Good Battery Power, Won't Start

Did you check each battery individually?

And look/clean all high current connections?

You may be showing good voltage, but what you are describing is a low voltage problem. What you are describing with the radio I have only seen happen when you disconnect power and then reconnect it.

You probably have a battery with an intermittent shorted cell in it. With no load they will show good voltage, but as soon as you put a load on it the cell shorts and kills all the voltage.

Check your grounds too. Matter of fact i'd just go thru every connection clean, scuff dielectric grease them. Battery terminals, grounds, starter, etc. I'll bet your problem goes away after that.

If they're the stock batteries then I'd say you got your moneys worth out of them.
Replace them, they're tired...

Cleaning all the wiring connections is a very good idea if you live in a wet or salty climate.

12.2 V ea., cleaned connections, still won't start.

Thank you all. I'll continue to chase connections while the batteries each charge individually. Unfortunately can't check specific gravity or load test here.

Rt. side positive had corrosion, mostly on auxiliary cable marked "A." Haven't traced where that goes yet.

Had significant draw when I reconnected the first battery. Key off, underhood light only possible thing on. Radio off, but still cycled through the cd exchanger when battery reconnected. Strange. Will disconnect underhood light when I reconnect things.

Was the motor cold when you tested it? It could have been the grid heater cycling if the motor was cold. That will draw some serious power.

Underhood light isn't your significant draw. My experience, the ECM draws a bunch of power at hookup time. It is temporary.

More than likely you have one bad battery. Charge them individually, and then check them individually. If you have one bad battery, its lower voltage will drain down the other good battery overnight, and then you have one drained battery and one bad battery = no start. Solution is to replace both if they are stock. If they are relatively new, you can get away with replacing only the bad one.

It was the batteries

Thanks to all who replied. Since there hadn't been any slow starting or signs of poor battery power, that wasn't my first suspicion. Each had 12.2 V.

After cleaning every connection imaginable, when it still wouldn't start, I took out the batteries and had them load tested. The second cycle of the test pulled one down to 9 volts. I replaced them and it starts fine.

FYI - The only symptom the truck showed was an intermittent dash display of the Electronic Throttle Control icon. I checked my wife's old receipts, and the only symptom it showed the last time they were replaced was the same thing.

Again, thanks!