Trouble with tachometer

You should start the troubleshooting at the sensor in the engine. Use a voltmeter in AC measurement position and observe the voltage at the two terminals of the sensor as you rev the engine up (have either an assistant or a long meter probe). If you measure absolutely nothing, the sensor is suspect (although you can check it with an oscilloscope if you can find one somewhere). If you obtained some readout, repeat the check at the instrument panel, at the blue connector mentioned (one pin is ground, one is +12 V, but the remaining two should exhibit the same behavior as the terminals of the sensor). If the readout is identical, the tachometer is suspect. If you obtained no readout, trace the wiring back through the bulkhead to the engine compartment and repeat the measurement at every connector it passes. If you found +12 V in any of the sensor wires, you have a shortcut somewhere.