HOW ABOUT SOME USABLE INFORMATION!
So, were you going to provide some or not?
What you are describing is the 'Excite' circuit of the alternator not working correctly.
If your engine starts and idles at a Very Low RPM
(less than 800 to 1,200 RPM)
And the alternator 'Excite' circuit isn't functioning properly, You will get EXACTLY those symptoms.
Revving up the engine will spin the rotor,
And RESIDUAL MAGNETISM
(no active electromagnet working, just what magnetism is left in the iron of the rotor)
Will eventually get the alternator charging,
Correct, self excitation.
Now, what normally provides the excitation? The lead from the ignition switch through the idiot light or resistor to the field terminal on the alternator.
At which time it will power the rotor it's self through the voltage regulator circuits.
So how does the charging current from the alternator “power the rotor”? By feeding back, through the diode trio, to the same point the field terminal is connected to, the one the wire from the ignition feeds.
Here’s a visual aid.
So how is it that current from the alternator output can provide excitation to the rotor but current from the feed to the field terminal can’t? Maybe because the connector to field terminal is dirty/corroded not making good contact?
Before you pronounce the alternator deceased, check and clean the connections and run a ground wire to the alternator case.
Just pull that plug and give it a little in and out. Yeah, the feed to the field needs a good ground return path too so a ground wire would be good rather than relying on rusted, painted brackets and engine block to do the job.
Hey, it don’t cost nuthin’ but a piece of wire and a couple connectors for that ground wire. Might save some time, trouble and money.