Go to a auto parts store and buy a volt/amp load meter ($20.00) before you buy anything. With engine off, Clamp meter on battery,( - black to black and red to red + ) should read 12 volts. With engine off, throw the load switch on meter and hold until wire elements in meter glow, should hold stedy at 400 - 800 amps. If either reading is not up to par, take off battery caps ( some times under a sticker on top of battery ) check to see if acid solution is up to the bottom of filler ports on baterry. Don,t worry if battery says maintance free. there is no such thing. if low, fill with ordinary tap water ( not recommened by manufacture but it will do) and put caps on. Charge battery with the lowest possiable charge on battery charger for a least three hours. Do not smoke or introduce any kind of flame when chargeing battery. When you pass electicity thru water it seperates the atoms (H2O) in the water and vents out of the battery. Hydrogen and Oxygen, same stuff they put the space shuttle into orbit ( very explosive ).
After chargeing battery with engine off, recheck voltage reading should read 12 volts or higher, no less. next flip the the load switch and hold till elements glow( about 10 seconds ), should hold between 400 and 800 amps. Its OK if it starts out high and drops as long as it does not fall below 400 amps. The higher the amps hold the stronger and higher Quality battery. If voltage or amp reading is not within these specifications, replace battery.
After checking and replacing battery if needed, start engine and check voltage. Voltage should read 14 -15 volts respectively. Any lower or higher alternater is not funtioning properly. If it is lower than 14 - 15 volts then you are useing the electricity in the battery to power the engine and related componets. If it is any higher then 14 - 15 volts then the internal voltage regulator is burnt out and alternater should be replaced.
If it is lower before replacing remove alternater from vehicale and bring it to a auto parts store. Most stores will check the alternator for free or a small fee, to see if it is working properly. If it is not working properly then replace it. Warning, Always disconnect negetive terminal ( Black ) from battery first, before removing alternater. If you don,t you can ether cause a fire,explosion of the battery or at the very least damage the electrical system permeantly!
If it is working properly then you have a wireing problem. Reinstall the alternater and using a multi meter, check that you are getting 12 volts on the field wire ( Large single Red wire )from the fuse box on the driver side fender under the hood to the battery and from fuse box to the back of the alternator. Put The pos terminal of a multi meter ( 30 volt scale DC ) on the field wire (large single red wire) on the back of the alternater. Put the negetive lead on bare metal of the frame of car. If voltage is not 12 volts than you have a fault in the field wire between then fuse box and the alternator. Remove the fuse for the field wire and useing the multi meter to check if you have 12 volts on one side of fuse holder. If you don't, use the multi meter ( set on continuity ) and check the fuse. if good check the field wire from the out bound side of fuse holder to the alternator for continuity. If good then useing the multi meter set to DC 30 volt scale check the field wire from the in bound side of the fuse box. If you don't have 12 volts, check the field wire at fuse box, for continuity, from in bound side of field wire back to where the field wire is spliced to the red Pos cable of the battery. If good, un-tape splice at battery on positive cable. you will see a smaller wire at the end of field wire where it meets the positive cable. This is a fuseable link wich protects the field wire from over load. Check for continuity on fusable link. You must seperate the field wire from the fuseable link first.
If the fuseable link is good reconnect. Before i tell you what to check next i need to give you a 101 class on how electricity is generated in a generater or alternator. Please be patient with me. To generate electricity you ethier have to rotate a copper coil around a magnetic field ( ie Magnets ) or rotate a magnetic field around a copper coil. Alternaters do not have magnets in them so you have to create a magnetic field for the copper coil to rotate around to generate electricity.
When you pass a electric current through a coil of copper wire, a magenetic field is created around the coil. In a alternater a small coil Called the primary coil or winding is energized when you turn the key on creating a magnetic field. A second much larger coil called the secondary coil or winding rotates around the smaller primary coil induceing a large current (60,100 amps ect
. ) into the larger secondary coil, which is connected to your field wire which in turn charges your battery and replaces any power be used by the engine and accessaries.
So we now must check to see if you have 12 volts going to the smaller primary coil. On the back of your alternater ( I have a 97 ranger and the positive wire of the primary coil is yellow with a green stripe), there is a two smaller wires next the field wire. Turn the key on and check the primary coil wire for 12 volts. If you do not have 12 volts, use the same procedures to check for any faults in the primary wire leading from the alternator back to the fuse box under the hood.
And that is that! I hope I did not confuse anybody. I also hope that this information will help you solve your problem. Good LucK.