Batteries dead as hammers
 

2001 GMC 3500 4x4. Yesterday morning, I found it electrically weak, but "alive." Jumped it and ran it for about an hour around farm; some 7 hours later, she's dead; I mean NO evidence of ANY electrical power when you turn key. I know it's a widely open question, but what could pull it down to NOTHING? Is this typical of a bad alternator? Or will I be chasing sparks for a shorted wire?

Thank you. --Bryant

Bryant,
to help you some more details are needed.
How old is the battery?Is it still the first one? Is it a lead-acid battery? Just have in mind that a depleted battery will not be recharged within an hour, it simply can't hold the charge.
Also what mileage do you have? A common failure are just simple worn brushes. they can be replaced for a couple of dollars- no need to replace the complete alternator.
If your alternator does not deliver any current to your battery, it won't be charged.

Remember, only with a voltage drop your battery will be charged. That means the higher potential (14V generated by the alternator) flows to the lower receiving (12V stored in the battery) battery.
If you don't have that difference your battery won't be charged.

If you have a multi meter check voltage at the battery, please.

You should have a voltage higher as 13,6 V, but not higher than 14,5V with a running engine.
Battery voltages less than 12.6V measured after 24hours of non use indicate a low charged or depleted battery.
If your battery voltage is lower than 10.8V your battery is complete discharged and empty.
Starting your engine needs a huge amount of amps. An already weak battery can't supply and support that.
Do not even try to measure amps- it will fry your meter.

I hope this little background gives you an idea how to solve your trouble.

Here is your personal check list:

1. Check the battery voltage( A acid battery older than five years has lost almost 30% of it's storing ability anyway- time to replace it)
If you have to replace the battery - don't cut the corner and get an AGM battery, they are recharged much quicker than an ordinary lead-acid.
2. Check the alternator output voltage
3. If both points above are ok- start to seek for current leakage from added consumers.

I forgot to mention that all connectors have to be in good shape and tightened- no corrosion on the battery poles or on cables!

did you happen to install a new radio or anything like it, some times they draw power when the key is off. the abs modual will keep recycling if it goes bad and run down the batteries also

Fixed :)
 

Bad battery; but replaced them both upon the recommendation of my German friend, Deezel Stink3r. Thanks, Brother. :)

Two new BIG ones and all is well. Alternator seems fine @ 14.5.

Kindest regards,

--Bryant