You may be able to fix it by replacing the brush holder assembly in the alternator. Yours may be different but on mine you can remove two screws and replace the old. Buy your parts here at Fastlane.

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1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T