Am confident someone out there can help. 1991 300D - Battery replaced and a few days later dead again. Alternator replaced with a re-built. Now a few months later drive with headlights on and the battery is dead again.

Mechanic measures voltage and no output from alternator, but when bench tested it's fine. The battery light doesn't come on (bulb ok) indicating no charge, but there is no voltage getting to the battery.

No obvious wires or connections bad - any ideas ???

Loose belts or a parasitic drain while ignition is off.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150

I would still recheck the wiring going from the alternator. Sometimes a female terminal opens up some causing a bad connection. If this is one of those Mercedes with the "Green" wiring harness that gets brittle you may have a broken wire.

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Bill Reimels
Now down to one:
1972 300SE 3.5 W109 (Euro delivery)

I'd agree that it most likely sounds like a bad conductor somewhere, but you might also want to check your main ground connections on the battery and between the engine and chassis. If the engine/chassis ground was broken, the alternator output would have no common ground with the battery and therefore no charging.

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1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown

But why wouldn't the battery light come on since no current is getting from the alternator to the battery??

Good point

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1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown

The battery lights are notoriously unreliable. My 124 completely wore out the brushes on the alternator without any hint of a problem until the battery went dead. The only time I saw the battery light go on was when it lit up with all the other dash idiot lights at startup, there wasn't even a faint glow when the engine was running. Give me a good gauge anyday vs. an idiot light.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold

Cause that is the wire affected??

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Bill Reimels
Now down to one:
1972 300SE 3.5 W109 (Euro delivery)

I am not a great mechanic, but had good luck replacing the voltage regulator on mine, although it was likely replaced on your rebuild, still might be worth a relatively inexpensive shot...

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1983 300D- silver blue, tan leather, 205,000.
1975 Datsun 280Z. Stock, ready to restore.
1983 San Juan 23. Dry docked.