If your battery is 9 years old, replace it.

It just cannot be the battery. It must be a connection. I had this same problem years ago and it was the connector on the battery cable; it was cracked where it was attached to the battery terminal and sometimes would make contact and sometimes would not. Was like either a 100% battery or a 0% battery; nothing in between. That sounds like the kind of problem you have.

People getting rid of cars for simple problems seems to be becoming a common trend. Let's not jump to hasty decisions here!

If your alternator had issues, your battery would die as you ran the vehicle - faster with more load like lights and radio.

You are having the car just suddenly die. As you said, you waited 10 seconds and cranked the car back up. If your alternator were bad and the battery had died, this is impossible. Not just on a Mercedes, but on ANY car.

You have a main battery connection that is broken, loose, or corroded somewhere to the point that it is losing connection. My hunch is on the negative strap from the body to the battery since you have everything in the vehicle dying out.

Problem seems to be solved...
 

Thanks for the additional replies!!!

Well we aren't getting rid of it, if she ever wants to sell it, I am going to buy it back. It took me years to find a 300SEL and only let her have this one at the time because she needed a car. :)

I am happy to report the car seems to be ok now. Thanks to ALL, but especially those that said it sounded like a bad connection. I told him what I had read here and he checked it all out.

I cannot always get a clear answer from my dad, but he said the connections were not very tight on the battery. He cleaned and tightened the cable ends and drove the car about a total of 150 miles today on two different errands and no issues. Normally it would do it every trip out. The battery (despite being 9 years old) checked out "good", though it was a little low on water and he put some in, as it does have removable caps. He added some water in it as mentioned and the parts store said the battery was still good to go for awhile. I didn't know a battery would last this long, I guess it being a Mercedes brand battery makes a difference, but the parts store said it would be crazy to replace it now, it still had good voltage/cranking amps and showing no sign of failure at this time.

Looks like all is well for now... time will tell I guess as it is driven more and we will be able to see if this resolved the issue. It just seemed odd to me, as I would think the alternator would supply enough power to keep it running, but I guess this car is set up different somehow?

Thanks again. :)

The alternator by itself isn't designed to run the whole car, it's designed to charge the battery from which everything else is powered. Think of the battery as a surge cell.

If the battery is removed from the circuit, the alternator can and will power the car - UNTIL the load exceeds what it is capable of producing. Too many lights, too low of RPM (idle), fan motor, etc will push it over the edge and you'll suddenly die. On a gas engine power MUST be present for the vehicle to run, not only for the fuel pumps (on injected cars), but also for the spark and ignition control circuitry to work. If it were a 61x or 60x diesel you probably wouldn't have been stalling since no electricity is required to keep them running.

For what it's worth, the general premise applies to ALL makes and models, though GM's (and possibly some other brands) will immediately die if the battery is too weak or removed (they are not self-excited).

Glad to see this resolved quickly and easily. Could a faulty ignition switch cause these symptoms?

Sixto
83 300SD
98 E320 wagon

Even if the ignition switch were faulty, the headlights wouldn't be going black.

Would be a bad earth conection .may be to battery like you said, or earth to the body needs cleaning up .And clean the fuses in the fuse box, do this for secondary measure