Measure the voltage of the battery when the engine is idling. You should have somewhere around 13.5 volts DC. This indicates that the connections are OK and that the alternator is putting out enough current to charge. The turn on the lights, accessories, AC, put the blower on HI, etc. Use as much electricity as you can muster. The voltage on the battery may dip a bit but should still be on the order of 13.5 volts or so. A reading closer to 12 volts indicates a bad alternator/voltage regulator or perhaps a loose/slipping belt. Check the quality of the cable connections to the battery, alternator and grounds.
If it appears that the alternator is working ok, turn off the engine and then restart it. Check the charging again. I had an alternator that worked when you first turned on the car but when the alternator's voltage regulator got hot it did not work any more on the next restart.
If the alternator is still working, then you have either a bad battery or a current drain in the car's circuitry. I replaced my Taurus' battery only 2 1/2 years after I bought the car. I have gotten into the habit of replacing them every 4 years even if there are no problem symptoms.
Remove the batter cable and let the car set overnight to see if it drains internally. If it does not then the problem is in wiring or an electronic component.
|