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
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
should be viewed as a potential problem with the
source and appropriate corrective action taken.
TROUBLE SHOOTING GUIDE
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
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
d. Erratic pulse signal adjust trigger threshold .