Here’s a technical explanation of exactly how the VW Ignition System works.
The coil gets power from the black wire- terminal 15 – that’s DIN for ignition on power – +12v- it comes directly from the ignition switch with no fuse whatsoever.
The ignition control unit (ICU) gets power from the same side of the coil, you can see that wire going into the ignition harness.
The ICU reads signals from the hall sensor in the distributor. The hall sensor is a magnetic pickup. The distributor has a metal shutter that comes between the hall sensor and a magnet. When the shutter is between them, there’s no signal. When there’s a hole going by, there’s a signal to the ICU.
Now, in an 8v, there is a counterweight system in the bottom of the distributor that makes the rotor move ahead of normal when it spins faster. There’s also a vacuum attachment that does the same thing, it’s attached directly to the hall sender and pickup to make them advance more. This is what gives the engine the correct timing curve.
In a 16v, there is a knock sensor and a knock box connected between the HALL and the ICU. There’s no mechanical or vacuum advance at all. Instead it’s programed into the knock box. When it sees the engine going faster, it advances it electronicly. It sends that signal to the ICU, that signal looks like a hall signal to the ICU, so it just fires whenever it’s told to.
The ICU has a ignition coil driver transistor. This is a high powered transistor that requires a huge heatsink, in the case of a stock scirocco – it’s about 2″x2″ – the entire ICU. That transistor switches ground on and off to the coil.
So when the hall sensor gives it’s signal to the ICU, the ICU sends a ground to the Coil. The coil’s circuit from +12v to ground is completed, it charges and fires in a split second.
You may also notice that this “Ground” side of the coil has a red/black wire that goes into the body harness. That’s the stock RPM pickup wire for the cluster.
The charge goes from there to the distributor rotor, where it is distributed to the plugs! 😀