How to Diagnose Automotive Alternator Problems

Alternators are powered by the drive belt or serpentine belt.

You may have an electrical problem if you are driving and your vehicle suddenly just stops running, or if your vehicle won't start at all. If the battery is discharged, then the alternator could be the source of your problem. The alternator sends current to the battery while the engine is running. A faulty alternator cannot charge a battery with the proper amount of volts, and cannot run the car with the proper amount of amps. Testing the alternator will tell you whether or not it is generating the proper voltage to run the car and charge the battery.

Things You'll Need

• Ohmmeter or multi-meter
• Open-end wrench set

Instructions

1. Raise the hood. Set the hood prop if equipped. Locate the alternator. The alternator is a metal-cased part with small vents in the casting. Visually inspect the alternator to ensure that there is not excess carbon (black dust) or any moisture on the alternator. Leaking power steering fluid or oil can sometimes enter the alternator, and this can impair its function. Visually inspect the drive or serpentine belt for cracks or looseness.

2. Check the battery leads to make sure they are secured tightly, using an open-end wrench. Visually check the two sets of wires attached to the alternator. Look for any blackened marks or burn spots on the lead ends. Black marks can indicate arcing or electrical shorts. Inspect the wire sheathing, especially near the alternator, to make sure that there are no exposed wires or rotted sheathing. Wires must be completely enclosed on your alternator to conduct the current correctly.

3. Check the charge on the battery with the car engine off. Set an ohmmeter to "V," which is the volts setting. Place the red lead from the tester onto the positive (+) battery terminal. Place the black lead onto the negative (-) battery terminal. The reading from your battery should be no less than 10.00 volts when the car is off. If the battery is not fully charged, you must charge the battery completely before proceeding to the next step.

4. Start the engine. Place the ohmmeter onto the battery again, and test the battery voltage. The battery should provide approximately 13.50 to 14.50 volts, depending on the strength of your alternator. If the battery provides a reading below these parameters, the alternator is not properly charging the battery. If the battery provides a number greater than 14.75 volts, then your voltage regulator may be damaged. Voltage regulators control the amount of charge the alternator puts out. A charge that is too high can destroy your vehicle's battery. Some voltage regulators are built into the alternators.

Tips & Warnings

• Some auto parts stores offer free starting and charging system tests, if you can get the vehicle to the store. Check with your local parts store to see if they offer this service.

• When the engine is running, there are many moving parts in under the hood. These moving parts can quickly grab or injure you or your ohmmeter. Avoid all moving parts.