Just acquired a 1991 420 SEL. Today the idle starts going in spurts (hi-low,hi-low...), all dash warning lights came on (except check engine), car stalled out- I jumped the car off & it started right back up but went dead minutes later, tried jumping it again and got nothing. Replaced the battery & it started right up & I drove it home. I would guess the alternator may be the cause, but it was replaced new (according to records) last month. Should I replace the alternator or check something entirely different. Any help would be appreciated!

Lots of other posts address how to check an alternator.

What is the voltage at the battery, engine off?

What is the voltage at the battery with the engine on and running?

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1989 420 SEL