98 Ranger 4.0 alternator not charging unless high rpm?

Hey y'all. 

I'm sort of stumped.  During my saga with the cooling system on my ranger I made a boo boo.  I was rushing and tried to remove the alternator before unhooking the battery.  Of course I know better, but again, just diving in and forgot.  I shorted the hot lead from the alternator on the body of the alternator.  I figured it was cooked and sure enough once I got the car repaired I had a low battery and a battery light.  

I first charged the battery and that did not work.  Then I got a new battery.  Then a new alternator and battery.  Same result- low on the volt meter on dash, batteries don't seem to be charging via the new alternator same as old.  

One strange phenomenon is that if I run the rpm high (it's a manual) the light will go off and stay off for a while if it's run at semi high rpm for a bit of time.  I don't have a tach on this truck- but it doesn't seem to be crazy high rpm.  The battery light will stay off until you turn the truck off.  Next time you start it....battery light on.  

I have been trying to keep battery charged with a small charger overnight.  

The last factor is this- this truck has a bad radio and I think in the past it may have drained the battery some.  So tonight I removed the radio to eliminate that variable- I need a new one anyway.  

I have checked all fuses, connections and wiring to the alternator.  

I believe the regulator is on the alternator.  Alternator is from O'reilly and it was not a reman- but I know those can be suspect.  I can change one out in less than 10 minutes at this point.  

Thanks for the help! 

If the battery light doesn't come on, that's the problem. Replace the bulb. If not, you probably burnt up the rectifier. It founds out against the case.

I thought so on the first alternator- but this one is doing the same thing. 

I'm looking at a diagram.... is the 175a maxifuse ok? It shows connected externally between the battery power and alternator output back into the underhood fuse block.

I've not checked the megafuse. Will do in the morning. Nice callout! 

100% megafuse!! Cost just under $6.  I'll be honest, I had never heard of that.  

Thank you Ranger! I guess you lived up to your name! 

You're welcome.

I've worked on Fords on and off for 35(?) years now. There is always a quirk to them.

That protection fuse seems to be HIDDEN in a lot of 80s and 90s Fords. It did it's job of preventing a meltdown.