Shortened my post intentionally. A couple of things perhaps not checked as far as I read into your posts. The integrety of the battery post to battery cable connection is a common failure . Did you clean both the terminals and battery cable clamps? Or with a friend put one lead from a voltmeter on the actual battery post terminal itself and the other lead on the clamp. Then moving the key into the starter position. There should be little voltage displayed in your case actually nothing. Do both terminals in this fashion.

The second thing was your description of using the battery jumper cable to the engine. I like to use a really good ground area like a clean alternator bracket or something simular. Anyways without verifying the battery post to battery cable clamp has little resistance first the use of the jumper cables from the clamp area can be meaningless. Now you very well may have taken care of these areas of concern already. But if not you should.

When I started my first post I quickly realised I was writing far too much information about the starter circuit down. I can add more later if the indication is I should. A friend that has more experience around cars than you can be an asset in itself remember. Starter circuits are very simular car brand to car brand usually so he or she need not have mercedes experience.

I took the starter back in to get tested and be 100% it wasn't the starter itself. Starter tested fine again.

I bought 2 extra ground straps and installed them. Now the starter is making a clicking sound and trying to turn over! So that's improvement at least. Seems like I still need to add some more ground points to reduce resistance to the starter. Either that or the battery needs to be charged from trying to start the car so many times.

It is tricky solving a problem like yours - you really need to get your ducks lined up so that it works - you could be fixing one link in the chain and then not realising that another link is now bust (if you see what I mean)...

...All I can do is to encourage you to be as methodical as possible with each stage.

EDIT:- Have you cranked the engine by hand yet?

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

No, I haven't cranked it by hand yet. Before, the starter wasn't even clicking so I figure cranking it wouldn't do any good as the starter gear wasn't trying to engage.

Be brave and put my mind at rest! Give it a quick churn - 27mm socket on the crankshaft bolt - it is all the way inside the pulley... Whilst you are at it do just like whunter recommends in the post above => 300D Part I.D. - Problems Starting

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

Well, I added 2 ground straps directly from the starter to the frame and viola! Starts like a champ now. Thanks to everyone for their input and suggestions.

Moral of the story is that ground issues are very tricky to solve! It's amazing that these cars only have one chassis to frame ground strap.

Thanks again everyone.