Ignition Tumbler or some relay?
 

My 1984 300 SD Mercedes Benz is giving me a problem that I never had before and I am confused if it can be from ignition tumbler or something else.
Here are the symptoms: when I go to start my car after waiting for glow plugs getting ready to start the starter does not go on even turning the key couple of the times. Then my only option to get the car running is to open the hood and manually close contact directly on the connector where the wires to the solenoid are connected. Doing this starter will engage and my engine will start.
One thing call my attention when it happen: this does not happen every time when I go to start my car. It can go for days without having this problem and sometimes it can go many times with this problem. Turning the tumbler to the on position all electric components of the car will work as supposed to work except car starter.
I do not know if there are some mini relay between tumbler and starter that can go bad and cause this problem. I know that all electric contacts on the connector near the fender are all OK.
Please give me your help to get this issue resolved.

No, I read there is no "starter relay", nor have I seen one, but check wiring schematics to be positive. All "starter solenoid" current runs directly thru the key switch and the neutral safety switch (on transmission). A bit strange since I think all U.S. cars since the 1960's have a relay.

Most likely you need a new ignition switch. Pop out the cluster (remove lower dash cover first) enough to see, pull off the switch connector (has a convenient pull tab), and with a mirror, you will see 3 flathead screws securing the switch to the metal stalk. It is much fun trying to turn those screws when you can't see them, and a stubby screwdriver barely fits (special factory tool?). A new switch is cheap.

Next time it happens, try moving the shifter to neutral and see if the starter engages.

As Manny suggested if moving the shifter around gets it to suddenly start it could be the Shifter Bushings and or the Neutral Safety Switch.

You have the Key, the Lock Tumbler that turns the innards of the Steering Colum Lock that at the same times turns the Ignition Switch at the bottom of the Steering Colum Lock. Between the Ignition Switch and the Starter Solenoid is the Neutral Safety Switch.

When you jump the Wire Terminal Block that is under the Hood you bypass the Neutral Safety Switch and any problems with worn Shifter Bushings.

If you end up replacing the Neutral Safety Switch look up how to line-up/index the Swich up properly when you install it.

As far as I know the 1st gen 126 and 123 are direct wired key -> NSS -> terminal block -> starter. No relays involved.

The second gen 126 and later 124s added a relay buried behind the cluster that interrupted the starter signal if the alarm had been triggered.

Ignition Switch R & R Tips
 

Like the others are suggesting...first make sure that the problem is not the safety switch / shifter bushing by trying to start in neutral, etc. Although the ignition switch R&R looks easy, it will test your patients.

The contacts in my switch were worn to where I had to wiggle the key to get the AC blower to work (even with new tumbler). I hear same can happen with starter contact within the switch.

Should you go the ignition switch R & R route here are a couple tips / photos.

1) No need to pull the key tumbler. But disconnect the battery.

2) When pulling the wire harness connector off the switch it needs to be "inched off" i.e. a wiggle at a time, or the plastic connector can break apart. It'll seem like it takes for ever (3 minutes or so) but it will come off. Don't try to grab it with pliers etc. 'cause you may crumble the connector & be in a heap of trouble

3) There are three flat head screws at 33* to one another. Like OP said a mirror helps get the picture but then its a feel thing. The screw head slot is narrow. I had to grind the flat head bit (see photo) so it would fit the slot. The screw is maybe 1/2 " long so it takes a while to back it out at 1/2 turn a try...patients, patients, patients.

3) A ratchet tool from sears ($3) allows for a little torque to be applied. I put rubber "o" rings on the flat head bit to keep it in the ratchet wrench. Screws are in kind of tight and I could not get the screws out with a cut down screw driver 'cause the limited space didn't allow me to get a strong grip on the screwdriver handle.

Overall took me about an hour...next time (if ever) 30 minutes.

The above procedure needs to be done with the key set to position 1.

Yes guys, the problem was the neutral switch. This morning I had the same issue and I did like I was suggested by mannys9130 and it worked great.
I never imagined that it was the problem.
I appreciated everyone suggestion.

Manny's suggestion means most likely that the NSS is fine and your plastic shifter bushings have fallen out. If so, you will hear rattles and feel the play. Many posts on replacing them. On my 300D's, I bought the special pliers but found it needed a larger OD backup washer to keep the bushings from splaying outward as I pressed. A grubby job so clean the side of the tranny first.