300sd challenge
 

The car isn't cranking at all but lights and radio come on. That's why I think it would be 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!

It may be ideal to cite the problem(s) you are experiencing. As in, are you trying to diagnose why the car isn't turning over? (my guess)

Have you tried jumping terminals 1 and 3 in the junction box (next to the alternator if it's a 126) or hot wiring the starter terminal directly from the battery?

This type of problem is so common on most makes of cars. I am really surprised your two mechanics could not run it down.

measure voltage drop to confirm that no corrosion is keeping it from turning. There has to be a bad part or connection or the system would work.

^ thiiiiiiiisssss

Does the engine turn at all when you try to turn it over by hand? Try turning it manually with a socket wrench on the power steering pump.

Most likely starter be no more good. Less likely possibility is that wiring from key switch to starter is failing to energize the starter solenoid. Check for voltage at starter solenoid with key in start position.

^^^

This.

You need two people.

Well as Kerry mentioned your mercedes has a very good provision to divide the problem into one of two segments.. Short the contacts together in the box on the fender he describes.

If starter works the problem is in the solenoid engaging circuit. If not it is in the high current part of the sector. The high current items are the feeds from the battery and the starter itself. The solinoid portion of the circuit is basically the neutral safety switch on the transmission. Plus the electrical portion of the key switch. Plus the solenoid. I suspect you changed the key switch mechanical part but not the electrical portion of the ignition switch by the way. Plus the associated wiring and connections. If you do the mentioned test and post the results others can guide you further if needed. This is not rocket science.