The ECU signal to the PTU is a positive voltage. The PTU has its own circuit that grounds the coil packs. This is why the PTU gets hot. It has internal transistors which are an equivalent of MOSFETs. They form a short circuit when switched on via the ECU positive voltage. That short grounds the primary winding of the coil pack. And it's actually the release of the ground signal to the coil pack that causes the spark plug to fire. The voltage in the secondary winding of the coil pack to the spark plug is created via inductance from the primary winding when the electromagnetic field in the primary winding collapses after it is turned off by the release of the ground signal.
