Survey; Which relay to power fuel pump.

[Accelo]

I have a choice of two relays to power fuel pump. Both are already installed.
One is part of the American auto wire harness.
The other is included as part of the computer, ecm, conversion LSX motor install. My fuel tank has been relocated to the rear.

The difference may be how they are switched:

The conversion auto wire system has a relay that comes on with the key. And stays on till you turn it off with the key. This relay is located on the left side and is powered through the firewall plug.

The computer, ecm, conversion harness for the LSX motor has the pump relay coming on for three seconds and then shutting off unless it sees the ignition signal. This relay is on the Right side next to the battery.

Pro & Cons;
The conversion auto wire set up as it has the fuel pump wiring included in the tail light bundle for a clean install.
The computer (ECU) conversion relay is on the wrong side for ease of wiring. I would have to run a new wire down the right side frame rail. However, this may be the safer of the two as the pump isn't running without the motor for longer than three seconds.

Question is which wiring scenario would you choose?

[pjmoreland]

I must say I like clean wiring, so this would be difficult choice for me. I am installing an electric fuel pump that will feed my carburetor. I will be installing an oil pressure switch that will be overridden by the yellow wire from the starter motor while cranking. I am also going to install a safety switch that will shut off the fuel pump in the event of a collision. It seems to me that these safety measures are more important with a carburetor than with fuel injection since the floats in the carb would allow fuel to dump out continuously in the event of a rollover. That seems like less of a concern with fuel injection, but you still could rupture a fuel line in a collision. Maybe a compromise in your case would be to add a safety shutoff switch to your AAW setup. This would allow you to use the clean wiring, and you'd have some protection in the case of a collision. You would still have the potential to spray fuel all over the place if a line ruptured for some other reason though.

Would there be a convenient way to tie the output from the LSX conversion harness relay to the AAW wiring? Maybe you could run a wire around the front of the engine compartment over the top of the radiator.

[Big Kev-O]

I would use the one that is powered by the ecm. Most EFI systems send a PWM signal to the fuel pump so it isn't running full speed when it doesn't need full flow. The ecu tells the pump to pressurize the system when you turn the key then stops pumping until you crank it over & start the engine. Controlling an pump with PWM reduces wear on your pump & regulator. If you want to use AAW wire that is in your taillight bundle cut it where it comes out of the fuse box and tie it in with the ECU up front.

[Caddylackn]

I would use the one on the ECU, there are safety features that use the relay to turn off the motor for safety.

One reason is low or no oil pressure while running, another is there may be an accelerometer sensor in the harness that can sense if the car has been in a wreck, then it turns off the fuel pump for safety.

[MDPotter]

The ECM needs to control the fuel pump. Run a 10 gauge wire back there from the relay.

[HO455]

The ECM control is the way to go. I believe this is so important I have put my money where my mouth is. I installed a aftermarket fuel pump controller for the fuel pump on my Burban.
You might be able to have both ways. I suggest looking to see if you can control the AAW harness relay with the ECM signal. Thus your extra wiring would be under the hood or dash.
Fortunately fuel pumps don't draw a bunch of amperage so the wire size doesn't have to be very large. Usually a 14 AWG will suffice.

[LS short box]

On my LS swaps I always let the ecm control the fuel pump. A said above when you turn the key to start the ecm will send a ground or positive signal (I forget which) to the fuel pump relay. If the truck starts the fuel pump will continue to run because the ecm sees a crank signal. If the ecm doesn't see a crank signal it turns the fuel pump off.
I also like clean wiring. Out of sight is what I look for. Could you run the fuel pump wire across the rad support down low and the go down the left frame rail with the rest of the chassis wiring harness?

[Accelo]

The oil pressure switch and a safety switch that will shut off the fuel pump in the event of a collision are both great ideas. I will power the pump using the ECM output on a relay. This is an older system without a variable speed pump driver. Thanks everyone for their input. Greatly appreciated.