A 120 VAC 60 HZ battery charger puts out an imperfect DC voltage, it is actually a pulsating DC voltage that corresponds to:

1800 rpm for an 8 cylinder tach,
2400 for a 6 cylinder
3600 for a 4 cylinder

To test a tach using the battery charger, hook all wires but the black wire to the charger's positive lead and the remaining black wire to ground/negative lead.

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