It's not turning over at all, but all the electrical comes on. From what I read the problem is mostly likely electrical in nature. My battery is new, within the last six months. It holds it's power just fine, there's no drain except for a tiny bit from the radio. The alternator is new. The power cable to the battery is new too, and the ground has been cleaned. The starter is new. There was no sharp clunk the last time in ran. The ignition switch is also new. And of course the shifter is in park. Power is being sent to and from the neutral safety switch. I've wiggled the shifter while turning the key and nothing happened. The back up lights work. The tumbler doesn't seem sticky, the key comes out just fine. The glow plug light comes on, all the glow plugs have been replaced in the past year. All the wires to the starter test 0 resistance. No fuses are blown.

Any ideas? I'm pretty experienced, and I've had two other mechanics look over the car and they're stumped too.

Thanks!

 

Connect a Remote Starter Switch to the Starter and see if it will crank for you. If it does not crank your Starter Solenoid is shot or there is a problem with your Engine to chassie Ground.

You could also see if you turn on the Key you are getting Voltage to the Starter Solenoid. If not start tracing the Wires Backwards the Ignition Switch.

 

check the battery connectors too... basically this exact thing happened to me last week... took an hour of checking, re-checking and wiggling to finally decide to pull the multimeter... starter had nothing, battery is brand new... checked the cable less than an inch from the terminal and nothing... took apart the batter connector, cleaned it, cleaned the power and ground cables, put it back together and sprayed some of the brake stuff on the terminals (can't rememeber what it's called - it's hot pink and is supposed to keep the terminals/connectors/cables from corroding)

 

I'm on my way out to try again! Thanks- I'll let you all know how it goes.

 

Nothing happens when I put the car into neutral.

 

Here's the readings I have: The battery is charged to 12.83. When I turn the key the ignition wire to the solenoid is 11.21, the battery to the starter is 11.66, and when the key is turning the battery drops down to 11.64. The starter isn't clicking, the only noise is from the glow plug relay.

 

If you are checking to see how much of a Voltage drop there is you need to disconnect the Electrical Connector at the Glow Plug Relay that has the small Wires going to it and is square.
The reason for this is that during Cranking your Glow Plugs are also on. Disconnecting the Connector keeps the Glow Plug Relay from turning on.

If you are getting 11.21 Volts at the Starter Solenoid Termina and it is not clicking your Starter Solenoid is no good; this assumes your Chassis to Engine and Battery to Chassis Ground Cable/Strap are making good contact.

I also suppose that inside of the Starter there could be something keeping the Starter Solenoid from moving.

If you disconnect the Small Wires from the Starter Solenoid and put your Meter over on the Ohm Setting you should be able to put one Meter Probe on ground and the other on the Starter Solenoid Terminal.
I don't have a resistance figure for you but the Starter Solenoid has a whole bunch of small wires wrapped inside of it. You should see some resistance reading. It might be there is a Motors Manual or Chilton Manual that could give you sort of a generic resistance reading for the Starter Solenoid.
By the way these Starter are similar in design to Delco Starters made up until the 1980s. Troubleshooting instructions for a Delco ought to work on the Mercedes.

If your Ohm Meter shows no reading at all that is an indication of an open circuit (as in no connection).

 

I am not sure this will be helpful at this pont but someone else might read this Thread. This is a Pic Terminal Block for a 300SD fromt early 1980s. It is supposed to be located under the Battery Tray.
If you Jump the 2 places the Red Arrows point the Startrer is supposed to crank.
Doint that bypasses the Neutral Safety Switch.

One side of that Block has Posative Voltage all the time I am guessing that is the right side.

 

Eliminate all possibility of a bad starter. Pull it back out and bench check it. Check that the engine turns by hand, but only turn it in the direction of normal engine rotation. Then check out your electrical system from the key switch to the starter solenoid. Keep in mind that just because it's new doesn't mean it's any good.

 

Every test lead back to the starter, although it has now be tested three times. So I replaced it anyway (under warranty free, gratefully) and now it's starting up better than ever! I wonder why it would test fine at autozone, but still wouldn't turn over the car. Anyone know?