Diagnosing automotive electrical problems is relatively simple provided you follow the rules and start from the beginning. Most problems with diagnosing electrical problems are caused by starting in the middle and trying to short cut the process.
The tool needed to make most electrical tests on a car is a simple digital multimeter (voltmeter). A good meter of this type for most do-it-yourselfers can be bought at stores like radio shack for less than $20.00. In order to properly diagnose and the electrical problem is to make sure that you start with a fully charged battery. This will be the first use of your new multimeter. Read the instructions and set the meter at DC Volts. Touch the red lead of the meter to the + of the battery and the black lead to the terminal of the battery without the engine running and everything turned off. A fully charged battery will read 12.75 volts.
If the battery is not this voltage charge the battery before going any further. If the voltage is 12.75 or higher turn on the headlights for a full minute, then turn them off and check the voltage again. It should still be near 12.75 volts. If it is proceeding to the next step, if it is lower have the battery load tested.
Once the battery is checked and passed, check all the cables to make sure they are tight and corrosion free, it is time to disable the engine so that it will not start. On fuel injected cars find the fuel pump or fuel injection fuse and remove it. The engine will still crank but not start.
With the voltmeter connected to the battery but where you can see it when you are turning the key to engage the starter, crank the engine for 15 seconds. While the engine is cranking, or if the engine refuses to crank continue to hold the key in the start position, the voltmeter should not fall below 9.6 volts. If it does and the battery has passed all of the initial tests it indicates a defect in the starter or an engine problem.
If the voltage is well above 9.6 volts and the engine does not turn and there is a single click when the key is turned to the start side it indicates a defective starter solenoid or a bad connection at the battery, the ground battery cable where it attaches to the car, or the cable running to the starter.
To check the engine for binding use a socket and a breaker bar on the bolt in the center of the front crankshaft pulley. The engine should turn over with less than 45-pound feet of turning torque.
If there is a click-click-click-click sound from the starter when trying to start, the most likely cause is a battery that will not produce enough electricity to run the starter.
Finally to ensure that the starter system will function correctly makes sure that the battery in the car is the right capacity. A small battery out of a small 4 cylinder car will not support the electrical needs of a car with a big 454 engine or a big diesel.
To test the alternator you must first do the battery testing and make sure the battery is OK. The alternator is a strange animal that requires electricity to be fed to it to convert rotary motion to electrical output.
A real simple test for alternator uses your new digital multimeter again. With the MM connected to the battery start the engine and turn all of the electrical accessories. With the engine running at road speed the voltmeter should read at least 12.75 volts. That indicates that the alternator will support all of the electrical loads and keep the battery at full charge. It would be preferable to see the voltage in the low to mid 13 range, however. If the voltage is below 12.75 the alternator is defective or in the case of an older car the separate voltage regulator is bad.
If the alternator fails turn off the engine and turn the key back on without starting the engine. Using a wrench or piece if iron that will magnetize (not aluminum), place it across the front of the alternator pulley. In most cars, except Chrysler built vehicles, the metal will magnetize and you will feel the pull. This checks what is called the field circuit. If you do not feel the magnetism find and check a fuse called Alt. If it is blown replace the fuse and try again. If there is still low voltage, the replace the Alternator.alternator