I have had the same on the landrover with a tacho driven off the "W" terminal of the alternator. In my case it was because the warning light had too low a wattage bulb fitted to pass suffecient current to excite the alternator, Most alternators will however self excite when they reach a high RPM,(even without a warning lamp) then they are fine until they are shut down. so if you give the engine a good rev the tacho suddenly springs into life.
The other thing mine does if the alternator belt is slightly slack the tacho will read 500 rpm and no more as the belt slips, then when the battery voltage recovers after cranking to start after a minute or so things return to normal.
Try tightening the belt and check the warning light bulb is at least 2,2watt and the correct voltage. some manufacturers fit a small wirewound resistor across the warning lamp so the alternator will cut in initially cut in at low rpm if the bulb fails or is replaced with say a 0.5 watt one.
If this fails then maybe a smaller alternator pulley and tacho recal is required?
Just to check it is lack of signal from the alternator, connect a DVM on AC volts between W and ground. the AC voltage should be present if the alternator is "excited" and charging and it should be around 14-16 volts depending on the type of diodes used in the alternator. I guess the voltage is insuffecient to trigger your tachos until you give it a good rev.
The tachos are 12 volt? maybe 24volt ones work....just?
I am usually not as green as i am cabbage looking.