Isolating a ground without factory parts is possible, but you need to know how alternators are built.
The portions of the circuits that need to be isolated are:
- The stator windings
- The diode trio pairs (what takes the "alternating" power created by the stator and turns it into DC
- The regulator
- The field winding (in the rotor, powered by brushes on the alternator shaft at the back of the case).
Normally the lower 3-diodes are pressed into the case or a cooling assembly, if this assembly can be isolated from the case, you have an isolated ground there. The stator coils connect to the diodes and are covered in an insulating varnish. The voltage regulator mounts to either the case or to the upper (12V out) diodes and lower ("gnd") diodes there is an ignition power supply to enable the regulator as well as a charge lamp connector for the alternator that only connect to the regulator - thus if the ground of the regulator is on the isolated lower diode trio mount, it is also isolated. The field coil typically gets positive power from the upper diode trio (12v out), and it is controlled by regulating the current to ground (the regulator should again be grounded to the lower isolated diode trio) - the brushes that feed the field coil should be in insulating holders so it should be isolated.
So, to make just about any alternator an isolated ground, the lower diode trio needs to be insulated from the case and a ground lug insulated from the case need to be provided to the lower diode trio, and the voltage regulator needs its ground reference to be the lower diode trio's holder. Other than that if your insulation on the other components are good, it's ready to go.
In a pinch, you may be able to take the rebuild kit for the size alternator you are running and cannibalize the positive post and insulator to make a new connection for the lower diode trio. You'd need a drill or drill press.
