Also give the connectors a tug to check that they are properly crimped. If everything checks out fine then reinsert the regulator's in-line fuses and proceed below. If the temperature sensor is connected, you could be seeing the effect of the temperature compensation. If the temperature sensor is connected, you could be seeing the effect of the temperature compensation. If the regulator just tripped to float, the batteries may be retaining a residual charge.
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 regulator reduces field current to a level consistent with the output demand, so that the batteries will not be overcharged. The regulator 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.