Is your battery or alternator light on? Here's the first step to determine if your alternator is not working.
You will need a voltmeter - I prefer a digital one - they are all less than $20.
Open the hood of your car/truck, set the voltmeter for 20 volts DC or whatever DC voltage setting it has between 15-25 volts DC.
Place the Black lead from the voltmeter on the negative post of the car battery - the black cable from the engine is attached to the negative side of the battery.
Place the Red lead from the voltmeter on the positive post of the car battery - a red cable from the engine is attached to the positive side of the battery.
Read the voltage: if it is around 12.5 volts, the alternator is not charging the battery. This may be caused by several things, it may be a loose or disconnected wire, a loose belt, the voltage regulator may be bad, or the alternator is bad. In many cars, the voltage regulator is attached to the back of, or incorporated into the alternator.
When my Ford F150 truck battery went dead, I pulled out my voltmeter and tested the battery. I was able to tell the problem was not a loose wire or connection because soon after I started testing at the battery, the alternator started to smoke, which is a pretty good indication that it needs to be replaced.
On many cars and trucks, if you have some mechanical ability, you can replace the alternator yourself. Look online for a discussion forum about your vehicle and search the posts for alternator replacement tips and tricks. When you go to your local parts store to pick up a new alternator, its ideal to bring in the broken one, most parts like this require a trade-in of the old part ( so they can rebuild it ) AND it is good to confirm that the new alternator is the right one. Save yourself a trip.
Tell us your alternator stories below, be good to learn other tips and tricks from you all.