Using 180sx factory relays to power new loom

Hi guy, first post here and struggling with figuring some stuff out on the factory engine fuse box for a Sr20det 180sx.

car is a factory 180sx Sr20det engine was removed with harness and ecu and have been replace with a sr20vet I’m now looking at making a new harness for.

what I’m struggling with was testing my factory relays for the “egi” and “ign coil” as I was hoping to use these to power the ecu and other components as needed. I’m sure there’s a million posts out there about this chassis that I’ve somehow missed. But is there any information that could be helpful. As I don’t have any of the old loom to check against. I’m struggling to find where the fuse power for the ign relay comes from as both switched wires of said relay come are in an empty plug on the other side of the engine bay. I assume this plug used to plug into the factory engine harness. I also assume from my testing that the other relays may have been switched via the ecu.

Just looking for a little input if it’s possible to really use these factory relays to control the ecu and related engine harness components or is it easier to add a complete new relay box inside the car.

might be over thinking everything any input would be greatly appreciated.

thanks team

Hello Jacob,

You are right, the OEM ECU provides ground to trigger the EGI relay (which will trigger the Coil relay). The connectors are located on the side of the fuse box. If you want to use these, you'll need to ground the relay pin when ignition is switched on. After that, you should have +12v on the Black/White wire for the ECU and Blue/Red wire for the coils.

What I usually do is add 2 new relays in the loom for the Coils and for ECU, injectors, IACV, boost solenoid, O2 sensor, etc... as using 30+ years OEM relays and wiring can lead to gremlins so I like to take them out of the equation. Also, the OEM wiring for the IACV provide constant +12v and create backfeeding caused by the flyback diodes in some aftermarket ECUs, so rewiring it to +12v switched will be needed to adress this issue.

Hi Frank,

I appreciate your reply. I did rewire both ign coil and egi relays to both be switch from the key ign power, I also have new Nissan terminals to put new wiring into the relays to provide power for all the components.

The new engine will be running a Bosch 82mm DBW throttle aswell. Would we think this should be an issue since next to no factory wiring will be used apart from the trigger of the relay considering I’ll be upgrading the wiring flowing through the relays? Only other concern being the wire size that will be the power wire that powers my r35 hitachi coils. (Trying to avoid ordering more gear) as I have 22, 18, 16awg milspec on hand.

thanks