Electrical System

The electrical system is ostensibly why I did this whole modification in the first place. A computer gets inputs of water temp, outside air pressure, etc. to give a precise fuel allotment. I've heard that installing the electrical system can be a really big headache. Many stores out there sell wiring harnesses for $500-$800 and even they still require to cut, modify, and solder wires. Instead I purchased a used 1991 Mustang 5.0 wiring harness for $125.

First thing: Print out this booklet: http://www.vintagebronco.com/newberry/pages/tech/efi/efi.html

Make sure you have all the wires. This is a '93 harness, so the colors may be a little different for previous years. It also has a fuel pump relay, which other harnesses do not. More on that later. Also, use their instructions for welding on the O2 bongs and also drilling the large oval hole in the firewall for the harness installation. Don't forget to get a baramotric air pressure sensor and a coil and bracket. $5 each at a junkyard.

The installation is pretty straight forward. There are about 8 wires going through the firewall on the driver's side. Find out which are "Crank only" "On and Crank" and "On" and use those as power sources. You may also choose to use the various posts on the starter solenoid for those sources. Speaking of starter solenoids: Make sure to use a '65 mustang soleniod!!! (Thanks, Glen)

From the wiring harness, you'll plug these without any modifications: Mass Air Flow, HEGO connector, TFI, Spout, Baro Press Sensor, and Coil. There are two grounds on the harness. The engine side (#26) ground must have a direct connection to the negative battery side. The interior cabin side ground (#38) does not. Make sure that fused wire (#34) goes to battery side of starter solenoid.

You'll have to split up the grey, green, and brown plugs and use their various wires from the ford booklet and my relay description below.

Follow the diagram below to setup the relays. The reason for all the relays is to provide a clean 12V voltage to the various components, so it is required, period. The EEC relay is already part of the harness, and you may be lucky enough to have a harness with a fuel pump relay. If not, then go to autozone and purchase two of them and their corresponding pigtails. Another choice is to grab 2 relays and pigtails from the junkyard. They are underneath the driver's seat in a '89-'92 stang.
I mounted all the relays next to the EEC relay. Use a 20Amp inline fuse for the O2 and FP relays. Make sure to solder and then properly seal all connection per www.fordfuelinjection.com

After that, you must connect the alternator to the harness. I'll qoute from Glen's 5.0 Mustang page: " The first thing to do is remove the old voltage regulator. I don't recall all that is involved, but I know that you need to have one main feed from the fusebox to the constant hot side on the starter relay, & a key on hot wire will run from the ignition switch to the alternator. These wires already run up to the voltage regulator. Use a test light to determine the proper one. At the alternator, you will have a feed to the hot side of the starter relay. This will power the electrical system & charge the battery. Then you will need the key on hot wire. Connect the alternator as follows:

There are two plugs on the alternator, one large & one small. Connect the 2 blk/red wires on the large plug to the yellow wire on the small plug & to the feed to the hot side of the relay. The white/blk wire on the large plug connects to the other white/blk wire on the small one. The red/grn wire on the small plug to the key on hot wire."

That's it! Not too bad, was it? It took me hours and hours to figure all that out. I hope this advice cuts that time down for everyone else.