Innovate Motorsports ST-12 User Manual

Page 18

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Use also when using inductive
clamp on spark wire or power wire
of Waste spark coil of 1 cylinder
Waste spark system:
1 coil for every 2 cylinders.

3 3


4 4


5 5


6 6


5.2.8 Using the ST-12 with the Inductive Clamp

The inductive clamp measures the magnetic field created around a spark plug wire when spark current
flows. If a metallic shield covers the spark plug wire, the inductive clamp may not work because the shield
would short out the magnetic field. Like all inductive clamp rpm pickup devices, some ignition systems
like Capacitive Discharge Ignition (CDI) or multi-spark ignition systems may not work properly with the
inductive clamp pickup because the pulses created may be too short in duration. Multi-spark systems
confuse the ignition timing measurement because the RPM converter cannot distinguish which ignition
pulse belongs to which crank rotation. The ST-12 will work only on the tach output of the ignition
system in this case.

The inductive clamp must be clamped around ONE lead only. Clamping it (for example) around all
wires of a coil-on-plug pack does not allow it to work because the magnetic fields of the wires
most likely cancel each other out.

5.2.9 Inductive Clamp Usage

- Plug the inductive clamp's 3.5mm audio plug into the RPM socket of the ST-12.
- Clamp the Inductive Clamp on the spark plug wire of one cylinder so the wire is completely surrounded
by the clamp.
- Make sure the clamp is completely closed.
- Start the engine.

The decimal point of the digit display of the ST-12 should light up steadily. This indicates when a valid
RPM signal is detected. If it does not light up, or lights up intermittently, reposition or reverse the clamp
(try clamping it upside down). If the decimal point out only occasionally, that is OK. The RPM converter
will still convert, though its output might be noisy. A noisy output has spikes or lengthy flat areas in the
data log. [Note: to work properly with the inductive clamp pickup the ST-12 must be set up for the
appropriate number of pulses per crank rotation.]

• For a 4 stroke engine without waste spark ignition, this would be 1 pulse per 2 crank rotations. This is

the factory setting.

• For a 4 stroke engine with waste spark ignition, or a 2 stroke engine, this would be 1 pulse per crank


• For a rotary engine, this would be 1 pulse per rotation. This is the same as for a 1-cyl 2-stroke motor.

Note: On any distributor-less ignition system you can alternately clamp the inductive clamp
around one of the power wires on the primary side of the ignition coil or coil-on-plug module.