Battery, starter, or fly-wheel. Or something else?

First of all, never, never, ever disconnect the battery while the engine is running. I can paste a copy of what can happen, but for now, if a mechanic got caught pulling that stunt, he MIGHT get one verbal warning, but for a second offense you can be sure he would be fired. Disconnecting the battery to replace it, or for any other reason, while the engine is running, has the potential to destroy every computer on the vehicle, and your truck can have over 20 of them.

A few decades ago uneducated mechanics who didn't understand how these simple systems work, removed one battery cable to verify the charging system was working, but that is a very inaccurate test, and those cars only had light bulbs that could be burned out.

What you described sounds typical of a failing charging system. You can start the testing yourself although the results can be misleading. Use an inexpensive digital voltmeter to measure the battery voltage while the engine is running. It must be between 13.75 and 14.75 volts. If it is low, suspect the alternator, but don't replace it until we perform a few simple tests to rule out other causes.

If the voltage is within the acceptable range, a professional load tester is needed for the rest of the tests. That involves measuring how much current the alternator can deliver under maximum load for only a few seconds. It must be able to develop its rated current which is typically 90 amps or more. A common failure is one failed diode of the six. With that, the alternator will only be able to develop exactly one third of its rated current. 30 amps from the common 90 amp unit is not enough to run the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down over days or weeks.

The load tester will also measure "ripple" voltage. If a diode has failed, ripple voltage will be very high. It's that ripple voltage that can trigger the charging voltage to go as high as 30 volts. The battery is the key component in damping and absorbing that ripple voltage and other voltage spikes, and the voltage regulator needs it to hold the charging voltage down to a safe level.

I'm really aggravated with the insane engineers who seem to have the need to hang an unnecessary computer onto every system that never needed a computer before. (My '88 Grand Caravan daily driver has one very appropriate Engine Computer, and that's it. I have all the toys, like power windows, locks, and seat, and none of them need a computer.

When a computer DOES fail, you'll see some function or system no longer working. When system voltage goes high enough to damage a computer, that one will be dead, and possibly blow a fuse or two. What usually will not happen is individual functions within that computer stops working. As an example, a Transmission Computer that's damaged this way will cause the transmission to stay in one gear. It will not cause something like shifting at the wrong speeds or skipping a gear. A Body Computer won't cause just inoperative interior lights. It will cause all the system it controls to be dead. That includes wipers, power locks, and in some cases, power windows and / or remote keyless entry.

To check the starting system, start by measuring the battery voltage, but the meter probes should be right on the battery's posts, not the cable clamps. A good, fully-charged battery will measure 12.6 volts. If you find it close to 12.4 volts, it's run down. If it's good but totally dead, it will measure closer to 12.2 volts. In that case, charge it at a slow rate for an hour, then recheck it. Once it's at 12.6 volts, it should crank the engine fast enough for it to start.

If you still have a slow-crank condition, measure the battery voltage again while a helper cranks the engine. Let me know what voltage you find during cranking. If it stays above 9.6 volts during cranking, move the voltmeter probes to those cable clamps and measure again. You should find exactly the same voltage. If it is considerably lower, one of those connections is loose or dirty.

If the voltage right on the battery posts drops below 9.6 volts during cranking, even after charging the battery, it is worn out. Normally there's no guesswork. With a bad battery, the voltage will drop real low. Also, it may measure around 11 volts or less when you're not cranking the engine if it has a shorted cell.

You might get away with 13.68 volts from the alternator, but the normal range is between 13.75 and 14.75 volts. With a fully-charged battery like yours, charging system voltage will rise a little, so I'd like to see more than what you have.

They missed one important test. That's current output from the alternator under full load. A common maximum current rating is 90 amps. As I mentioned a few days ago, if your alternator has one bad diode of the six, you will lose exactly two thirds of that capacity, and 30 amps is all you'll be able to get. That's not enough, and you'll have a battery too run down to start the engine within a few days again.