Digital Tachometer SELECTRONIC^®

Typical Wiring Diagrams for MT90

Terminal 2: connects to battery (-) or ground

Terminal 3: connects the back-light to battery (+), (back-light can only be used when powering from battery)

Terminal 4: connects to battery (+) or power from magnetic pickup or alternator

Terminal 5: RPM input signal from magnetic pickup

Terminal 6: RPM input signal from alternator

NOTE: Gap from face of gear tooth must be enough for gear to move. Rotate gear completely to be sure of minimum, no-touch clearance. See instructions supplied with the magnetic sensors. To get minimum of 4 VAC RMS, gap tolerance is critical. Turn the pickup in until it stops against the face of a gear tooth. Back the pickup out only enough to allow rotation of the gear. Rotate the gear, if any tooth touches the pickup, back it out to clear the tooth. After clear rotation secure the pickup locking nut.

NOTE: Gap from face of gear tooth must be enough for gear to move. Rotate gear completely to be sure of minimum, no-touch clearance. See instructions supplied with magnetic sensor.

Calibration

To calibrate the MT90 for your engine, remove the rubber seal on the back of the tachometer and dial in the correct number.

• Magnetic Pickup: dial in the number of teeth on your ring gear. Set the switches from left to right. For example, if the engine gear has 125 teeth, dial a 1 on the left switch, a 2 on the center switch, and a 5 on the right switch. Using this setting, the MT90 will count the passing teeth and convert them into engine RPM.

• Alternator: Multiply the ratio of alternator to engine pulley diameter, times, the number of poles of the alternator divided by two, to determine the correct calibration number (also see “Other Calibration Methods”).

Other Calibration Methods (teeth or ratio unknown)

For setting the calibration switches when the number of pulses per revolution are not known, set the rotary switches on back of the MT90 to 0-6-0 and read the tachometer at a known engine RPM. This can be read by a hand-held tachometer or any means which tells actual RPM The reading is the input frequency in hertz. Multiply the frequency times 60 and divide the result by the engine RPM. Set this number into the rotary calibration switches. NOTE: If pulses per revolution are not a whole number, for example: 21.5, a setting of 021 will read slightly high and a setting of 022 will read slightly low.