How to Troubleshoot a Chevrolet Alternator

The biggest contributing factor to alternator failure is the amount of draw due to equipment or faulty batteries. A Chevrolet alternator has a maximum amperage output rating. This rating implies that the alternator can produce this amount of amperage on a flash basis or for a small amount of time without overheating. A Chevrolet alternator should not be operated at more than 40 percent of its rated output on a continuous basis or it will heat up and fail prematurely. The most common failure is the diode trio. Diodes allow electricity to flow in one direction only, converting the alternating current to direct current for use in the vehicle.

Things You'll Need

• Volt and ohmmeter

Instructions

1. Test the battery by connecting the voltmeter to the battery terminals with the red lead to the positive terminal and the black lead to the negative terminal. The battery voltage should be 12.5 to 12.75 volts. If the voltage is lower, the battery is undercharged so it must be determined if the battery has a bad cell or the alternator is not charging sufficiently.

2. Use a helper to turn the key to start the engine while observing the voltmeter for amperage drop. When the starter is engaged, the voltage should not drop below 10.5 volts if the voltage was correct to start with. If it does drop below 10.5 volts, the battery has a bad cell and needs to be replaced. If the voltage was low to start with, the battery should be charged first before further testing.

3. Start the engine, then check the voltage on the meter. A good alternator should charge at 13.8 to 14.5 volts. If the alternator is charging under 13.8 volts, a diode pack is bad and the alternator is operating at reduced efficiency and should be replaced.

4. Test the voltage regulator circuit if the battery was charged fully and the voltage was within specification. Turn on all the equipment such as headlights, wipers and air conditioning and re-check the voltmeter. It should show very little drop. If the voltage drops below the 13.8 figure with the accessories on the regulator is bad and should be replaced.

5. Check the belt tension if there are any noises such as squealing which indicates that the belt tensioner is weak and needs to be replaced. It is allowing the alternator to slip when under load. If the alternator bearing is squealing or grinding noises can be heard, this again requires replacement before it locks up.