How to Diagnose a Starter, Alternator and Battery

You just drove the vehicle yesterday and it was fine. Today, it just will not start. There are many things that can cause a no-start - this article will cover three of the most common no-start problems: the starter, alternator and battery. It will also cover a situation where the vehicle may shut off while in motion.

In order to follow these instructions and correct this problem you will need to have a voltmeter. This is a simple device and can be purchased at any place that sells auto parts. You do not need to spend a lot of money on the voltmeter - it needs only to read volts and ohms. It is one of the most valuable tools in diagnosing an automobile's electrical system.

The first problem we will deal with is when the car stalls and will not restart while driving. Check to see if the engine will turn over when you turn the key to the start position. You should hear the engine turn. If it does, then the problem is an engine related problem and will most likely have to be towed.

If the engine does not turn over, then the problem is with the alternator, since the battery starts the vehicle and the alternator operates the car - mostly independent of the battery. What has probably happened is that while driving the alternator failed and was no longer charging the battery. From the time of the failure, you were driving on the battery alone. The alternator also charges the battery, so when it fails, the battery no longer charges. A fully charged battery will operate a car for approximately 30 minutes. If it does run longer than 30 minutes, it will not be much longer. The more accessories you use while the battery is not receiving a charge, the less time you will have before the car shuts off. Accessories include air conditioning, lights, radio, heater or any other electrical component you have on the vehicle.

Jumping the battery, if it works at all, will not get you far since the alternator failed and ran the battery down. It takes considerable time to charge a battery to a full charge - at least four or five hours. You can try charging the battery on the roadside and attempt to drive to your destination - it is possible to get another 15 to 20 minutes out of a short charge. If the battery will not hold a charge, you will have to call a tow truck.

In this next situation, the vehicle will not turn over once it is shut off for a short time. The first thing to do is turn the headlights on and see if they are bright. If they are, then check the battery terminals to see if they are tight and clean. Next have someone watch the headlights and see if they dim when you attempt to start the car. If the lights dim when you attempt to start the car, the battery is low. If they stay bright there is a starter problem.

If the lights dim, set the voltmeter to volts and check the battery for twelve volts. If it shows less than twelve volts, charge the battery and start the car. After starting the car check the battery again with the engine running. The voltmeter should show 13.5 to 14.5 volts if the alternator is working. If the alternator is working, replace the battery. If the alternator is not working, replace the alternator. Recharge the battery.

If the battery passed the test and the car still will not start, then look to see if the black negative cable is tight at the point that it is grounded to the frame. If the negative cable is tight, turn the key to start and listen for a clicking noise indicating the starter is trying to work. If you do not hear the starter click, open the fuse block and check the fuses. If the fuses are good, check the starter relay. In most vehicles, it is located on the driver's side fenderwell in the engine compartment. It should click when someone turns the key. If it does not click, the relay may be bad. Look for an identical relay for something else in the relay box and switch them. If you hear the relay click, the starter is bad. Make sure to replace the bad relay if you swapped two relays out.