Volt meters use a small current to check for continuity and should show "0" even for small gage wire. They can barely turn on a red led due to the low voltage and current. A voltage drop should be checked from the engine ground to the ground terminal on the battery when at max charge and the plus alternator output should also be checked for voltage drop to the plus side of the battery.
4 gage is .25 ohms per 1,000 ft so .25 volts for 10ft at 100 amps.
6 gage is .40 ohms per 1,000 ft so .4 volts for 10 ft at 100 amps
8 gage is .64 ohms per 1,000 ft so .64 volts for 10 ft at 100 amps
Excessive voltage drop will prevent 1 wire alternators from achieving full battery charge. Remote sense alternators can compensate for the voltage drop.
I would expect 14.2 to 14.4 volts output at 2,000 rpm and above, if less the alternator should be checked if the belt(s) are in good shape and properly tensioned.
The local Autozone spun mine for free and it was found to be dead. If running at partial power, could be 1 or 2 of the diodes. Install a ZAP stop diode to prevent diode damage.. A small investment/ insurance.
Cal 9.2 #19 SilverSwan