What You Need to Know About Upgrading Foxbody Mustang Fuel Pump Wiring 

You try to reduce the length of small gauge wire as much as possible. 18” of 16 gauge beats the heck out of 36 feet of it though. This method does allow you to remove the tank easily and doesn’t require you to remove the tank if you are retrofitting the wiring after the pump install. As a general rule, try to limit the length of factory wiring in the power circuit as much as reasonably possible.

Well, it's set up for a big fuel pump anyway!

Thank you! Cheers!

The connector carries circuits for the fuel pump and fuel level sender. The grounds are directly opposite the positive leads for each of those circuits, so if you find the fuel pump wire, which is normally a red wire in this harness, the black wire directly across from it on the connector should be the correct ground for that circuit.

Thanks for watching!

Thanks for your kind words and your support!

Glad to be of help, and thanks for watching!

Thanks! I have lots of tech advice on this channel - I hope you will stay and have a look around!

Cut both the power and ground wires going to the pump. On the chassis side of that cut, connect the power (usually red) wire to pin 86, and connect the ground on the chassis side of that cut to pin 85. Run a new 10 gauge feed from your battery, with a fuse in it, to pin 30, then connect pin 87 to the power wire (usually red) on the pump side of that cut in the chassis harness. Connect the ground from the pump side of the chassis harness to ground directly. This way the original fuel pump circuit is intact but only triggers the relay, and the fuel pump power now runs through the new 10 gauge harness and directly to ground after the pump.

It can happen for sure. I used this same technique in my Coyote equipped street rod - triggered my own heavy gauge power circuit with the control pack. If you really want your big fuel system to work, this is the way.

@@tecmotiondyno Would be nice if Ford noted the potential need for larger wiring in the control pack instructions. My tank is an aeromotive stealth gen 2 and I probably tossed the instructions lol!

Thanks! I’m used to taking a lot of pictures but shooting and editing video of these jobs does take a fair bit of time and effort so I really do appreciate encouraging feedback like this. It’s probably worth it if it helps guys out. Maybe at some point I’ll gain the 1000 subs I need to remonetize this channel, but even when it was monetized before I only earned $100/year out of it, so the real reason to do it will likely always be to provide some genuinely helpful or interesting content.

@@tecmotiondyno Absolutely man, doing stuff like this is always helpful. I've also been considering doing helpful videos like this as well but I know nothing about editing videos.

@@fox5oh302 it’s a bit of a process for sure. I always feel like they run too long, but then I’ll post a 20 minute video and someone will say “you forgot this or that”, and you left it on the cutting room floor because you thought it made the video too long for people to watch. I’m told you can hire guys in India to cut video for you inexpensively but I am not convinced that it’s a good idea for technical content like this.

@@tecmotiondyno yeah I agree with that, some video I hate when they cut out stuff like how they did certain things

The voltage drop is a function of the resistance in the wire and that is a function of the gauge and length of circuit. 18” of smaller gauge wire WILL add resistance but it is much less than say 32 feet of the same wire. This method gets most of the problematic resistance out of the circuit while maintaining serviceability and you can do it without dropping the tank. If you have a good power source in the trunk you can feed off that, but be careful that your amp wiring is large enough to carry the amperage for both the fuel pump and amplifier. Generally, I suggest a separate 10 gauge circuit, even to a trunk mounted battery or rear mounted master power switch.

Good feedback. I will try to include diagrams in future electrical videos. If you have a specific question about the wiring I will also try to answer that for you.

Wiring problems are one of the worst issues with Fox Mustangs. Over the years most of these cars have had an endless array of accessories and modifications cut into the wiring and lots of those wiring cuts are 30 years old which has given them plenty of time to deteriorate.

If I have any doubts I will run a resistance test from my proposed ground point to the battery negative terminal to check the ground quality too.

@@tecmotiondyno Hi thanks so much for making these videos your knowledge is very helpful. When I’m shift between 3 and 4 manual and I keep a steady cruise the car hiccups like it wants to lose speed. Could this be the fuel system? Please help.

@@alvaropimentel779 It's so hard to give a diagnosis over the phone or internet based on a small amount of data. Yes, it could be the fuel system I guess. I would guess that under the circumstances you indicated it is more likely to be something else, like the ignition system. There are a couple of videos on this channel on wiring diagnosis, and things that are commonly wrong with these cars that might give you some things to check into.

@@tecmotiondyno Thanks this gives me an idea on ware to start. Thank you for your response.

@@tecmotiondyno Hello again Mathew I decided to order fuel injectors and start with that 19 lb performance EVs from LMR. I’ll probably need a new gas tang overhaul. 1988 foxvert I bought February last year with 147,000 miles everything original only thing that the previous owner did was new radiator and replaced clutch twice I’m the 2nd owner. I’ve done tune up and flowmaster dumps that’s it. I want to keep it original as much as possible. But I would like a little bit of upgrades like headers and cam with that being said the back passenger window won’t go up and when I press the button there’s a fast clicking noise do you have and idea of what could be?

It doesn’t actually matter. It will activate the relay either way.

@@tecmotiondyno thanks!

I buy the relays on Amazon because it is a reliable and reasonably priced source. They always have some more or less irrelevant Chinese company name like “ehdis” attached to them. Search for 80 amp relays on there and look through the results for ones with a 10 gauge harness. In the past I have bought parts like this from a specialty truck place like Gregg Distributors but they are much less costly and easier to get on Amazon. At first I was skeptical of a Chinese part like this, but I started out using them in my own cars and have had no problems. In reality, the parts you get that sound like they are American or Canadian made are probably identical. One other thing I always keep in mind is that this circuit should run up to about 15 amps continuous, so the relay rating is 5 times overkill, which means that if it isn’t quite as good as the seller claims, it will still be a lot better than using a standard 30 amp relay which usually has a 14 gauge harness. I had a 30 amp relay in my Drag car and used to carry a special jumper in the glove box because it would fail occasionally and I could still jumper it to get it back to the pits.

It might be an interesting test to see what you really get with each stage of the upgrade.

a pump too large will cause irregular pressure readings because the 1/4 return line becomes the restriction. it will start backing up on the regulator. This causes drivability issues. the only restriction in the fuel system is the fpr.

I have never seen a 255 lph pump melt the wiring or cause a fire when used with stock wiring, but the voltage drop at the pump is significant and results in a significant reduction in deliverability. If you put a 255 in a car that only needs 150 lph you will be fine with stock wiring. If you actually need the flow, a wiring upgrade will make it happen.

@@tecmotiondyno awesome. Thanks

Direct to the battery, or add a power distribution lug to create a new pickup point for main power harness connections.

@@tecmotiondyno Thanks that was very helpful.

Question Tecmotion. I just purchased a power distribution so I can connect the positive terminal of my battery to it. Once I connected ground to some metal pieces of my car will the Fuel pump prime and then stop priming. I want to AVOIDE the fuel pump to continue priming with out stopping similar to jumping it directly from a positive and negative battery.

If you have a stock computer and wire it the way I show in this video, the fuel pump will run the prime cycle and then stop when the key is turned to run exactly as it does from the factory. The key is that you use the original fuel pump power wiring to trigger a new, heavier relay, and when that relay is activated it will power the fuel pump via the new heavier (10 gauge usually) circuit, but behave exactly as it did stock otherwise.

@@tecmotiondyno Thanks makes sense and thanks for that info.