it'll just keep getting worse. doubt it will just up and die on you. i have experience with three diesel starters going bad. it'll start giving you just a click maybe once every twenty starts. then every ten, then every five. then it'll do it every time, but it'll eventually turn. then it'll eventually turn if you bang on the housing with a hammer. then it'll be completely dead.

you may have some other problem. ground strap corroded or loose? corrosion at the battery? corroded or loose negative cable?

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1981 NA 300D 310k miles