I did not think IL6 motors were made in 1990, but that is not germane,
Those motors had a voltage regulator, driven by the stator. The stator supplied an AC current which was rectified and then regulated, based upon the condition of the battery.
Simple test is to disconnect the yellow stator wires from the VR. Now put a voltmeter set to AC onto the yellow stator wires and start the motor on a flusher device. If you get an AC signal, the stator is likely good.
Now reconnect everything. Put the voltmeter on the battery, engine off and note the DC voltage. Now start the motor and read the voltage with the engine at high idle. The DC voltage should read higher. If not, the VR is likely bad.
