Warning: This is for RETURN fuel system equipped mustangs only. If you need to understand what that is, or how to do that, see my other posts.

Part of the complexity of making more power is the fuel requirements that run in parallel to the needs of the engine. Most add additional fuel by way of larger fuel pumps and injectors. While that's all well and good, that hardware change does require wiring improvements to go along with it. Most seem to forget this fact, and it bodes for unfortunate times when you forget.

To put this to rest with facts, a factory Ford Mustang FPDM is rated at 13.9amps full-tilt. A single large fuel pump (340lph+) can be as much as 30amps. If you happen to have two of these pumps, well you're in for a bad time. The method below will provide as much as 40 amps, and can be expanded to provide even more if you duplicate the relays.

I am unsure of why the providers of MS3PnP's don't offer this information in their documentation (that I could find anyway) but thru time and effort I've found it's relatively easy to maintain OE key-on behaviors in lieu of the common key-on always-on fuel pump that seems to plague PnP cars. Lets begin!

To start off, you need some items. It's up to you on how robust you want to make this, but this was my recipe and it's worked well so far. I am also going to list a cost estimate so you all can see how cheap it really can be.

Parts list:

• 5 Pin 40amp Relay - I bought a pack of 5 with brackets and weather tight pigtails for 25 dollars on Amazon - 5$

• Fuel Holder - I also purchased a 5 pack of these for about 15 dollars on Amazon - 3$

• 12ft 10 Gauge red wire - Something I had, but I'll estimate this at about 8$ for a roll

• Heat Shrink, a couple terminal loop ends, and solder (or crimp connectors if thats your fancy) - You should have this!

As-is, cost estimate of about 10-15 bucks in total. I won't estimate your beverage of choice cost, but for me 2 coors light did the trick at a whopping 2 dollars!

How-to:

First we need to understand how relays work for those unfamiliar. The best way to understand it is comparing it to a draw bridge. If the bridge is open, the circuit is open, and no traffic gets across. If the bridge is closed, cars (and continuity) flow across the bridge.

While this diagram does have some wording on it, it's not mine, so here's what I wired and how:

Pin 30: This should be connected to your 10 gauge red wire from the battery up front. It should also be the circuit/wire that has your fuse holder in place with a 30-40 amp fuse.

Pin 87: This is to your fuel pump(s). Connect to Pin 10, or the BN/PNK (brown with pink stripe) wire out of the factory FPDM connector. Effectively this is your throughput when the relay closes that feeds the fuel pumps power

Pin 85: Connect this to pin 9, or the PNK/BLK (pink with black stripe) wire out of the factory FPDM connector. This is 12v hot with key-on, so don't lick it

Pin 86: This is a ground circuit. Connect it to Pin 1, WH/RD (White with red stripe).With the MS3Pro, we will be turning this circuit into a ground signal with a settings change below.

You will also need to ground the fuel pumps. Take Pin 3, and put a terminal loop on it. Clean your ground contact that the factory harness uses and add this new terminal to that as well.

Okay, so you're all wired up now. You can rest easy. But wait! There's more.

Jump into Tunerstudio. Go to your Fuel Settings, Fuel Pump and Pressure control. Then change the fuel pump functionality to on/off

Doing so turns the white/red wire from a PWM controlled circuit to a purely ground controlled circuit. All it does is ground the fuel pump relay for key-on prime, and start/run when the car fires up.

Hopefully this guide will serve you well in your efforts to improve the overall functionality of your ECU adventures. I wasn't happy with the always-on fuel pump that occurs with the traditional wiring methods found on the internet. Most do that because on a factory ECU, you have no choice. White/Red is not a powerful enough ground signal for a relay to work correctly. But with the MS3, it is!