The issue with the brushes is that as they die the car will experience periods of NO charging. It can be intermittant and when not occuring the voltages would be OK.

Another point is that the starter alternator wiring harness on those cars is also one of the noted harness failure points.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician