Hey Joey,
Testing alternators is often performed at higher RPMs. (as in, a load test)
So, they could be saying it it working....but in the idle condition it's NOT. Something else might be amiss.
So,
GRAB THE MULTIMETER. Report the battery voltage readings at rest; at idle; at 2000 RPM; and 2000 RPM with the lights and interior fan on HI. (Loaded)
+++++++++
My anecdotal experience:
Our '88 suddenly showed a low/wavering voltage while driving. (Older Esprits have a voltmeter, are you jealous?) I brought the car to an auto electric shop, and they said "the charging system is fine, the alternator is providing 70 amps"๐ฎ
Our Esprit has a 90 amp alternator!!!!!! ๐ฌ The issue turned out simply to be a loose alternator belt. (Anybody noticed that modern belts don't squeal when loose? )
So, if your multimeter doesn't show 14.5VDC over 2000 RPM, first look at the belt's adjustment. THEN see if the alternator is putting out at idle. Original 4 cyl Valeo alternators don't...
Do you have the Caddy Catera alternator fitted?