your current key fob is turning opaque due to the ageing of the plastic. If the Infra-Red diode's light cannot be transmitted through the opaque plastic, the EIS will not release.
You can use your cell phone camera to see if the IR diode is transmitting. Multiple posts on this in the archives.
Search!

 

Good point about the plastic. You can also order a replacement key case online and swap the guts into it.

 

The W210 will start even if the key fob battery is dead. It does not need power. So, not sure if checking if the IR diode is transmitting or the state of the plastic is relevant.. It could however very well be a key fob issue. Maybe a bad connection or component inside the key fob?
Key fobs can be expensive from dealer, but I seem to recall that there are vendors who can copy an existing one without breaking the bank. I can't imagine having just one FOB. You are absolutely dead until car gets towed to a dealer and they get you another one for many hundreds of $$$$!

 

Have you seriously searched on this issue?
Multiple posts on an opaque plastic front end of the key fob blocking the IR data transmission. Simple toothpaste is one of the abrasives used to remove the degraded plastic.
The W210 used IR transmission to a receiver on the door handle so that depressing the "Unlock" button would unlock doors, and open the windows and sunroof. Depressing the "Lock" button would close the windows and sunroof. Dead batteries prevent this.
Purchase another key fob. From where is your choice.
I'm not going to spend time explaining the DAS3 system. Read the attached document. I suggested the easiest (not the cheapest) solution. The batteries in the key fob have nothing to do with the EIS; the batteries are for the door locks. Again, dead batteries in the key fob have nothing to do with the EIS (Ignition switch). Use the mechanical key, open the door, put the key fob in the EIS, start the car. Ask yourself, from where does the power come to enable the key fob to communicate with the EIS? Why is there an antenna inside the key fob?
Read the attachment. Start at page 59.
Let us know the solution for others having the same issue.

 

He is having trouble starting the engine, not opening the doors!

 

We have owned our 98 E320 for about 27 years.Have had Fobs malfunction or fail several times. The fob may not be your problem, but because it does start the engine intermittently, there can't be too much wrong if this is a fob problem. By the way, does the FOB work for opening and locking doors and trunk?

The Fob and the slot it fits into on the dash can wear. That slot can also get dirt in it. For a start, I would clean that out best as you can. Then experiment to see if there is a sweet spot where the engine does start. The way the "switch" works, is that you turn it to the right, the engine starts and the fob automatically springs back. Our's has at times got sticky and diid not spring back.

The other issue I once had, was more difficult to correct. There is a coil inside the FOB that acts as an inductor for battery-less ignition. This allows the car to be started even if FOB battery is dead. The coil is a common failure point especially after fob is dropped or is just old. I have repaired mine, but it is not something I would recommend unless you have a spare.

First step is to try the cleaning AND get a spare FOB made. If that works, then the old one can be repaired. Often by the same shops that make the replacement fobs.

At same time, check your fuses. Especially the one for the ECU. If still good, just the pulling and re-inserting can help!

 

Read the technical documentation on DAS3 and the owner's manual. You missed my point. You got very close to the correct understanding in your first post.
-Key fob in EIS. No IR diode, no start. (I have a post on this in the W211 AMG sub-section.)
-Key fob in EIS. No power to the IR diode (via the induction antenna), no start.
-Key fob in EIS. Good IR diode and power to IR diode, "hand shake" between modules enables the start signal. Batteries not required with key fob inserted into EIS.
-With batteries, however, it is very easy to confirm the IR diode is working. If the IR diode unlocks/locks doors and while opening/closing all windows/sunroof, the IR diode is good (this requires a minimum 3-second depression of the open/close button while pointing the key fob at the door handle's IR receiver. Hold button until all windows/sunroof are open or closed.) So, that's one test of the diode there. The other test of the IR diode is to observe the end of the key fob with a cell phone camera. With the key fob out of the car, you need batteries in it to check the diode.
-In the preceding case, one needs to read the owner's manual to know about this function. OP, it is possible the car does not have this feature. If the car does, easy check to perform.
-Nonetheless, if the plastic tip of the key fob is turning opaque, it attenuates the IR signal and can cause an intermittent start condition. Several posts on this on the forum.

A second key fob greatly simplifies the troubleshooting process.

 

I searched for intermittant no crank and intermittance no start....didnt find alot on that search...as I wasnt sure if it was a key, EIS, gear selector etc issue.

Thanks for the info. will begin to read what you sent over

 

-Shifting the transmission into gear includes the brake light switch. It actually contains two switches; one for the brake lights and the other to release the lock on the shifter so you can put it into gear. This, however, would have nothing to do with your intermittent start issue.
-One potential issue is a bad solder joint or bad connection to the EIS. Even I don't want to have to pull the EIS out of the dash to troubleshoot.
-Could also be an intermittent relay in the front SAM or a bad connection on that circuit. This can be verified with a new relay and, if necessary, a manual switch that replaces the relay. Just remember to flip the switch to "off" once the engine starts or you will destroy the starter motor.
-Potential intermittent starter solenoid issue should be considered.

That is why I'm trying to point you to start with the easiest troubleshooting techniques. If cleaning the end of the key fob doesn't work, and cleaning the IR window in the EIS key slot doesn't work, the next step is a new key fob. Expensive, yes.

FWIW, if I'm looking at a Mercedes and the owner only has one key fob, I'll drop my offer by $500 immediately because now I have to go to the trouble of obtaining a replacement key fob.

If you can find a good diagnostic system it will make your job easier by showing you which circuits are enabled or not.