Usage:
7N7412 Sensor Assembly Verify
NOTE: For bench testing, connect a voltage source with a range from 8-40 VDC (24 VDC preferred) across the electronic speed switch (ESS) terminals ESS-5 and ESS-6. Terminal ESS-5 is the battery negative terminal, and ESS-6 is the battery positive terminal.
INSTALLED SENSOR ASSEMBLY (Position shown is for RH rear of engine)
1. Tachometer drive housing. 2. 7N7412 Sensor Assembly. 3. Sensor harness connector. 4. Sensor drive end.
1. Disconnect 7N7412 Sensor Assembly (2) from engine tachometer drive housing (1). Set the voltmeter (6V3030 Multimeter or a voltmeter of same accuracy) voltage scale to a scale higher than 6 VDC. Connect the voltmeter to the 5N1955 Electronic Speed Switch with the positive (+) voltmeter lead connected to SW1 (ESS-1) and the negative (-) voltmeter lead connected to terminal ESS-5 (-).
SW1 CHECK
A. Slowly turn the shaft of 7N7412 Sensor Assembly (2) one full turn in a counterclockwise direction [as viewed from drive end (4)] and watch the voltage indication. A 4 to 6 VDC reading should be seen for all but one small area of the shaft rotation. Mark the position of the shaft where the voltage reading was zero volts. This point is called SW1 0° reference point.
NOTE: If no voltage is indicated between terminal SW1 (ESS-1) and terminal ESS-5 through one full turn of the sensor shaft rotation, check speed switch for proper voltages. A voltage of 4 to 6 VDC should be seen across terminals ESS-4 and ESS-5. If speed switch voltage is correct, then either the sensor is defective or the wiring from the sensor to the speed switch is defective.
After finding SW1 0° reference point (the position where zero voltage was measured on the sensor), an additional zero volts indication will be measured once for every 90° of shaft rotation from the 0° reference point by using the sequence that follows:
B. Connect the positive (+) voltmeter lead to SW2 (ESS-2) and leave the negative (-) voltmeter lead connected to ESS-5. Now slowly turn the sensor shaft counterclockwise. A 4 to 6 VDC reading should be seen for all but one small area of shaft rotation at an interval 90° from the SW1 0° reference point. Zero volts will be indicated at this area.
SW2 CHECK
C. With the negative (-) voltmeter lead still connected to ESS-5, connect the positive (+) voltmeter lead to SW3 (ESS-3). Again, slowly turn the sensor shaft counterclockwise. A 4 to 6 VDC reading should be seen for all but two small areas of shaft rotation; one at the 180° interval and one at the 270° interval (both measured from the SW1 0° reference point). Zero volts will be indicated at both of these areas.
SW3 CHECK
2. If all four zero volt levels are recorded in the proper sequence and at the proper degree of rotational position (90° apart from each other), then the 7N7412 Sensor Assembly operates properly.
3. Before installing 7N7412 Sensor Assembly (2) back on the engine, start the engine and check to be sure that the drive shaft in tachometer drive housing (1) is turning properly.
4. If tachometer drive shaft is in rotation, stop engine and install sensor assembly.
NOTE: The tachometer drive housing has two output drives (one marked A and the other marked B). Be sure that sensor assembly is always connected to output drive marked A.