I have a 1985 300CD. When I turn the key to the start position i get just a click. Headlight and horn indicate plenty of juice. I hook up a charger and put on boost and it starts right up. I sounds like a solinoid click. Sometimes if I hold on the start postion it with kick in the starter. Are there contacts that can be cleaned within the solinoid? Can I replace just the solinoid? Could it be something else?

Sounds like it is time for a new battery. Take the battery to Macparts and have them load test it for you.

Tinker