If you get 13.0 v running with either a cold or warm motor, somethings not up to snuff. It's been my experience that once you start seeing < 13.5v on a warm motor at idle, it's time for brushes/alt/etc. Cold motor voltage reading should be more like 14.0v or better...up to about 14.4v max.

My 2 cents.

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'91 300-SEL