Testing Hall Sensor. Failure Modes. The Hall Sensor distributor can fail through broken wiring insulation, a broken wiring connector at the side (this embrittles over time due to engine heat), or a failed Hall Sensor.

Quick Hall sensor test:[Note from Dave] the tachometer needle does jump slighty as you crank engine over if the Hall sensor is working, but the needle lays dead if the sensor is not working. 

Full Hall Sensor Test: Undo the distributor connector. When the ignition is ON the Voltage between positive terminal (red lead -Nr. 3) and ground should be approx. 11V. Voltage between (blue lead - Nr.2; middle one) and ground should be 5V although if you put it on a scope you would see the sharp rise and fall from 12v to ground as the distributor rotates. Pulse frequency varies as rpm is increased. . Resistance between (black lead - Nr. 1) and ground should be close to zero Ohms. Undo connector from the control unit (above the brake pedal) and remove the sealing washers (plastic inserts on the side of the connector). Replace connector without cover or sealing washers. 

- Disconnect the/red and blue leads from ignition coil. - Measure voltage between terminal 24 at rear of control unit connector (blue lead) and ground. NOTE: Connector must be attached to control unit. Switch on ignition. Turn crankshaft by hand. Voltage should indicate OVER 1.8V each time a vane passes Hall generator. Instrument should read approx."0" (0 - 0.7V) each time an opening passes Hall generator.  The correct voltage is less than 0.7V or more than 1.8V.