The alternator and voltage regulator are the two main components on the charging system of your vehicle. However, pinpointing the cause of problems like a dead battery, battery overcharge or malfunction-indicator lamp warning may be difficult. If you suspect your voltage regulator, perform these simple tests with your voltmeter to find out if it is performing as it should or if you need to replace it.
Determine how to bypass the regulator on your alternator. On some models, the alternator case provides a small hole at the back. Inserting a screwdriver through this hole shorts the small tab inside to the alternator frame or case. Other models provide two terminals marked "BAT" (battery) and "FLD" (field). Shorting these two terminals with a small jumper wire bypasses the regulator. Still on other models, you need to unplug the regulator plug terminal and short wires "A" (battery) and "F" (field) using a jumper wire. However, you might need your vehicle service manual to identify these two wires.
Connect your voltmeter probes across the battery terminals to check the condition of the battery and record your base voltage reading. Make sure to observe polarity: the voltmeter leads should connect to their respective ground and power terminals on the battery. Your voltage or base reading should be between 12.4 and 12.6 volts to perform the next measurements.
Turn off all electrical accessories and start the engine. Let it idle at about 1,500 rpm and connect the voltmeter probes to the battery as you did on Step 2. Your voltage reading should be 0.5 to 2 volts higher than your base voltage. Then turn off the engine. If your reading is the same as your base voltage or more than 2 or 3 volts above your base voltage, go on to the next Step. Otherwise, go to Step 5.
Perform the same test as in Step 3, but this time bypass the voltage regulator using a screwdriver or a short jump wire depending on your particular alternator model. If your reading is now 0.5 to 2 volts higher than your base voltage, replace the voltage regulator. If not, check the alternator and wiring to find the problem.
Start the engine and turn on all electrical accessories like air conditioning, headlights, wipers and radio. Let the engine idle at about 2,000 rpm and connect your voltmeter probes to the battery as you did on Step 2. Your reading should be .5 volts higher than your base voltage, indicating your alternator and voltage regulator are working properly. If your reading is less than .5 volts over base voltage, go on to the next Step.
Perform the same test as in Step 5, but this time bypass the voltage regulator using the screwdriver or a short jumper wire. If your reading is now .5 volts over your base voltage, replace the voltage regulator; otherwise, take your alternator to a service shop for repairs, or replace it.
