A malfunctioning voltage regulator on an alternator can cause an incorrect amount of voltage to the battery. The voltage regulator is located on the outside of the alternator on some vehicles, and internal to the alternator on others.
Voltage regulators do exactly what their name says, they regulate voltage. Most vehicle voltage regulators keep output voltage at approximately 13.5 to 14.7 VDC to the battery while the vehicle is running. The 13.5 - 14.7 volts is the average amount required to charge a battery under normal load conditions.
Test Voltage Regulator
Step 1- Start the vehicle and locate the battery.
Start the vehicle and be sure the parking brake is applied. Open the hood and find the battery.
Step 2- Set the voltmeter to the proper range.
Place the voltmeter in the 20 VDC range and carefully place the black meter probe on the negative battery terminal, and the red meter probe on the positive battery terminal.
Step 3- Review the test results.
You should read about 14.5 Volts on the display. If the reading is much lower (under 12), then your alternator may not be functioning correctly, which causes the battery not to charge. If the reading is higher than 15 Volts, then your voltage regulator might be faulty, or may need adjustment.
Adjusting External Voltage Regulator
If you find that the voltage regulator is out of adjustment, then there are methods to adjust it on some vehicle models. Check your vehicle's repair manual, as it may have listed procedures for your type of voltage regulator.
Note: Not all models can be adjusted, so be sure you research your particular model. Here is one method that may work on your vehicle.
Step 1- Locate the adjustment screw.
Find the adjustment screw on the regulator and insert a screwdriver.
Step 2- Watch the meter and start the vehicle to idle speed.
Have a helper watch the voltmeter that is hooked up to your battery. Start the vehicle and let it run at idle speed.
Step 3- Dial in the voltage regulator's adjustment screw.
Adjust the voltage regulator's screw until the desired voltage is obtained at the battery. A good range of DC voltage to the battery is 13.5 to 14.7 VDC.
If the proper voltage cannot be obtained by adjustment, then it is time to purchase a new voltage regulator or alternator.
More Steps to Try
Another reason you may have low voltage at the battery is a loose or corroded ground. Look really close at all the grounds for tightness and corrosion, as this will cause high resistance in the charging circuit, and the alternator may not function properly.