Good morning everyone

I began having problems with my 300 SDL, the battery light popped on this week. I had the car checked at Advance Auto Parts and was told the alternator was bad. I purchased a new 70 amp alternator from Bap Geon. After having it installed, the alt/ battery light came back on.

Took the car back to Advance Auto Parts and was told the battery was weak. I purchased a new battery and the light came back on. Had it checked a third time and was told the alternator is bad.

While driving home the dash lights did come off and on. The alt/ battery was also coming on intermittently. The brake lights also stopped working on the way home.

Today I went to have the old alternator rebuilt, and was told it was a good alternator. My question is where should I start and what should I check?

Thanks

Ron

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Wiring and all connections in the alternator battery circuit. Depending on your electrical experience you may or may not have good methology to do this.

Clean the ground strap points between the engine and chassis and battery to body shell first. That means unbolt them clean them well and apply a little grease and bolt them back on.

Although with your starter able to work well you would think they cannot be the issue. But they can as the high current demand of a starter can make a marginal ground connect temporarily.

Thank you Barry

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Directly off of the positive battery terminal is a power junction that will have several wired connections both bolt on and slide on tabs. These get a little junked up after a while.

My 86 sdl did the same thing as yours....turns out these connections needed cleaning. You also may have ground and positive battery terminal cables that look ok but have deteriorated within the insulation casing. Probably should replace those...fairly inexpensive.

Also the wiring connection to the alternator's voltage regulator may need cleaning.

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1986 300SDL 440,xxx

Thank you for the very helpful information. I will look into all of it.

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YES The outboard connection

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Thank you TX6513 for the 2nd on the wiring connection cleaning.

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X2 on checking the integrity of the wiring insulation. Worn Wire in my 123 was touching chasis.

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