12v at the battery at rest is too low. Should be 13v + ! The cheepie harbor freight meters give a good DC voltage read.

Obviously your alternator tension needs help. That is the 1st thing to do. Hate to see you replace a starter for a loose belt which = insufficient charged battery.

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1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."