With engine running measure voltage between the post behind engine fuse box where the + cables join and a green wire at any one of the fuel injectors. Slide a paper clip wire or a pin into the rubber seal at the injector connector 5-7 mm and you will feel the metal terminal and can use this as probe...

Voltage drop test. Digital voltmeter is best. Any power supply circuit using relays as the switch should have no more than 0.5 volts. Less is better. Relays have switch contacts and the metal contacts can get burnt surface that lessens the ability to flow current. The rely will often have soldered c...