BRZ fuel pump wiring question
 

Hello,

I recently was re-doing some wiring on my BRZ, trying to figure out why my second relay would not power the second pump. I noticed the "Green" wire at the fuel pump area that would be considered the trigger wire only has ~5 volts with the key on. Is this suppose to have 12 volts? If so, does anybody know what the issue could be? I haven't messed with any other wires that I know of.

Is there a place I can grab this 12v + trigger wire anywhere "sooner" maybe directly out of the fuse box?

Battery is fully charged and brand new. Car runs and drives great and grounds are all in place where they are needed as far as I know.


Thanks for any advice you can provide.

https://i.imgur.com/Sh9n4lM.jpg

I found this relay, the red wire is coming from the front of the car and supplying 12v to the relay. The green wire is the one going to my factory fuel pump hanger and is supplying the 5v. Does this mean the box is not working properly? Or is it suppose to be 5v coming out.

If it's suppose to be 5v, what is the proper way to bypass this relay but also retain all the other functions from the other wires (unsure what they do, maybe fuel level sender wires)?

That is the Fuel Pump Control ECU/Assembly (G12) on page 98 of the wiring diagram. I'm not good enough at interpreting wiring diagrams to give you a good answer, nor do I have my car torn apart to test the circuit for you.

Yea i've been through the pdf multiple times, I know what it is but for the life of me I can't find anywhere if it's suppose to spit out 5v to the pump or 12v. The green wire.

I’m sure someone has a multimeter and easy access to it. Isn’t it buried behind a lot of plastics? I’ve seen them sold online used for cheap.

I got rid of that altogether. Did wire in an inertia switch just in case. Pretty sure just spliced the red to the green,, then ground the gray.

I'm pretty sure the fuel pump is PWM controlled so you would need a oscilloscope to see 12~14.5v.
The ECU would send a signal to tell the controller what duty cycle to run the fuel pump.
I will look at the wiring diagram when I get time.

I have done the same quite a bit in other cars but for the BRZ and a lot of Subaru's in particular, the fuel pump would constantly prime with the key forward from what i've been told. Without this relay, there's nothing stopping the pump from priming/running indefinitely.


You may be onto something for sure. My biggest issue right now is i'm not sure if Motec needs to see 12v to trigger a ground switch (for my second pump) or if any voltage is enough (5v for example) to trigger the ground switch circuit.

If you used solidstate relay it should run your 2nd fuel pump the same as the 1st. If your using the factory ecu since it controls the controller.
Attachment 189542

I would think motec would have PWM fuel pump output

It probably does, but currently its setup to run via the factory/oem controller and my bet it it's utilizing the factory PWM. I am not trying to dig that deep into the bullshit that is Motec M1 again.

And as far as a relay goes, I am running a standard 40amp 4 wire relay. The type of relay i've ran on 15 other cars in the past without issue. This one is the only one giving me problems. it's related specifically to the Motec ecu ground switch. Either it doesn't like the 5 volts its getting, or the wire from the relay to the ecu is somehow bad, even thought it's brand new.

Things change.
Hope all works well.

I think i'm going to try and run a second 12v ignition source trigger to my second fuel pump relay. I currently have the oem 12v trigger (the one that happens to be at 5v at key on) split into both relays. This was suggested to me by somebody else i'm working with on this. Will update to see if it resolves my issue. Don't know why it would though but we will see.

Here's exactly what I got going on.

https://i.imgur.com/7iuZZNH.png

Pin A30 is set to ground out the circuit given a certain parameter is met. Regardless of that parameter, when I test the circuit voltage goes to 0V which means it is grounded, but fuel pump doesn't turn on. When I manually jump the fuel pump it kicks on no problems.