Field Coil Draw Test

On 114 amp. alternator, connect a jumper wire between field terminal of alternator and positive terminal of a fully charged battery. Connect test ammeter positive lead to other field terminal of alternator, and negative lead to battery negative terminal.
On other alternators, connect jumper wire between field terminal of alternator and negative terminal of a fully charged battery. Connect test ammeter positive lead to other field terminal of alternator and negative lead to battery positive terminal. Then connect jumper wire between alternator end shield and battery negative terminal.
With alternator on an insulated surface, slowly rotate alternator rotor by hand. Observe ammeter reading. Field coil draw should be 4.75-6.0 amps. (114 amp.) or 4.5-6.5 amps. (all others) at 12 volts.
A low rotor coil draw indicates high resistance in field coil circuit (brushes, slip rings or rotor coil). A high rotor coil draw indicates possible shorted rotor coil or grounded rotor. No reading indicates an open rotor or defective brushes.