2006 Chevy Trailblazer 6 cyl Two Wheel Drive Automatic 85,000 miles
Out trailblaser check engine light comes on with a trouble code which says " engine RPM lower than expected" and has for a while. My wife has also complained that the headlights dim and brighten while sitting at a light for some time. She left it running one night with the lights on and the heater blowing for a while and the motor cut off with a dead battery. The vehicle runs fine otherwise by the way. The alternator charges fine until you turn electrical loads on while at idle, lights, rear defroster, or heater fan(but not any single one by itself)which will cause a discharge of the battery. If you bump the throttle it will go back to charging and then discharging.
Is the alternator bad or is something letting the rpm get low under electrical load conditions which is too low for the alternator to charge?
I believe the manual says rpm should be 550 which it idles at preety good but will dip below when you turn an electrical load on.