Had the following problem start up the other night...... Went and started my truck and was warming it up. All of a sudden charge and brake light came on idle increased 500 rpm. Note very cold -20c. Drove to work volt meter showing 12v. Lower than normal. Came home checked system with reliable meter 12.5 v at 2000 rpm. Charged batterys (running two batteries) Figure its alt. buying time until the weekend to change. Now tonight coming home lights and heater on volts started dropping fast, tach doing crazy things, lights dimmed etc. Limped home, engine missing, 5 volts on meter. Let it sit 3 hours figuring I'd have to boost it to get it into garage. Nope, fired right up 12v on the gauge lights working everything normal except charge and brake light on. (realize volts low). Both batteries are new and equal size 850ca. Even with pooched alt why low voltage and dead in the water then 3 hours later she starts perfect and 12v on the gauge. This situation does not compute with me.

It's going to be tough to tell you what that is remotely, but 12.5v while running indicates a problem. Ideally you should see 13-13.7 or so at the battery.
I'd have my battery and or alternator checked...
Course, I've got no idea of what it's like running a truck at -20c

the power in the batteries is low, put a charger on them now so they don't freeze tonight.

the 12 volts just means that the cells are working, not how much power is in the battery.

they recovered chemicaly and started your truck right up.. then ran so low on power that the voltage dropped.

like a telsa coil... heck of alot of voltage, but no amperage. high voltage, low power.

I'd try some new brushes first.

If that 12.5v is leaving the alt then I agree. If that 12.5v is at the battery I would trace back to the alt to verify all the connections are still good before tearing the alt apart.

I can't help only sympathies. Last week my truck did the exact same thing, charge light went on. Volts went down to 5v over the battery. Tach went down to zero, even thought engine was running. All the lights went dead. My truck stopped running about a mile latter. IÆm guessing you have the same problem as I do, which is a bad alternator. I replaced it about a month ago and think I just got a bad unit. The battery will hold a charge so I donÆt think itÆs that. And even if the bad doesnÆt hold a charge the car should still have at least 13-14V from the alternator to use for lights and what not. The other issue it may be is a short in the system. If it is I wouldnÆt have a clue on how to track it down. Actually I think IÆm going to post a question about tracking down a short to just to find out how to do it.

yeah, I'm pretty sure alt is gone, I've been charging overnight to buy time to the weekend, but I've never seen or heard of a battery going dead so that starter wouldn't turn over just click and then a few hours later I might add at -20c come back to life like nothing happened. Thanks all for advise.