Dead battery

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One way to check the alternator is disconnect the "FLD" wire at the voltage regulator and apply 12 V to it. The alternator should then put out full current. If not, assure you measure 12 V across the field terminals at the alternator. Often the problem is you don't get a good ground connection from the low terminal which connects to the case in the early alternators (1 field wire). If so, run a ground wire to it or remove the alternator and sand the corrode aluminum body where it contacts the brackets.

Notes:
If old-style mechanical reg, you can actually leave FLD connected when you externally power it, but that might damage an electronic type.

If you don't have a multimeter, they are often $2.50 at Harbor Freight Tools. I have one in every car.

If the alternator puts out full, the problem is your regulator.