PT-E85:
E85 BIOETHANOL
It is basically what pretty much all of the flexfuel boxes do:
Two wires from ECU goes into the box.
There is a circuit that sniffs the pulses. It does'nt matter if the two wires are reversed or not. Basically a logic IC makes it understandable to the core (atmega).
Atmega sends identical but extended pulse to the N-FET, and that controls the injector.
The injector gets positive straight from the battery, and negative from the device.
And it does'nt matter if the wires are reversed or not, because injector does'nt care about polarity.
My engine uses common ground.
EDIT: Oh, i just searched from internet, and someone claims that some injectors might have a flyback diode. And if the wires are reversed, it might do that the MPGuino can't get signal.
Just a wild guess. Reading more...