This is sometimes necessary if the generator, battery or regulator has been changed.
To polarize a generator, connect a jumper wire from the (DF) terminal on the generator to the generator frame. Remove the fan belt, then connect a wire from the positive terminal on the battery to the (D+) terminal on the generator. The generator shaft should start to spin.
Note: Don’t run the generator this way for more than a few seconds to avoid overheating.
The generator will now be properly polarized. If the generator did not spin during this process, the generator is most likely defective.
Put the fan belt back on and re-test the generator voltage with the (DF) terminal grounded. If the output voltage is still low, the generator is defective.