Thanks for reporting back. It sounded like the battery from your first post: "The cranking sound got weaker and weaker".

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1982 240D 4spd, 327,000mi

You can do a simple test on your alternator with just a multimeter. Check the voltage across the battery terminals with the engine not running for a reference. You should get somewhere around 12.5 volts. Next, start the engine and leave it running. Check the voltage across the battery terminals again. If you get the same voltage as you did when the car wasn't running, your alternator isn't working. You should get around 14 volts with the engine running.

If you don't have a multimeter, get one. Harbor Freight sells them as cheap as two or three dollars. Most parts stores also have cheap (under 20 bucks) meters for sale. They make some very nice multimeters that do lots of fancy things and sell for four figures, but you don't need one of those to work on a W123 diesel.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.