Thanks! My motivation to lay that all out is that a lot of GMT400 owners view the electrical system as
semi-random spaghetti, especially when it fatigues to the point of acting up. Then they search for a shop
that troubleshoots electrical issues, and IF they can find one, they charge $$$/hr due to supply
& demand. And then after some owners experience a loss of reliability, a few actually give up & bail
on their vintage workhorse.
Obviously my belief is that the GMT400 is the best combo of affordable Functionality and long-term
DIY maintainability. What I mean by this is that the square bodies are also simple, but due to supply and
demand, the price of admission is steep. Meanwhile, the newer-generation trucks are ever more complex
and less DIY friendly.
So here we are. The GMT400s have great engineering, but the wiring systems are starting to show their age.
(Or rodent damage. Or PO hackery. Or attempted theft. Or undiscovered marginal spots inflicted upon the
harness from long ago fender benders. Or simply the shared, rusted grounds.)
But your problem was a perfect excuse to highlight how understanding the basic architecture of the
electrical system can help us minimize the amount of real estate that we have to inspect before we
even pop open the hood. And we do this using SDD. (Symptom Directed Diagnosis.)
And the last thought I have on this troubleshooting subject is that another way of saying the above is
to "Use the complexity of the system against itself". You see, the way that all the electrical circuits on
the positive side are so carefully separated from each other via all the fuses, circuit breakers, fusible
links, and even which circuits are on at each individual Ignition Switch position -- if we understand
this then we can use this intelligent layout to eliminate troubleshooting 7/8 of the pie, and just focus
on the remaining suspect slice.
And if logically-unrelated functions & circuits affect each other? Well, that's a shared ground issue
gone wrong.
****
Especially with the access to the General's FSM Electrical Manuals plus the folks in this forum, I believe
that anyone who wants to keep their GMT400 electrically reliable can do so if they are willing to learn
the basics and keep their electrical connections clean.
Fun stuff. Glad to hear that you discovered the root cause, and once again thanks for following through,
closing the loop, and sharing the fix with the rest of the forum.
Happy 4th of July --
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