I have a 99 dodge durango, I have recently replaced the alternator due to when I turned on the headlights or heater the check gauges light would come on. Once again even with the new alternator, when the headlights and/or heater were turned on the battery would still die. Went and had the battery and alternator tested and they said the battery was great, but the alternator was charging on the low voltage side. Once again we replaced the alternator and the battery. Tonight I drove it, and guess what happened? Yes that dreadful light came back on, what else could it be, could it be as simple as the battery cables or clamps?

 

Could be the voltage regulator

 

i would start with the battery/alternator cables and clamps, they CAN get corrosion under the insulation which you cannot see and which can cause all sorts of problems.

 

+1 Sharps

Also could be the battery temp sensor located under your battery. It controls the output of your alternator due to battery temperature.

 

+2 on the connections and the corrosion. It really does rob some power if youve got corrosion just settling on a connection.