Indicates the battery and or charging system is not keeping it above a preset limit that will trigger the light. Could be the battery is slighty weak but if it starts ok thats not that likely. Also if the charging system is going out it is dropping voltage as well. If it is the charging system the light will come on when you turn on the headlights and prolonged running will deplete the battery. If it seems to be related more to the PTO being engaged and not other electrical loads I would guess your pto is engaged with an electrical soilnoid and it may be going out on you. If it is drawing too much current and is close to grounding out that would pull the charging system down and could cause a charging system malfuction indication. A key would be what electrical loads cause the light. If you can have it running at a decent rpm, not wide open but a little bit above idle and put a meter across the battery and see what you are getting, would think around 14 volts on a 12 volt sys if the battery is fully charged. If that is right start turning on loads one at a time and see if any one load seems to drag the voltage down and causes the light. Would really look hard at the PTO circuit if it is electrically engaged and see what I could find in that area