Lets assume Alt and Battery are good... (Likely not but say so for now... Have alt tested at auto part store.)
If some fool put a "power pulley" on the crank then can spin the alt too slow cause low amps regardless volts @ battery even w/ stock alt pulley.
Alt mounts are ground for the alt. crap/paint on alt, bracket and engine can F'd that ground. Chrome on any of that doesn't help.
See my Cave, Watt Story
and rest of alt pages then read more electrical if needed.
Check w/ Ω meter won't help much if any. Wire sizes are too big to use standard meters including "Pro" meters like Fluke 77 etc.
Look up "voltage drop test" instead.
Should see ~ 14.7v anytime the engine runs but if belt slips or have power pulley the alt won't make Amps even is get right volts.
Full Charge battery should be 12.6 - 13v after running the engine.
You likely need a AC/DC Amp Clamp or other method to read high amps. You will blow the fuse or fry most standard meters w/ 10 - 20a range.
these cheap options have guide/marks on the back so can read just by holding wires on the marks
Can test HL loads and others but might not read loads < 10a.
May not be best but give you ballpark to show problems.
Don't bother getting Used Amp Clamps... almost always they are AC only clamps use by Building Electricians dealing 120vac and high and won't work.
High Watt HL eat a lot of Amps. If Alt has issues, this only make more problems.
If Rad Fan and other E-motors see low volts for any reason them will try to pull more amps to makeup for this. See my Cave, Electric Motors
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
The Ogre's Fiero Cave