General Technologies TD101 SmarTach + D: Gas and Diesel Tachometer with Ignition Voltage Tester - Manual (Page 11)

DIS Ignition Systems
Distributorless ignition systems (DIS) may cause significant
differences in Spark Analyzer readings. DIS fires spark-plugs
with both negative and positive pulses and there may be a
wide difference in voltages between the positive and negative
pulses.
For this reason, when performing the Ignition Peak test on DIS
systems, the comparison should only be made between spark-
plug wires with the same polarity, either positive or negative.
Unbalanced voltage readings between wires of different polar-
ity may not indicate a problem in this type of ignition systems.
SPECIAL CASES
Shielded Spark-Plug Wires
Some vehicles may be fitted with aftermarket “high perfor-
mance” spark wires. These wires have solid copper cores and
are shielded by metal jackets which prevent transmission of
the radio signals. The SmarTach+ antenna needs to receive
these transmissions in order to perform its tachometer and
spark analyzer functions.
Depending on the efficiency of the shielding it may be possible
to make some assessment of spark voltages sufficient to see
any out of balance situations. In the tachometer mode it is
possible in some cases to pick up a signal if the sensor hook is
used right on the spark wire. However this will give an inaccu-
rate reading because the SmarTach+ will be reading only one
cylinder, whereas you have set the unit for multicylinders. In
case you wish to read RPMs in this way, it is permissible
to do so providing you first set the “CYLINDERS” to 1.
Often it is possible to read RPMs with shielded spark-plug
wires by holding the antenna over the distributor. This way,
the antenna can sense all the sparks and there is no need to
change the “CYLINDER”setting.
Inboard Marine Engines
Many inboard marine engines are supplied from the factory
with shielded spark-plug wires, because of the risk of explo-
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