You may have studied circuits, but you should troubleshoot current paths. Circuits get complicated, and when you troubleshoot current paths, you simplify the complicated. There are only two current paths that you will work with regardless of the circuit load(s) when you look at circuits from a current path viewpoint. The first current path is always between the battery positive (+) terminal and the input(s) to any load on the vehicle.
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The top load (Figure 1) represents a single load switched to ground circuit. The bottom loads represent loads in parallel that are switched to voltage. When you look closely at a wiring diagram (Figure 2), you can pick out the inputs to parallel current paths. This example is a power train control module with four voltage inputs, two receiving battery voltage at all times and two hot when the key is in RUN and START. I look at these current paths no differently than I look at the current paths in my simple drawing or any switched to voltage feed (for example, to a headlight or taillight). They are dealt with in the same manner when you troubleshoot current paths, and not circuits.
The second current path in any circuit is always between the output of any load and the battery negative terminal (Figure 3). The current path from the output of the two different circuits provides ground from the output side of each load all the way back to the battery negative (-) terminal. The parallel loads could be two backup lights, or two brake lights or two ground outputs on a transmission control module.
I hope you are getting the idea that once you begin troubleshooting current paths you will not be intimidated by complicated wiring diagrams. It is the current that gets the work done in any circuit. Current (electrons) flows, voltage does not flow. Voltage is the difference that exists between the battery positive plate material and the negative plate material. The positive plates are void of electrons, have lots of holes in their atomic structure that are seeking free electrons. The negative plate material has lots of excessive free electrons just waiting to find a mate on the positive side of the battery.
Engineers hook all kinds of loads between the battery terminals, place protective fuses and circuit breakers in for safety in the event of a short circuit, and provide some means of control which could be a mechanical switch, electromagnetic relay, or solid state driver, whatever engineering can dream up. The bottom line to all of this is that nearly all of the potential difference (battery voltage) should be used to push (or pull) electrons from one side of the battery to the other passing through some useful load to get some work accomplished.
In some cases, nearly all of the voltage existing between the battery terminals is used to do work. In other cases, some of this voltage is dropped intentionally to limit current by placing resistors in the current path. In the case of our power train control module shown in Figures 2 and 4, current limiting resistors are inside to do just that. Even though current is intentionally limited inside control modules, troubleshooting a module’s voltage feed side and ground side current paths is the same as troubleshooting the voltage feed and ground side of a headlight or taillight.