Thread: What is the purpose of the ECM relay?

Looking at the diagrams, it just seems redundant. If white wire is constant power from the battery, why is there a need to have a relay to distribute the same power to the ECU? The ECU is already being fed power from the white, and that is the same power to the relay, so why the need for the relay? Is it for an audible diagnostic function, as in you hear the relay click so you know there is power being distributed to the rest of the circuits inside of the ECU? Or am I just missing something?

Just wanted to clarify - the ECM relay gets power directly from the battery, it does not go through a switched source like the ignition switch. The same wire that feeds the ECM relay also feeds directly to the ECU without a switched source or another relay. The ECM relay is powered by the white wire and makes a connection from (again) the white wire to the orange blue wire, which then in turn feeds the rest of the ECU circuits. This is how I understand it, which would mean the ECM relay is on all of the time, as long as it's connected to the battery? if so, why not wire the ECU directly and just remove the ECM relay altogether?

Because Nissan didn't think it thru and probably someone made a mistake and they just left it like that instead of issuing changes to the assembly line and all the crap that goes a long with that. You're right, it doesn't make sense. Nissan seems to go 90 percent of the way with their vehicles but leaves you hanging just a bit with certain things. Blower resistor motor failure, 5th gear popout. Another example, the OEM white face gauges in the 98-99 200SX SE-R/Sentra SE. They only made the speedo and tach white. Left the fuel and temp gauges black. Makes no sense.

Wait, don't you hear an audible "click" at that relay when turning the key from first position to second position? I always assumed this is when the relay fired and powered the ECM. Coincidentally that is when I get radio interference in my SE-R and have assumed it was caused by the Stillen/JWT upgraded ECM.

Interesting. I'll have to eyeball the FSM...

I haven't looked at the diagrams in a long time, but I believe the ECU gets constant power for some low-draw memory settings (which you can clear by unhooking the battery for 10 minutes). Without this source, the "learned" maps can't keep their settings. So that's why the ECU gets constant power. This is a low amp source and draw to a specific pin on the ECU plug.

When the ECU needs to run the car, it needs way more power, and to different pins, and that's when the ECM relay comes into play.

What Ben said. But I'm still going to eyeball the FSM when I get home, and before I get liquored up.

This would make more sense, but it seems strange to me that a PCB with a minimal amount of components would need anything more than a couple amps to run. It leads back to my original question, why a relay - why not reduce complexity and wire power directly. It's not like the power wires to the ECU need a ton of ampacity...

Yah I guess that's a different kind of question. Who knows? Maybe for ease of replacement in what they thought would be a high failure spot? I'm sure there is a decent reason for it. Maybe someone else knows.

If your ecu was getting full power 24/7 you would have a dead battery every few days, maybe even hours.

Not to mention having hot power running 24/7 would open up a entire new door of issues when replacing sensors and what not.

I dont see any benefits of the ecu being on 24/7. I think the relay was a great choice. They are very simple componets (only 4 wires) anyhow so there is no need to complain about them.

Looking at the 1991 FSM wiring diagram it appears that it is the ECM itself which fires the coil in the ECM relay allowing power to the ignition system, "air flow meter", knock sensor, etc. through the relay. So the ECM would be powering the relay, not the other way around.