The whole system needs a reference so that the ECU knows whereTop Dead Centre (TDC) is and therefore, when to trigger the spark’s current. There are two main types of trigger wheel — shown is an aftermarket system, which will convert any non-fuel injected engine; such as a Crossflow or early Pinto, allowing it to be bolted to the front pulley wheel with a bracket for the pick-up.
It works by having one tooth for every 10 degrees of crank rotation, which the pick-up sensor counts.The 36th tooth is missing so the ECU then has a reference and knows when to start its cycle. Some engines — Zetecs for example — already have a trigger wheel built into the flywheel, with a pick-up mounted in the back of the block.
Some ignition systems still use the distributor as the means for distributing the spark although that’s its sole role and it’s empty of points or electronic ignition.