Wiring the s14 dash harness from scratch and I ran into this little problem. The ECU grounds the ECU relay through pin 4 to allow power to feed into the switched ecu power pins 38/47 as well as feed power to the MAF and CAS.

My question is under what conditions will the ECU ground the ECU relay in order to power itself. I’d assume it has something to do with the accessory or ignition power but how would powering the harness provide ground? This process takes place through the ECU so I have no idea what’s going on inside. I’ve also removed all the dash harness as well as the harness that runs to the headlights fans etc. not sure how to play this. It could turn out that I can throw it in with all these wires hooked up and it’ll somehow work but I don’t want to risk an ECU. Anyone have experience on this part of the wiring? The Rest seems straight forward as I’ve mapped it out and it seems to check out with continuity testing. Any help would be appreciated

The ECM only needs to self-enable for vacuum-testing the fuel tank while the car is parked (EONV test). Otherwise it self-disables about 2 seconds after you key-off.

I think I’ve solved it. The ecu gets power from ignition, a fairly thick wire. Does this Power the ecu enough for it to activate a solenoid/relay/switch type thing in pin 4 that allows it to ground it’s and receive main power after grounding the ECU relay?

With that EONV are you talking about the fuel pump priming for a couple seconds before turning off? when the car is turned to ignition but not started?

No, EONV stands for Engine-Off Natural Vacuum, and it's done under specific circumstances to make sure the fuel tank and the two valves that seal it (Purge and Vent) are working properly. On most cars the tank needs to be between 1/4 and 3/4 full, the tank needs to be above a certain temperature when shut off, and then the tank temperature needs to fall by a certain amount over a certain period of time. Under those circumstances, with both valves closed, the tank should generate a natural vacuum as the temperature falls. If it doesn't, the ECM knows something is leaking and throws a code before going back to sleep. That's why "gas cap" codes always light the MIL after the car has been sitting for awhile, never while you're traveling.

I see. My question was more related to how the ecu relay is grounded by the ecu. Does ignition power allow it to do this? Thank you

Well, it needs minimum "sleep" power to be able to do anything, including wake itself up. That's provided by the battery backup wire. Find the diagram for "Main Power Supply" in the EC section of the vehicle's SM, one of the feed wires will connect straight from the battery. That's wakeup power. If it fails or isn't connected, the ECM will still wake up with the key on ignition power, but all transient data like LTFT will be lost every time the key is cycled.

Yeah that’s pin 46 and 109 backup power and current return. Besides these and ignition power pin 36. The ECU doesn’t need anything else to retain memory and start up?

Nope, that should get you going.

Awesomeeeee