A car not turning over or somewhat turning was causes concern and a need for diagnosis. Determining what the issue is before heading to a mechanic will save you time and possibly money. Knowing your alternator is bad most likely eliminates the need for a new battery, but not always.
The quickest and easiest way to test your alternator requires you to disconnect one wire. Get the car started. Unhook the positive cable from the battery, located under the hood of your car. The positive terminal is red. The negative terminal is black. Be sure to disconnect the positive cable only. If the car stops running, your alternator needs replaced. A good alternator will keep the car running even if the battery is not attached. Working with a car battery can be scary and when done wrong can result in injury to either yourself or your vehicle. If you do not know what you are doing, do not attempt this method.
An alternator goes bad gradually. The last stage of deterioration carries with it a noise akin to a grinding noise and is loud. If you hear this sound when starting the car, you know your alternator is already bad and needs to be replaced immediately. When uncertain, an auto parts store will likely test your alternator for free to see if it is actually going bad.
Purchase or borrow a voltage tester. Voltage testers come in handy for your car troubles, just like jumper cables and emergency kits. Connect the voltage tester to the battery. A good alternator puts out 13 to 14 volts. Anything under 12 is an indicator of a deficient and nearly dead alternator. Try turning on the A/C, the lights and the radio. A good alternator will stand up under this test and remain strong at 13 volts, even with all the power draining.
