There are several factors. No it can't be bench tested at least by a store tester
"Not enough" at idle might be pulley sizes or design of the alternator. The older "round back" were worse at low RPM than more modern "square back" and some of the more current designs (Denso etc) kick that *** even more
Too high charging voltage is USUALLY voltage drop in either the hot side harness or can be the ground
The way to check for drop:
1....Turn key to run, engine stopped. Put one probe of your meter directly on battery + post. Put the other probe as close as you can get to the B+ (ignition) terminal of the regulator. Do not unhook anything in this test. If you still have a ballast, use the "key" side of that, or probe the blue alternator field wire
You are hoping for a very low voltage, the lower the better. More than .3V (3/10 of 1 volt) investigate. You are looking, normally, for drop in the bulkhead connector, the ignition switch connector, and the ignition switch, or anywhere else in the "path" from battery + to the VR IGN terminal.
2....GROUND. Make this test with engine warm, battery normalized, and first with accessories off, and again with lights, heater, whatever you have powered on. Stab one probe of the meter into battery NEG and the other stab into the mounting flange of the VR
Again you are hoping for a very low value, zero is perfect.
The total of these two readings is ADDED to the regulator set point. In other words if the VR would be charging at a "normal" nominal 14V (13.8--14.2) and say, you have a 1.2V drop in test 1, then the battery will run right at 15.2V
Other causes of too much voltages is (rarely) a bad battery, and once in awhile, a bad VR.
GROUNDING. Look at the front of the pass. side head. You have that/ those same bolt hole (s) at the rear of the driver side head. Buy a "eyelet to eyelet" starter cable, bolt it to the rear of the head, and either a bolt/ nut through the firewall, or something like the master cylinder stud. This will provide a VERY solid ground from battery/ block to body, "chassis."
FLICKER. This can be caused by a flaky mechanical regulator. In other cases it is sometimes caused by a weird "feedback" situation. First thing to try there is the ground cable mentioned above.
If you cannot easily fix voltage drop in the hot side of the harness, one way around it is to cut the blue "ignition run" wire coming out of the bulkhead at some point BEFORE it branches off. Use the end coming out of the firewall to trigger a Bosch relay, power the relay through a breaker / fuse from the starter relay stud, and hook the other cut end of the blue "IGN1" to the switched contact on the relay. This will then provide a solid voltage referenced to the battery for underhood loads, including the VR
