Alright, I've never encounted a problem like this one before, so I'm not sure where to start.

My mother was driving her GMQ at around 25 to 30 mph, everything seemed as fine as it normally is on that car, when it just completely died.

The electrical system is almost entirely non-functional. No four-ways, no headlights, no horn, no power seat, no dome lights, no ignition.

Here's the really strange part-
When you open the driver's door (didn't check any others) the battery light and oil light come on. When you turn the key to "Run" they become slightly brighter, but no other changes.

Tried hooking up the jumper cables from my car just 'cause, but no change.

The car is a VERY early 1992 MY GMQ - actually manufactured in 1991, odometer froze up at 187k over two years ago. I was told the car also has been experiencing a shudder between 25 and 30 mph for some time now. Not sure if this has anything to do with anything.

Bad ground(s) and / or bad battery cables. Measure voltage @ the battery, with/without a load on it, if the battery is good, check ground connectons, the ones under the hood by the driver side hinge tend to have problems on some of the older cars.

Check those and post back.

DING!

Cables DO corrode from the inside, with the outside looking fine... I'm betting there's a bad cable to/from the battery, as Bull mentioned.

Ok...

Minor corrosion visible near the terminal on the pos cable, but good power and good ground throughout the system.

Continuity check between block and neg term - good
Between body and neg term - good
between block and body - good

Checked voltage at battery, power dist. block and alt. power cable, all read 11.8.

Here's another strange thing. The car has been sitting since yesterday afternoon. It was cold last night, so the engine is cold, BUT the alternator is warm to the touch. WTF?!

What kind of alternator failure could have caused these symptoms? I'm wondering if the voltage regulator failed and fried the computer, but I don't see any signs of overcharging on the battery like I've seen with other voltage regulator failures.

You have just found your problem. The ALT field coild is shorted to the case and making a "heater" literallty. When this happens, 3 things will occur; the alt is toast, the cable that runs to the dist box will generally have a fuse link in it that could have blown, and you will have a blown fuse in the relay box that will cause you exact electrical symptoms. The book will not detail which fuse is blown, I had to pull every one and check.

Remove alt from vehilce and replace with a lifetime warranty autozone or equivilent, replace the fuse and check fuse link. I just checked mine and the fuse link is almost directly near the post for the dist box. One wire comes from the batt cable, the other goes from the dist box to the alt, the fuse link in under the shrink tube right by the post. If you cant find it I will post a pic.

This happend to me on my uncles 95 GMQ, left his daughter on the side of the interstate. Alt was so hot I had to wear leather gloves almost 5 hours after it happened. His actually had a nasty odor coming out of it, so it was probably worse than yours.

All fuses in both boxes were good.

I'll go check the link now.

If fuses & fusible links had blown, the alt wouldn't be hot any more. Certainly not after sitting for a day/night.

So it's more likely a bad diode, which will ONLY kill the battery after a while. But it has nothing to do with the car dying & not starting. THAT's still most likely a bad ground.

No link.

It's just a straight cable from the terminal to the dist. box.

I double checked the voltage at the low current fuse block. Voltages at the fuses are showing between -35mV and 1.2V.

The interior lights under the dash are glowing dimly, but just not enough that I could see them in the dark last night.

So what would interrupt power between the distribution box under the hood, and the fuse block under the dash?

Still betting on the alternator?

The battery is still at 11+ Volts after more than twelve hours, so there can't be much of a drain.

Where would you start looking? What would be poorly grounded?

I did notice a ground wire that looks like it was installed by a prior owner running from the body to the neg terminal that appears to have overheated and burned through, but the body still has a good ground to the battery terminal.

You need to Ohm out your grounds, from the battery to the ground(s).

But it sounds like you need to fix the low battery and warm alternator first. What I would do first.....

Disconnect the 12V+ cable from the alternator, charge the battery, see if the car runs W/O the alternator hooked up, if it does, that may be your only problem, otherwise, you may still have a calbe problem.

The PCM grounds are on the driver side fender, almost all the way back to the firewall, you may want to double check those, to make sure they are tight and not corroded, also, Ohm them out from the battery negative to the ground point that they are tied to, high impedence will cause you tons of issues.

Disconnect, clean, & tighten every terminal on the heavy black (-) battery wire. While you're working on it, inspect it carefully for signs of internal corrosion (blistered insulation; stretches when you pull as hard as you can), especially at the battery clamp terminal.

Might as well do the same to the (+) while you're at it.