Hi Sylvain, looks very much like the wiring from the ignition switch to the connector at 5 is too thin and you are getting a voltage drop as a result. It's very strange that your pump stops working at 10 Volts, I think maybe that it has some electronics built in to it as normally a pump would just run a bit slower at reduced voltage. Anyway, there is nothing to stop you running an extra wire as long as the pump remains under ecu control. You do not show an inertia switch in your diagram - are you running injection? It is vital that you have some device to turn off the pump if you have a crash, otherwise you could end up pumping fuel in to a burning engine bay. Anyway, putting an extra, thicker wire from the ignition key to the right hand side of connector 5 will do no harm and should fix your problem.
Best of luck, let us know how you get on.
Jaevlar Huite
