Resolved Power Transistor Function

[rdeis]

I found this great thread on what the power transistor actually does: https://www.dsmtuners.com/threads/what-does-power-transistor-do-and-control.379361/

And I have more questions...

Does this mean that the factory coils are basically dumb coil like on an old distributor system then, with no control system in them, and the power transistor does the control function by grounding the coil like an old school distributor would?

I've worked with coil-on-plug in other cars, and I've also read about people poorly engineering coil on plug DIY that actually weakened their spark, I want to get smarter and not repeat whatever mistake they were making. If I'm understanding all of this correctly, coil on plug units have their own control logic built in, so they get switched 12v power from someplace, ground to the block, and all they need is a fire signal from the ECU. Is that right? No downside from using just the two ECU signals and wasting spark on one of the coils?

Sometimes the coils have 4 wires, Three are +12v, ground, and signal. What's that other one do?

What coils are people using in this application? (I have a set of coils from an Evo-X handy...)
Thanks, all!

 

[luv2rallye]

The power transistor unit (PTU) is the electronic switch (literally 2 power transistors - one for each coil to handle the high current) that cause the coils to fire (by providing a ground pulse to the coil's primary negative lead at the time the ECU tells it {which is based on the CPS signal}). The other side of the coil is always connected to +12v so the coil builds up energy {increasing magnetic field} when this side gets the grounding pulse and then coil fires when the grounding pulse is removed {magnetic field then collapses across the coil's secondary inducing the high voltage}. The PTU also provides a tach signal for the ECU and tach gauge.

 

[steve]

Small correction, while the PTU or the Tach Gate on a 90 provides a signal to both the Tach and the ECU, the ECU only uses it to confirm that the coil fired. It has a much better sensor for actual RPM, the CAS or Crank sensor.

The ECU has all the timing information and knows what the coil dwell (charge time) should be so it schedules the PTU activation for pulling the coil low and when to release it to fire the plug.

The issues with DSM COP's is either splitting the coil current by wiring them in series or drawing too much by wiring them in parallel and getting the proper dwell since that a parameter in the ECU software. The typical work around is use them with a CDI or a standalone ECU.