All vehicles have a component called an alternator or generator that uses a diode, rectifier, stator and rotor to produce electricity. The alternator assists in running the vehicle accessories and electrical systems, as well as maintaining a charge on the battery. An alternator that has an internal failure will stop producing voltage, allowing the battery to exhaust its reserve supply. A dead battery and stranded driver results. Testing the alternator stator requires disassembly of the alternator, "splitting the cases", and using a multimeter. A confident do-it-yourself repair person can perform the task by using some special tools and following a few simple steps.
Set the transmission shifter in park or neutral, depending upon your vehicle type. Apply the emergency brake. Raise the hood. Hook a multimeter up to the battery, by connecting the positive lead of the meter to the positive lead on the battery. Connect the negative meter lead to any engine ground source. Have an assistant turn all the interior accessories off and start the engine.
Read the voltage output on the meter. It should be between 13.5 and 14.5 volts. Any reading lower will indicate a charging problem with the alternator. A higher reading will indicate a battery overcharge condition. Have your assistant turn on all the accessories and raise the idle to 2,000rpm. Look for any under or overcharge in the system.
Disconnect the negative battery cable with a socket. Use a socket and wrench to loosen the upper and lower alternator bracket bolts and shove the alternator toward the centre of the engine. Slip the alternator belt off the pulley. For the serpentine belt configuration, use a socket to turn the tensioner pulley bolt to relieve pressure on the belt, then slip it off.
Remember the belt routing configuration, or refer to your owner's manual later during reassembly. Use a small socket to remove the eyelet wires on the back of the alternator -- remember their location.
Remove the upper and lower bolts to the alternator with a socket. Pull the alternator off and take it to a work bench. Use a scratch awl to scribe a line across the alternator case, so you can assemble the case in the same orientation. Remove the four long-bolts that hold the two cases together. Pry the cases apart with a screwdriver. Gently pull the rotor out of the case, separating it from its bearing seat. Look at the stator winding, the large copper ring.
Look at the three wires that join the stator winding to the diode pack. The wires have small nuts holding them in place on studs. Set your multimeter to ohms, and attach one of the meter leads to the first terminal and the second lead to the next terminal. The meter should read 1 ohm or less. More than 2 ohms indicates a shorted stator
Place the leads of the multimeter in all combinations across all three leads. Any combination that shows more than 2 ohms or indicates infinity, means the stator has a defective open circuit. Replace the stator or alternator.
Insert the rotor back into the case, if you have replaced the stator. Join the two case halves, with the awl scribe mark aligned. Replace the four case bolts and tighten them with a socket. Place the alternator back into its mount and screw in the upper and lower bolts tight with a socket. Only "snug" the mounting bolts tight.
Slip the belt back on, or use a socket to turn the tensioner pulley bolt and slip the serpentine belt back on. Adjust the non-serpentine belt by pulling the alternator outward to gain tension on the pulley belt. Tighten the alternator mounting bolts with a socket and wrench, leaving 1/2 deflection in the belt. Reconnect the wire eyelets on the back of the alternator case, in the same location you removed them. Reconnect the negative battery cable.
Refer to your owner's manual for the proper disassembly of your alternator. Removing the interior alternator parts varies for each make and design.