If it is not a loose connectiom, ... With a freshly charged battery, with the sled running at 3,000 RPM, what voltage do you see across the battery? You should see > 13 volts. If you do not see that, then the sled is not charging.
The voltage regulator connection may be a good place to start. But if you not seeing > 13 volts when running, then the regulator or the stator is not working. The stator can be checked by measuring resistance across it's connector. Once you have the correct resistance there and if you not seeing > 13 VDC, the Voltage Regulator has to be bad.
The Stator's Charging Coil connector has 3 white wires in it. Put one ohm meter lead on a wire connector near the edge of the connector. The resistance from that connector to both of the others should be .19 - .24 ohms for each of the other white wires. If it is not in spec, it is bad.