The great majority of ECUs no longer use that half of the dual resistor Think of the coil resistor. That part of the circuit is unchanged from the breaker points days. In fact, in an emergency, if the electronic system fails, you can plug a breaker points dist in, hook up the dist primary wire to the coil, and unplug the ECU and off you go
Each side of the dual ballast gets switched "ignition run" power (IGN1) The coil side goes to the coil + and the ECU side goes to the ECU. Again, most of these are now unused
A 4 pin ECU can use either a 2 or 4 pin ballast
A 5 pin ECU must have a 4 pin ballast
You can NOT tell a 4 from a 5 pin ECU as many still have 5 physical pins. Two ways to tell: On a working system, if you unplug the ECU side of the ballast and it still runs, it's a 4 pin ECU
If you do resistance checks on the ECU and the 5th pin is open it's a 4 pin.
