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How many relays should I have?

EFI Wiring Fundamentals

Discussion and questions related to the course EFI Wiring Fundamentals


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Hi all,

I am in the process of building a SR20DET S14 and am nearing the time to start my wiring. I have already removed all of the chassis wiring and will not be reusing any of the factory wiring, except the harness that goes to the rear of the car. I am having a hard time figuring out how many relays I will need. I know I will need 2 for the ignition and EMS, 2 for the fans (Hi and Low), and 2 for the fuel pumps (I will be running two), and one for the wiper. What I am having a hard time deciding on is if my radio, window, accessories, and lights all need a separate relay. I also plan on installing a kill switch but keep everything else running off triggered by activation of the key barrel. I plan on using one of the Eaton Bussmann power distribution modules, because I believe this will provide the cleanest look as well as ease of diagnosing any issues and an actual PDM is a bit too rich for my blood right now.

Lastly, how would you guys go about wiring the items in the car that require a constant 12v even when the car is shut off and kill switch is off, such as the radio? In an effort to keep from having to completely reset the radio every time the car is shut off.

Cheers,

Kevin

Unlike the switched outputs from the ECU, many of the circuits in the body wiring will be switched on and off by a switch that can directly handle the current flowing in that circuit. What I mean is that if you take the brake lights as an example, you won't need a separate relay just for the brake lights. You'd supply power to the brake lights via a relay (which is probably also going to supply power to a few other circuits) and the brake switch on the pedal will complete the circuit to earth. In this way you can group your circuits sensibly and just make sure that the expected total current in the group of circuits is below the maximum current rating of the relay.

For the radio you will usually have a permanent 12V and a switched 12V powered when the key is in the ACC or RUN positions. You can run the permanent 12V through a fuse directly to the battery.

Thank you Andre! That helps out a lot. Upon further consideration, I think I am going to suck it up and pick up the PMU16 from ECUMaster, because I already have an EMU Black. This helps a ton though and will help in the wiring of everything immensely.

hey, a bit off topic but I am also using ECU Master Black on my S14a SR20 (although mine is a uk car) and am currently in the process of finishing up my engine harness. Do you have a build thread somewhere as I'd like to follow!

Regards,

Gabriel

Unfortunately, I do not, once I get a camera, I plan on keeping a build log on youtube. Once I start doing that, I will be posting them on my channel "CrabFabPerformance"

I don't know when I will be able to start that because I am pretty busy with work right now.