Here are the symptoms:
- Battery was bench tested last month and deemed good.
- 2001 suburban wouldn't start the other day. My wife jostled the battery cables at the terminals and it fired up
- Driven around today with no problem, but after greasing the steering shaft it wouldn't start. No crank.
- Cleaned the terminals and tightened the connections. Now at least I have power but still won't turn over.
- Voltage tested at 12.5v at the battery terminals while not running. A bit weak but still should be enough to at least hear a crank, right?
- Voltage tested at 12.8v at the battery terminals while running and with headlights, dome lights on. Hmmmm. Makes it appear to be an alternator problem, although I just put a rebuilt unit in a month ago.
- Alternator tests at 14.8v from the alternator to the negative battery terminal. Good alternator? Weird. Why do I get 12.8 at the terminals but 14.8 between the alt. and the battery? Bad cable?
- Jumped the car and it fired up but died a few times. Finally got it running and let it idle for a few. Took if for a 5 minute drive. No problems. Ran as normal but battery needle kept fluctuating erratically up and down. Another indicator of a bad alternator?
- When I parked it and shut it off, I tried to immediately restart but it wouldn't even crank. Not even a clicking from the starter motor.
Thoughts?

 

A battery is typically 12.6 volts when fully charged. If by "bench tested", the battery was load tested, then yours is perfectly fine. Why was the battery reading 12.8 volts with the engine running? Because you had some power robbing accessories turned on. With 12.8 volts at the battery, it won't go dead.
"My wife jostled the battery cables at the terminals and it fired up". This is where you need to do some more digging. It appears as though you have a bad or loose connection on one of your cables. Check where the negative cable(s) goes to ground. Check where the positive cable(s) goes to the starter and/or junction block. Are the terminals at the battery in good, un-corroded condition?

 

do a voltage drop test on the insulated/power circuit and grd circuit. This will tell you if the wiring is ok or not. Its a simple 10sec check.

 

SOLVED!
Thanks for the suggestions. Here's how I resolved this today. I went to do a voltage drop on the positive cable but the way the 01 Suburban was designed made it impossible to get to the connections on the solenoid with my mammoth manly hands. So I pulled the starter. Starter fired up. But the voltage drop on the cable indicated a major drop. Pulled all the positive cable connections (starter trigger, starter amp wire, battery junction box, battery connection and the fuse box) and cleaned everything with a wire brush. Put some anti-corrosion gel on the things and reassembled. Then did the same with the negative. No issues there but just wanted to make sure the grounds were solid (engine block and frame). Charged the battery while doing all this. Fired right up.