Saturday night I drove the truck to a local tractor pull, after a morning heater core replacement. All day the truck started fine with no issues, alt. gauge showing +/- 14 volts. Parked it at 8:45PM, watched the pull went to start it at 12:45AM. First thing I noticed was when I hit the unlock, the interior lights were very dim. Starter wouldn't even crank once. No headlights or auxillary lights were on. Took 20 min. to get enought juice to jump it (on a good set of cables). Upon starting, the alt. gauge never read above +/- 13 which it's never done before. It ran for an hour and then I hooked a trickle up overnight. The charger was still showing max. draw after being on the truck for 10 hours. However, it started fine and has been doing OK since.

Any ideas as to what could've happened?

I have an amp in the truck, with a signal wire on/off. Could a small short kill two virtually new batteries in 4 hours and then somehow repaired itself? I did check the batteries, they're running fine. Heck, it takes my daylighters 2 or three hours to kill the truck. Is this one of those chalk it up to a mystery and wait for the prob to come back?

 

That's a high draw to drain 2 batteries , if it were a P-pumped I would check the fuel shutdown solenoid , I forget what your truck uses ?
But do not forget to check connections , pos. & grnd.
Having access to a load tester is a must for something like this .

 

if there is a short it didn't fix itself! it may have rubbed through somewhere and intermittently be laying on the short, but it will come back (and probably at the worst possible time).
wiggle the signal lead to the amp and see if the power light flickers, maybe it's a problem in the amp relay or switch?

 

So you had the dash assy out for heater core replace. There are only 12 dozen places that could have secured wiring damage. Look as possible under dash for a fubar or three. My guess is problem will reoccur and severity of damage will increase. Might be looking for redo of dash job. Sorry

 

Maybe it is just coinciding with the dash/heater core replacement - that the mechanic ran drained the batteries down to a point where one of the batteries was on the verge of and now has a shorted cell. I had an issue where one shorted battery cell, pulled down both batteries to where I couldn't get cranking amps. Since the batteries are in parallel, try disconnecting the positive battery post wire that connects the batteries electrically, then measure voltages. If one of the batteries has a shorted cell, both batteries will be adverse effected. If one of the batteries measure 12+ volts after some recovery time, and the other is dead or near dead voltage, you'll be able to rule out what maybe a bad battery?

 

Its true the dash is completely fubar'd, I've been looking at a redo on that for a while. Its a piece of junk. I was fairly careful around all connections and grounds though. If it happens again I guess I'll drop the column again and start looking for a short somewhere under there.

The batteries were fine right after the heater core replacement, fired it up to watch for leaks in the lines and connections.

I'm hoping that somehow the amp had an internal switch that turned on and started the power draw (though it doesn't seem that it could pull both batteries down that fast). Its really old and has plenty of bizarre quirks.

Also going to re-program with the smarty this afternoon. I sure do hate waiting for something to happen, and fail catastrophically. There are few things worse than electrical demons.

 

I would start by testing the batteries. I had a cell take a dump as mentioned above and it caused all kinds of havoc. If they're good..
Put the ammeter in the battery circuit and start pulling fuses.. I doubt it's wiring damage sinse a hard short will burn wires or blow fuses before they drain what, 1500 CCAs?
It's not to uncommon to have two problems at once.

 

I had the grids stick on and kill the batterys, but usually this problem should only happen in cold weather.