As stated above, mechanical diesels will run without a functioning electrical system once started. Troubleshooting battery and starting system problems is pretty tough without a multimeter. Fortunately you can buy one good enough for that purpose from Harbor Freight and other sources for around ten bucks. A battery with a good charge on it will show a voltage around 12.5 volts with the engine off. Once started, the alternator should put out around 14, a bit less if there are other electrical loads present and the engine is at idle. Load testing a charged battery will tell you if the battery is still good. This requires a load tester, which is a different tool, but many parts stores will load test a battery for free.

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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.