Ensign No. 735 Digital Dual-Tach Sync
Aqua Meter Instrument Corp.

Testing the Tachometer's
Trigger Threshold Adjustment

Your Ensign's tachometer trigger threshold has been factory adjusted to the optimum level for the vast majority of marine engine pulse sources. However, under some circumstances readjustment may be required.
After installing your Model 735 or 750 and setting the pulse rate and other calibration switches, start the engine. Keeping the transmission in "neutral", advance the throttle from idle through maximum RPM* in 500 RPM steps and then gradually return the throttle to idle.

*DO not increase RPM beyond engine manufacturer's rated maximum RPM.

During this test, monitor the tach reading. It should increase steadily as the throttle is advanced to maximum RPM, and should decrease steadily as the throttle is backed down to idle. If you are checking out an Ensign 735 installation, repeat this test for the second engine.
  If at any point during the test, the tach readout suddenly begins to display rpm readings that obviously are erroneous, the Trigger Threshold must be adjusted. Usually the erroneous reading will be a sudden jump to 2 or 3 times the actual rpm, and then a resumption of correct readout when rpm is raised.
  The primary cause of this sudden erroneous reading is a "noisy" pulse signal from the engine due to dirty alternator contacts, worn brushes or ignition points, poor connections or other pulse source fault. Therefore, instead of a clean pulse which the tach counts as "one pulse", a noisy pulse may appear as two or more pulses -- all of which the tach circuit counts. Both the Ensign 735 and the 750 have a very sophisticated input filter network that "cleans" all but the very noisiest signals which will result in the symptom noted above. If this check is completed satisfactorily no adjustment of the Trigger Threshold is necessary. However, if Trigger Threshold adjustment is needed, proceed with the following steps:

1.) Disconnect power to Ensign.

2) Remove the Back Plate to gain access to the Trigger Threshold control which is identified as "R8' on the printed circuit board. It is located to the left of the calibration switches (see drawing below.)

3.) Depending on the nature and characteristics of the noise on the signal pulse it may be necessary to increase or decrease the Trigger Threshold level from the factory setting. Therefore, start by rotating R8 clockwise a few degrees using a small insulated screwdriver and repeat the check out procedure noted above. If the condition improves continue slight clockwise adjustments of R8 until Proper operation is achieved. Obviously, if clockwise rotation of R8 worsens the condition, proceed from the factory setting with counter-clockwise adjustments of R8 until proper operation is achieved.

4.) Replace the back plate. 

NOTE: No further adjustments of the Trigger Threshold should be
            necessary unless there is a significant change in the pulse
           signal characteristics. Also such a major pulse signal change
           should be viewed as a potential problem with the engine's pulse
           source and appropriate corrective action taken.


1. Unit inoperative - no LCD segments activated.
a. Ignition switch not on (especially with twin engines, make sure ignition switch connected to 735 is on).
b. Check fuse or circuit breaker in the power supply circuit
c. Check power connection for correct voltage (11 V. min. 16 V max) and polarity  (negative ground).
d Make sure connections are clean and free from corrosion.

2. No tachometer reading.
a. Check connections to engine tach lead(s) and engine pulse source(s).
b. Adjust Trigger Threshold -

3. Tachometer reading obviously incorrect (i.e. lower or higher than actual engine rpm).
a. Incorrect pulse setting (switches A1, A2, and A3) for the engine used. See page 5.
b. Low power supply voltage (below 11 VDC).
c. Weak pulse signal from engine pulse source due to poor connections or defective pulse source.
d. Erratic pulse signal adjust trigger threshold .

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General Use of Switch "A"

A3        (SEE BELOW)
A4        TEST (Off = Normal; On = Test Mode)
A5        RESOLUTION **
                 Off = 10 rpm
                  On = 20 rpm
A6        Not used Leave in OFF position.

    ** The tachometer readout RESOLUTION can be set by internal switching to either 10 or 20 rpm increments. This permits precise engine idle speed adjustment at the 10 rpm resolution setting, and a 20 rpm setting for a more steady tachometer readout during turns and rough water operation when the engine(s) are working at greatly varying loads.

It is important to match the tachometer pulse setting to your engine(s). With the ENSIGN's power lead disconnected, remove the rear cover to expose the small DIP switch. Find your engine type in the table below and set the switch accordingly. For specific outboard model numbers see separate matching chart enclosed.

   GASOLINE ENGINES                                          TACHOMETER PULSE
                                                            DIESEL        PULSES PER       SETTING.
IGNITION                ALTERNATOR    TACH       CRANKSHAFT         "DIP"
2 cyl.  4 cyl.                                                     2                  on   on   on
3 cyl.  6 cyl.                                            --       3                 on   on   off
4 cyl.  8 cyl.                                0.5:1              4                 on   off  on
                            10 Pole Alt.                          5                 on   off  off
6cyl.                    12 Pole Alt.                           6                off  on   on
                            16 Pole Alt.     1.0:1              8                off  on   off
                                                 1.5:1              12                 off  off  on
                         --                      2.0:1              16                off  off  off

* For diesel engines with industry standard (SAE) 7/8-18 thread tachometer takeoff use, Diesel Sender Assembly No AS-DO364 for each engine. Will operate with most other diesel engines including Volvo, contact factory for specs.

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