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alternator test - diesel engine
a real simple test to check an alternator is to take a piece of metal (screwdriver etc) and hold close to the back of the alternator while it is running. If it is producing electricity it will be creating a magnetic field and you will be able to feel the pull on the metal. (be careful not to hit the output post).
The surest way to test output though is with a multimeter (Radio Shack about $25). with the alternator running check from the output (+) to ground (-). reading should be above 13.5.