My son has a 2004 mercedes c230K coupe with 1.8L 4cyl engine. We changed the alternator on the car because his battery kept going dead and we finally had it tested and we were told it was bad. After completing the change we tried to crank the car and now it wont turn over all the way. When you turn the key, it tries to turn over but won't do a full rotation. I am trying to determine if this is the starter, the crankshaft position sensor, or another problem entirely. Any help would be greatly appreciated. I tried to add a video but it seems you can only add a photo.