General Technologies TD101 SmarTach + D: Gas and Diesel Tachometer with Ignition Voltage Tester - Manual (Page 7)

Special Considerations
Until a few years ago it was simple to decide whether the cycle
setting should be at 2 or 4. With few exceptions, only some
motorcycles, lawnmowers and outboard motors would have
needed a 2 cycle setting on the SmarTach+. Most motor ve-
hicles use 4-stroke engine in which the ignition system supplies
pulses to the plugs ONLY near the tops of the compression
strokes. This means that each cylinder receives a spark pulse
every second revolution of the crankshaft.
Today, many of the newer vehicles employ DISTRIBUTOR-
LESS IGNITION SYSTEMS (DIS) where each plug is fired
every time its piston approaches TDC (Top Dead Center). In
these systems, a plug fires not only near the top of the com-
pression stroke, but also again a second time near the top of
the exhaust stroke. This second spark is sometimes referred
to as the waste spark because it performs no useful function.
Because the SmarTach+ detects ALL spark pulses, if the
SmarTach+ is set at 4-cycle, it will show correct RPMs on the
traditional distributor systems, but will show TWICE the correct
RPMs on a motor with a distributorless ignition system. For this
reason, when setting up the SmarTach+ to test a motor having
a distributorless ignition system you must set the cycles at 2,
although the motor is operating on a 4-stroke cycle.
In case of doubt concerning whether or not the motor is “dis-
tributorless”, an experienced technician will easily detect an
incorrect cycle setting by taking a reading at idle speed. Most
motors are set to idle at about 600 RPM and the sound of this
idle speed is familiar to the experienced ear. If the SmarTach+
is indicating a speed of about 1200 RPM it should be reset to
the 2-cycle setting for a distributorless system.
After set-up of the SmarTach+ for cylinders and cycles, check
the instrument’s display. If it shows “0000” you are ready to
start. If the display has switched off, press TACH briefly to
switch on again and wait while the display switches through
“cylinders” and “cycles” to show “0000”.
(Note that when the SmarTach+ switches off, the last settings
for “cylinders” and “cycles” are retained in the instrument’s
memory and will appear again when it is powered up again)
Taking tachometer readings
1. To take RPM readings after the set-up has been com
pleted, extend the instrument antenna and hold the
SmarTach+ by its plastic case. Do not touch the antenna.
2. Place the end of the antenna near (but without touching) a
place were the spark-plug wires are grouped together, like
a distributor cap, coil packs, etc. At this point the display
should show “0000”, as the signals from the ignition
systems are overloading the instrument.
3. Move the antenna slowly away from its position, until you
obtain a steady RPM reading. If you do move the antenna
quickly, the instrument will be unable to adjust to the rapid
changes, and as a consequence it will not provide steady
readings if any at all.
4. After obtaining the first reading, you may want to continue
to move the antenna further away, in order to locate the
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