300SDL, engine turns over for 1-2 sec, then 'thud' sound and stop dead
 

Need some ideas to tackle this problem.

It happens a few times, normally in the most inopportune times. Turn the key, engine turns over for 1-2 sec then stop dead 'in-flight', nothing! I can get it started by doing the following:

1) Turning the key again. It will be OK 2nd or 3rd try.
2) One time my wife got stuck and it would not start with a number of tries. Jump the battery and still the same. When I arrived, it started at the first try ?? Car is afraid of me?
3) Wouldn't start this morning. Try it a few times and no luck. Put the car in drive and roll it backwards a little bit. Cannot roll it forward, curb is in the way. It started right away. It may or may be related with rolling the car.
4) Battery swapped from another car, same problem. So battery is ruled out.

Wife does not want to drive the car as it is doggy. What could be the problem? My plan is to take the starter out and take a look to see anything obvious. What shall I be looking for? Will replace it in any event.

Probably you have a bad connection in the starter. The initial try at starting arcs the bad connection & then there is no connection. Best start at the battery with plenty of boiling water to clean the terminal & clamp. Then both ends of the earth strap. Next the big terminal on the starter. If that all doesnt fix it, time to remove the starter & clean it out.

If you replace the starter it will almost certainly fix the situation...
this has had many threads done on it....
and mine did the same thing...
when you have a chance to take it apart you probably will find that the screws on the inside of the starter which hold the supports for the brushes are loose....
As layback40 said....

it's the "thud" sound that has me wondering... possibly ring gear issue?
describe the sound and when it happens... DURING cranking, and it goes thud, and stops...

'Thud' is not a sound so to speak. I try to described how I feel. The engine started to spin/crank then it just stopped suddenly after 1-2 seconds.

It does not happen all the time, only when you want to get home in a hurry. I will definitely inspect the ring gear and turn the engine to look at the fly wheel.

ahh, then I'd focus on the ignition switch. it's common for it to wriggle loose, or fail.
simple test, if it will not start with key, bypass the switch using the jumper block for the starter.
not sure where it is on the SDL though.

Neutral Safety Switch - Shift Lever Bushings
 

Additional thoughts:
How are the shift lever bushings?
Also may a worn Neutral Safety Switch

Have you checked the engine to chassis grounding? Mine had similar symptoms, a few years back, of one time start fine to the next time of a couple of revolutions then dead. The ground strap being loose was causing very intermittent problems.

Took out the starter and the starter look good. Cannot open up the solenoid to see the contacts condition. The lip was pressed on. Will get a new starter. I came across this doc

http://www.gbcnet.com/mercedes/docs/OM603%20Starter%20Motor%20Removal%20and%20Installation.pdf

and the 'Bosch WR-1 Starter Motor Relay Kit'.

Basically it is to add a relay to the ignition switch wire to reduce the current thru the switch and the NSS. I can make one myself as I have loads of relays.

Why anyone needs one as the car/ign switch/NSS has done OK for a long time? The way I look at it is it adds another failure point into the starting circuit.

Anyone has done it before?
Is it any use?

Thanks.

Did you look inside the starter ? Like at the brushes and what holds them in place .....or the condition of the armature ? The mica between the copper which touches the brushes is supposed to be undercut when servicing it... the copper wears down to that height after a few decades and less than great contact occurs... but the cage holding the brushes is suspect at that age...

Can you turn the engine continuously by hand?

Sixto
87 300D

You shouldnt meed that relay.
I am with others on a connectivity problem.
The alloy end cover is sometimes a problem. It needs to make good contact with the body of the starter & the bracket that has the brushes in also needs to make good contact with the end cover. Problem is you have alloy contacting steel. The alloy corrodes at the junction & the oxide does not conduct high currents for long.
Best not just find one bad connection & think its fixed or you will have a repeat very soon.

1 bad connection = all connections are bad !!

Found the problem - not very good news
 

The problem is the head gasket. The car is smoking more than usual but it seems the HG deteriorated real fast. I put a new starter in there and it started a few times. I thought the problem is fixed but it hasn't. It happened again tonight and the starter just stopped dead. I tried to crank it by hand but it will not budge. I removed the injectors and cranked it, water was sprouting from the holes. :( The engine was hydo-locked. So end of the line for the HG

That was what I was going to suggest it might be.

The starter solution did not explain the dogginess nor the sudden stop once it was already cranking.

I would try to check to be sure you did not bend your rod too while the head is off.

It is always 20/20 after the facts. The rod should not be bent as the hydro-lock was caused by coolant seeping into the cylinder while the car is stationary for a long time. I doubt the starter do not have the energy to bend anything.