(edit) it COULD be the ring gear. Hopefully not though, hope its just a sticking solenoid from brush residue build up, try squirting a lot of WD40 in it, you best remove that darned heavy cursed thing (can you tell, me and starter motors are enemies!? look into the opening with a mirror and see if teeth are missing on the ring gear. The starter can be activated using jumper cables. It should slap closed and the gear move into position when the solenoid is jumped to +

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!