Are we taking this into consideration?
The second potential clinker is that feed wire to the pump is tied to the Engine Computer. The computer turns the fuel pump relay on, then the 12 volts that gets switched on goes to the pump, and it goes to the computer to verify it got turned on. Often the computer will send out a small voltage, then watch the resulting current flow, to verify the integrity of the circuit. Once the fuel pump relay gets turned on, the 12 volts that shows up makes that little signal voltage from the computer irrelevant. You could have a relay that is not turning on, meaning you would have 0.0 volts at the pump, but the 6 volts you are seeing is coming from the computer.
The biggest issue is you must find 0 volts at the fuel pump when the engine is not rotating, (cranking or running). That is the safety feature on all cars with electric fuel pumps. If a fuel line got ruptured in a crash, the pump would dump raw fuel onto the ground, creating a serious fire hazard. Instead, the engine cannot run with no fuel pressure, so it stalls. Once stalled, no signal pulses show up from the crankshaft position sensor or the camshaft position sensor. It is the lack of those pulses that tell the computer to turn the fuel pump relay off. What this means for you is other than for one second when you turn on the ignition switch, the fuel pump will not get 12 volts until you are cranking the engine. In fact, that one second can provide a clue. If you can hear the hum of the pump for one second when you turn on the ignition switch, that entire circuit is working, and there is no point in diagnosing anything in it. Often you cannot hear the pump over the noisy chime.
Saturday, October 12th, 2019 AT 3:34 PM