Electrical fault tips
If you would like more information on the basics of reading electrical diagrams there are many very good articles available on this site. Once you understand how to read those diagrams the next question becomes how to use them to speed up the diagnostic process. When looking at the diagram you have to be sure not to get tunnel vision. In other words, make sure you remember the component(s) you are testing don't operate in a vacuum, they are likely connected in one or more ways to other components on the vehicle. I like to start at the component itself and then work outwards. I typically follow the positive side wiring on the diagram and find a junction point, connector or splice where there may be additional components supplied from the same voltage source. If you find any common positive side wiring like that it's as simple as activating those components that share the wiring to see if they work or not. If they do work, then you know the problem is not further back in the circuit. If they don't work either, then you know the problem must be further back in the circuit. This type of electrical diagram analysis isn't meant to replace pinpoint testing, but rather to reduce the time spent doing that testing. A very simplified example would be: Wire "A" supplies wires "B," "C," and "D." A component connected to wire "C" is inoperative but those connected to wires "B" and "D" still work. Given that fact there is no reason to test wire "A" because a failure in that wire would have resulted in failure of all connected components. In essence you validated the integrity of wire "A" by using the diagram and the other components that also receive power from wire "A." This type of diagram-based pre-testing work can significantly speed up your diagnosis.
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|How do you know what voltage each wire should have without a diagram?|
Once you've narrowed down the parts of the circuit that will need to be tested as much as possible the next step is to perform a thorough visual inspection. This doesn't mean disassembling the entire vehicle, opening up all of the wire looms, etc. but you should perform an inspection on parts of the circuit that are readily accessible. Things you should be looking for include signs of physical damage, corrosion, previous wiring repairs and improper routing. Previous repairs should be inspected closely as it's not uncommon for a poor-quality wiring repair to fail again.
Of course, once you've narrowed down the parts of the circuit you need to test, you'll need to know what the normal readings should be at each point you'll be testing. This may seem simple to identify which wires should have battery voltage and which ones should have ground but remember to take into account the effects of switches, etc. Also remember it's best to test the circuit without disturbing it if at all possible. Any time you disconnect connectors for testing purposes you run the risk of covering up or temporarily fixing the problem you are chasing.
Even after you've done your testing and are fairly confident you've found the root cause of your electrical problem you still aren't quite done with your diagnostics. The last step in your diagnostics should be to attempt to bypass the problem area and confirm correct circuit operation which accomplishes two things. First, it lets you verify your diagnosis before beginning a permanent repair. Second, it allows you to actuate the rest of the circuit to ensure there aren't any other problems present that are being hidden by the failure you've already found.
Once you've identified the failure and repaired it you should do a final verification of the circuit. It's important to not only see if a component works, but you should also verify the integrity of the wiring to/from the component. That testing is best done using a voltage drop test on both the positive and negative side of the circuit while the component is operating. If the voltage-drop tests pass you can feel confident in your repair and not have to worry about a potential comeback due to an underlying problem that was missed.
This same process can also be used to help speed up the diagnosis of check engine light problems related to things like actuators or sensors. It's a similar process where you can use a wiring diagram to check for electrical commonalities in the components. If, for instance, you had a code for a single fuel injector circuit you could use the wiring diagram to verify which parts of that circuit are potentially shared with other injectors that are still working. Any parts of the circuit that are shared with other components which are still working don't need to be tested.
In the grand scheme of things, if you can explain and test the electrical integrity of a simple relay circuit you can test virtually any 12V circuit. The electrical tests on other components/circuits follow the same exact principles, but you do need to effectively interpret wiring diagrams to know what "normal" is for each test being performed.