When I was a 2nd year apprentice Mechanic in the late 70s I left the mining world and got a job up north at a busy truck shop, the oil patch was booming and Mechanic jobs were easy to find (even for rookies like me). The shop rate was $27/HR and I was pulling in $10/HR. If there was one mechanical problem I resisted (no pun intended) the most it was electrical faults.
I'll never forget a White Western Star that came in and it had positive ground, you had to reverse your train of thought when it came to thinking through the circuitry.
Anyways, back then series parallel switches were very common (12 volts running voltage and 24 volts cranking voltage). You just had to remember how the wiring went back together while the customer was standing there looking at his watch.
Through all that experience up to now and hundreds of shorts, no starts and charging system problems I've come to one conclusion.
Keep it simple and don't over-think the problem. If wiring continuity is disturbed with resistance, short or open circuit you're going to have trouble.
SIDEBAR: If you ever have one or more circuits either not working or operating sporadically with relays or solenoids clicking on and off, there is a very good chance you have the blue stuff (corrosion) making a home in the connector or inside a module. It's happened to our equipment many times. The environment will take down anything that exposes itself!
If you look at an electrical system and think too broad you'll lose yourself. Think of the fault and the circuit, nothing else. A human friendly wiring diagram is most essential!
Here is a perfect example of a wire that had rubbed through the
insulation. Oxygen goes to work fast on this exposure. Luckily this
problem was obvious to the human eye and not nestled inside a connector
or module. One other method we use is grab the wiring harness while the fault is active and shake the hell out of it. Sometimes this works to fish out a problem. A short might all of a sudden quit or an open circuit might reconnect while you're at a certain point at the harness or at a connection point.
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