Hi, just got a 1987 300D a few days ago. Ran well until my driveway but the day after the battery was dead and (after having changed it), no way to have it start.
After a few investigations: power arrives to the ignition switch when I turn the key and the starter works (12V on the main wire and when I connect it to the smaller wire, the starter cranks).

So there is something wrong between the ignition switch and the starter!

Question:
1. Any idea?
2. Does anybody have the correct wiring diagram?

Thanks!

Are you saying that you turn the key and the starter does not engage? If so, that could be the NSS (neutral safety switch) gone bad or it could be worn shifter bushings (ie the vehicle does not know exactly what gear it is in). If you can find a sweet spot with the shifter where the starter fires, then it is likely shifter bushings.

If you have to jump it with two screwdrivers from the jumpbox under the hood, it may be the ignition switch.

Just a thought from my experience with my '81 300SD over twenty years of ownership.

Good luck. Others will chime in with better info.

X2 @ tyl604. Neutral safety switch on side of transmission or worn shifter bushings. Try putting your foot on the brake and starting it in neutral.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Thanks, well this is a manual transmission... anybody knows if there is a similar security as the neutral sensor does not apply! I keep searching!

Look at the top of the clutch pedal for a neutral safety switch. Test for power to it in the crank mode and power out of it when the pedal is fully depressed. If you jumper the switch wires to test it be careful because the car will always crank even in gear, this its a safe issue. Don't leave it jumpered/bypassed.

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer