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Fuel pump relay wiring from ECU

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I know this is a noob question and the answer is somewhere but I cannot search anymore, for my health :p

I'm wiring x2 aem fuel pumps to x2 relays to my AEM EMS v2.

Is it correct to run x2 Low side outputs (PIN 59b and 68b in this case) as switched grounds to trigger them (relay pins 85 or 86), and then any 12v source to relay pin 30a or does the 12v need to also come from an ECU output? If so, what do I use?


The 12V can come from wherever. Although powering and grounding sensors at the ECU is always good practice to avoid ground loops and such, triggering a relay should be no problem.

So it would appear then, that when the ECU grounds 85 or 86, 12v will be allowed to flow through the ECU; minus whatever voltage drop occurs between the coil and run of wire, and this is ok?

current flows. When active the voltage at the ECU output pin will be 0 volts,when in-active (pump off), the ECU output will float and measure as 12V. There will be no current through the relay trigger coil since the voltage is the same on both sides. You do know that when you connect the ECU to terminal 85 on the relay, you should connect terminal 86 to 12V (you can swap 85/86 on most relays). It takes a voltage differential to trigger a relay.

When the ECU output is grounded, there will be a slight current through the relay trigger coil. The ECU can sink that amount of current.

Ok perfect, I understand now. My problem was nested in an error understanding the physics side of current.

Thanks for the help! :)