Got the battery problem somewhat narrowed down. Battery loses no charge when truck is just sitting, with pos cable on or off - tried it both ways in case of short.
Starting truck, causes 15-25% drop on battery......is that normal?

After driving, battery is nearly dead and has to be jumped or charged.
Does this sound like it must be the alternator for sure???? Alternator tested good at Schucks.

 

If the voltage regulator is not an alternator/regulator combo, you might check that. there isn't really much else in the system unless your belt is messed up.

 

Put a new voltage regulator on it, and the alternator tested good???

 

Jumpstarting a battery takes a little out of it everytime. you may just have some dead cells in the battery.

 

This is the 3rd battery in around 6 months. I need to find out what's killing it, before i replace it.

 

Have you measured the voltage at the battery and at the alternator "BATT" terminal with the truck running?
A good check is to start the truck and run it at @1500rpm, turn on everything, headlights, heater, etc and you should still have at least 13 volts at the battery +. If not the alternator isnt putting out enough and as a result the battery is being drained to keep up with the electrical load.
15-25% to start the truck.....hhmmmm....depends on how long it takes to start and how much load the engine is....meaning high compression is harder for starter to turn.
Just wondering how are you measuring the percentage used? If you are going by the meter on the charger I wouldnt put too much faith in its accuracy...... but that is just me. Best way to check a battery is with a load tester.....should be able to get it checked at a Parts Store...... I would think if they sell batteriesthey would have one...but whether or not they will test it for you at no cost.....
When all is said and done you may have a wire problem.

ps
Check out this diagram from Franklin2. It shows how the system is wired.

https://www.ford-trucks.com/user_gal...=16603&width=0

 

I am havin the exact same problem. Ive had at least 5 different batteries in it, changed the voltage regulator, the starter, here in a few hours we are going to check the timing. if you look in the hayes manual, the diagram shows the wire that goes to the choke in frankllin2's diagram also goes to the s terminal on the voltage reg, battery was to dead to start this morning, but i am going to try wiring that up

 

Mine is a 68. If you find anything let me know, and I'll let you know if I find anything.

 

That is how it is wired if you have an "idiot" light instead of an ammeter. Sorry I am so used to ammeter trucks I should have shown both diagrams.

https://www.ford-trucks.com/user_gal...=28796&width=0

 

Working on it right now. Cold outside, so it died after initial start, then wouldn't start again. Charged battery back up to 100%, only took 5 mins, started right up.
The battery is 875 cca; being only down to 85 - 90%, shouldn't it still have enough power to start truck? It won't do it unless it's at 100%.
I can start it by jumping it with my other car which only has a 660 cca battery.

 

Yes it should have started.......the only thing that comes to mind is a dead/weak cell in the battery, it will show 100% and you will have 12 volts just wont have the 875 cranking amps available.
If possible fully charge the battery and get it load tested, that will show if the battery is weak.....any Parts Store that sells batteries should be able to test it.
Another thought.....is it possible that your truck is overcharging, running at higher than 14.5volts??? That will kill a battery by boiling the electrolyte, since you have gone thru a few batteries it could be your problem. Check the voltage output of the alternator with the truck running, even better hook your voltmeter to the cigarette lighter or some other 12 volt source in the cab and go for a drive...............watch the voltmeter and see how high the voltage gets. Do not stare at it though as you may hit a tree or something :-)

 

I recently had a alternator problem that I used a voltmeter plugged into the cigarette light to find. Here's the story.

1. Started to leave on long trip for Thanksgiving. Got about 100 miles from home and battery light came on. Turned around and went home.
2. Checked voltage at battery with engine running. Got less then 12 volts with lights OFF.
3. Bought remanufactured alternator. Checked voltage again. 14.4 volts with lights on.
4. Headed back out on trip. Car quit 20 miles later with Check Charging System message on message center. No Battery light this time.
5. Car towed home. Took other car.
6. Get back from trip. Voltmeter hooked up to battery shows alternator working.
7. Buy Voltmeter/Clock/Thermometer from Radio Shack that plugs into cigarette lighter.
8. Use cigarette lighter voltmeter while driving.
9. Found that alternator worked until engine heated up. That suddenly quit. Took about 5 minutes. Voltage dropped to less then 12 volts.
10. Took alternator back and exchanged.
11. Exchanged alternator working flawlessly now for 1 week.

When I took the alternator back the parts guy was going to check it. I told him that it would probably check ok since it was now cold. He would have probably insisted that it was ok if he had been able to find the correct plug to hook it up to his tester. I was getting ready for a big argument.

 

Mine could be a mystery wiring problem. I don't have a heater fuse, but the heater works. I don't know what's been done to the wiring over the years.

 

Well you could always re-wire it according to the factory diagram. It isn't very hard all the wiring is under the hood except one wire that comes from the ignition switch.

 

kioolt

That was a good story and good troubleshooting methods fixed the problem.