The voltage should correspond to the voltage on the battery. If there is no voltage then the wire is open-circuit. Check the connector and wire. Also give the connectors a tug to check that they are properly crimped. If everything checks out fine then reinsert the NEXT in-line fuses and proceed below.
Some tachometers operate from a terminal on the alternator.
The magnitude of the tachometer signal is dependent on the amount of current in the alternator field. Output current is also proportional to the field current, resulting in a conflict when field current needs to be low to regulate properly. Under these conditions, the NEXT reduces field current to a level consistent with the output demand, so that the batteries will not be overcharged. The NEXT is designed to be kind to the batteries first , and kind to the tachometer second. While special software in the regulator attempts to both regulate and still supply enough field current to keep the tachometer operating, the tachometer may still become erratic or even stop.