Battery Light On after ignition key swap?

Yep, let's find out what is wrong which might be the alternator got popped during the event. With the engine running let's check the voltage at the batteries.

here is a guide to help us see if the alternator is charging or not:

https://www.2carpros.com/articles/how-to-check-a-car-alternator

If not let's check the fuses in the under-hood fuse panel: IGN E and SEO EGN let me know what happens.

Working on it. So far this is what I have. Prior to starting, battery No. 1 (Driver) 12.89V, Battery No. 2 was 12.63. While idling ~ 1500, batt 1 14.28 was read, over on batt 2 it was 13.60. This is what I have so far.

Okay so it is charging but once the batteries looks like it might be bad causing the system to give you the waring light, I would disconnect the battery #2 and have it load tested at the auto parts store. Let me know what they find.

10-4, wilco.

Over and out!

Okay, we are able to come back full circle on this. All fuses (SEO IGN, SEO B1, SEO B2, IGN E) are all good. Both batteries were load tested today, and they are both good. Still Red battery light stays on.

Keep in mind that this is after a stolen attempt and a key cylinder and switch had to be replaced.

Lets check for power at the fuses with the key on, this guide can help you. Also, I have included the charging system wiring diagrams so you can see how the system works.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Check out the images (below). Let us know how it goes.

Okay, update. Voltages are for SEO-B2/IGN E/SEO IGN/SEO B1 in that order. At the fuse block KOEO 12.54/11.57/11.57/12.63, KOER 14.06/13.97/13.95/13.96. All fuses are good. Voltage at the Passenger Battery KOFF 12.28, KOER 14.13, Driver KOFF 12.29, KOER 13.97. Battery light still on.

So the light was not on before the repair? Seems strange because the ignition lock does not factor in for the charging system. This truck does have two alternators, correct?

No it was not. One alternator.

I am wondering if the alternator has lost a diode, I would use the volt meter to see if while the truck is running you can measure for AC voltage, the guide above will show you how. Does the ignition switch/lock seem to work okay?

The switch/lock seems to be working fine. We had issues getting the ignition switch gears to line up with the cable, but we finally got after 2-3 times. That is all that we can remember about that. We keep thinking "Did we miss a tooth? on the alignment", but we figure it would not work if it was off a tooth (inadequate key travel). We will check the alternator and report back.

As you can see by the wiring diagrams there are not that many wires or items for the system.

Okay, update. I went to test the alternator (single) and got 14.3x on both batteries. When I quickly switched from direct current to alternating, it flashed 35.x for just long enough for me to see a value, then it went to 0.0. Now before we went to test it, we reconnected the batteries (otherwise, they would drain and die) to start it up so we can move the truck as we needed to. Within minutes, the gas pedal was effectively neutered and the message 'Reduced Engine Power'. We read the ODB Codes and it was littered with codes (all along prior no codes were present) in this order P0192, P0201, P0203, P0205, P0207, P0602, P0604, P0612, P0643, P0652, P0699 and P0802. We took images of the ODB reader and cleared the codes. As of now, they have not come back. One other thing we noticed was the grounding strap from the firewall to the hood was severed. Not sure if that comes into play here. So, the question here is does the 35.x AC voltage (Scale 600V - lowest we had on that multimeter) on the alternator test present a problem even for a flash and can a signal like that blow up the ECM and produce these codes?

I think the alternator is okay it is common for the voltameter to flash voltage then give you the reading. It is starting to look like the PCM is bad, lets send it in for testing. Can I ask how old the batteries are? Sometimes a battery can test good but have a weird internal error fyi. You can find a place to test the PCM if you search google or ebay. Check out the images (below). Please let us know what happens.

Both batteries are new (and they both pass load tests) to eliminate them as the culprit. I went ahead an purchased and new alternator and will install it next week. I will check on the PCM.

FWIIW: We replaced the FICM module about two years ago. It actually was fried on the CB and was not repairable. Paid an arm and leg for it at the time for another one.

Yep, that is the logical next step, seems strange it is charging with no AC voltage but you never know.