5.3 starter issue, and parasitic battery drain.

[rockola1971]

Just putting the key in and switching to accessory is dropping voltage down to 9.5V, to run drops it down to 8.9V and to start it goes down to 5V.

I thought I found something to do with IGN B, but I insert the key and sometimes get a voltage drop from 12.6V to 10V but it's momentary drop.

At this point I would suspect the mechanical ignition switch has fried internal contacts. Its a fairly easy swapout too!

 

[Fless]

Although your measurements showing the cluster drawing that much when the truck is asleep indicates a parasitic draw, it's not enough to cause the no crank issue with a good/charged battery.

There's something else going on here. You may have a bad battery cable or two, or bad grounds, or both. The battery cables tend to rot from the inside out and it's not always apparent when they go bad; they can have low resistance but can't carry a load. There are also many important grounds on these trucks and they should all be clean and secure.