Trigger Polarity – Why is it so important?

It’s not uncommon for us to receive a question on a no-spark or mis-timed spark problem, and often the solution is to switch the position of the trigger wires.  This begs the question, why is the position of these wires so important?  We’ll get into that here and hopefully provide some valuable insight on these components themselves.

First, what is a “trigger”?  For one thing, it can go by many different names such as a “pickup coil”, “crank position sensor”, “signal generator”, etc.  It is a part, usually consisting of a metal base plate with two screw-holes and a raised plastic or potted portion with a magnet in it that is bolted to the cover next to the flywheel.  Wound around this magnetic core inside the trigger is a coil of very thin wire.  Here is a cross-section of a typical trigger such as Rick’s part# 21-502:

The magnetic face is designed to work with a section of the outside circumference of the flywheel known as the “reluctor”.  This is simply a raised portion of metal that will pass very close to the magnet.  When it does, it will create a fluctuation in the field of the magnet inside the trigger.  This fluctuation will in turn induce a small electromagnetic potential difference between the two ends of the coil (this is referred to as the “Hall Effect”).  Since the flywheel is bolted to the crankshaft and the changing field only occurs when the reluctor is passing the magnet, the signal from this will always be at a particular degree of crankshaft rotation.  This is how the CDI times the ignition.

Inside the CDI box is a capacitor across which a static charge will build up due to applied voltage from either the stator’s source coil (AC-CDI, “stator ignition) or the battery (DC-CDI, “battery ignition).  For more detailed info on CDI boxes. check out THIS blog postThis static charge will remain across the capacitor until it is discharged to the primary side of the ignition coil (hence Capacitor Discharge Ignition).  This timing is controlled by a semiconductor component called a thyristor, or SCR (Silicon Controlled Rectifier).  This component is in an “off” state naturally; it must be “turned on” by applying correct voltage to its gate terminal.

The trigger is connected to the gate, and the signal being generated is responsible for opening and closing the gate.  As a semiconductor device, the SCR requires not only the correct voltage applied to the gate but the correct polarity as well.  If it’s backward, the gate will actually increase in strength when it should be decreasing.  Doing this causes significant timing failures, if not stopping the output altogether (this will vary depending on the vehicle).


Hopefully this answers your questions on why the position of the wires matters so much on the trigger coils!