SM4 Manual
by
MRM-Racing
V3.0
122
Trigger Devices
The output of Hall Effect, Optical, or Magnetic inductive (needs internal reluctor) sensors
could be connected directly to the input of the SM4. Many distributors used in original
equipment manufacturer’s computer ignition system would be fitted with Hall or Optical
trigger. They normally have a chopper wheel attached to the distributor shaft and passes
through a gap in the sensor. These sensors require three wires:
•
Ground
•
Power Normally 5-12 volt
•
Trigger out signal
If using stock distributor components make sure they do not are fitted with mechanical and
/or vacuum mechanisms. These mechanisms have to be defeated or removed, and the
timing edges set correctly, then they can be used to trigger the SM4.
A third type op pickup, the magnetic inductive is used in distributors as well as on crank
trigger units. It is now possible to directly connect this trigger to the SM4 and use the
internal reluctor adapter to convert the signal to square wave required for the SM4.
The magnetic trigger is basically a piece of wire wrapped around a magnetic core and
attached to the sensor face, which is called the pole piece. The physically size and shape of
such trigger can take many forms. This type of pickup has two wires, one positive (+) one
negative (-) or three wires with the third being shield. Sometimes the wires are marked as
such, but more often than not they are unmarked. Below is an example from a Bosch
trigger found on Volvo, BMW, Opel and more.
If you cannot tell which wire is the positive from markings or wire diagrams, then it would be
necessary to check the signal going into the SM4 with an oscilloscope.
When ferrous metal is passed very close to the pole piece, the pickup will generate a
voltage (similar to figure below) on the positive wire. The amplitude of the signal will vary
with the speed at which the ferrous metal passes the pole piece. At low speeds the signal
may only be several hundreds mV, but at high speed it could be more than 20 volts. The
sudden drop from positive to negative corresponds to the metal being directly in line with
the sensor. As can be seen, the signal is a sine way type. The voltage input of the reluctor
inside the SM4 must first go to a positive peak, which arms the circuit. When the signal
raises approximately 75-80% of the peak value the reluctor will generate a falling edge, and
when the signal crosses zero volts a rising edge trigger the SM4. If the wires are reversed
Note:
You can use any combination of Hall Effect, Optical trigger or Magnetic trigger for
both Cylinder Pulse and / or Cylinder reference.
Pin out for Bosch type
Magnetic trigger
Pin 1 Positive
Pin 2 Ground
Pin 3 Shield
