While troubleshooting a different vehicle it reminded me of your recently acquired '90 2500 freebie with the electrical gremlins.
In this thread, so far we've put a lot of focus on the grounds, and deservedly so. But at the same time we have to remember that
each of our individual electrical loads only work with the 'difference voltage' between the power feed on 1 side, and the ground
return that *they* see on the other.
And not the net voltage when the device isn't working (ie: yet to be called upon by the human) but more importantly, what is the
voltage across that electrical load while the current is being drawn? (ie: the power windows are moving, or the heater/AC fan
is on high, the headlights are on, the snow plow is being lifted, etc.)
In a perfect world, each individual device is under the impression that it has the vehicle's +12V Battery sitting right next to it,
connected by short, robust leads with no voltage drop across them. But as we all know, this beautiful power distribution vision
is not always the case, especially in mass-produced vehicles that have racked up this many miles & decades of service. (!)
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Apologies for the long setup, but just trying to emphasize for those new to automotive electrical troubleshooting that we only
have a total of ~12 volts to work with. (Actually 12.6v to 14.5v nominal, off/running, on a healthy setup.) In reality, a brand new vehicle
right off the assembly line will experience a tenth or two drop at the power window motor while it's operating, but that's to
be expected given the wire size compromises made during the design phase. And since it works well enough, no problem.
But once the lights are dimmer than they should be, or the defroster fan is noticeably slower, or the engine cranks slowly
even with a new battery & starter, now it's time to get out the multi-meter and see what's going on. Personally, if I see that
upwards of 1 volt or so is missing between the battery measurement vs the electrical load being troubleshot, I've got to
stop and bone up on the circuit.
Is this 'normal' operation, or have I discovered a marginal path that I need to restore to new functionality?
I am bringing all this up because the OP's vehicle is a '90, and therefore the '88-'94 wiring harness design rules are in play.
More specifically, between the battery and the fuse box is something called the "Battery Junction Block". It's on the firewall,
doesn't draw too much attention to itself, but between the underhood environment and all the vibration over the years,
the connections (nuts) loosen up, and voila! we end up with a voltage drop when electrical loads try to pull current through
these marginal connections. Here's a screen snap of a GMT400 owner who had cleaned all the grounds, and yet was still
chasing electrical gremlins...and he eventually made it all the way back to this pre-fuse power distribution interconnect:
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(credit: Here's a link to this YouTube video. As Hendrix would say, "Are You Experienced?" If so, probably redundant, but IF you are still learning this stuff, it's a good broke/fix story.
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Just wanted to share this with @Additivewalnut -- I really want to see him take his freebie electrical basket case
and make it reliable again. Also, if anyone else is reading this and has an unreliable GMT400 electrical prima donna,
maybe this thread, plus the relevant FSM, a decent meter, and some quality time on the wiring system, and you can
get the machine (re)conditioned to work on *your* schedule.
Hope this sheds some light on +12 volt electrical systems.
Cheers --
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