If battery is below 11 volts the light will come on. Double check engine running at battery with meter on DC, then check with engine off. You should see 13.4 volts with running and 12 volts with off. If voltage is low it maybe the regulator on the side of the engine. This will be on the blower housing and be in a rectangle shape. Two bolts hold it on and there maybe a ground wire on one of the bolts. There is three wires with in one black plug. The regulator should be plugged in to the black plug. The regulator will need to tested at a dealership in a tester to see if it failed.