Project Rehab: Replacing A 5.0 EFI Harness With Ron Francis Wiring

Whether you’re resto-modding a car, like we are with Project Rehab, constructing a new build, or just doing general repairs, replacing worn, and damaged wiring will not only make a vehicle more reliable, but also safer. That’s why we turned to Ron Francis Wiring for a new EFI wiring harness for Project Rehab.

The Ron Francis CobraSD-75 harness included high quality OE connectors and cross-linked SAE automotive grade wiring, with screen printed insulation that detailed where every wire went.

Modified Harness

Ron Francis Wiring (RFW) doesn’t currently supply a direct replacement complete EFI engine harness for the Fox-body Mustang. They do however supply harnesses that allow the EFI 5.0 HO engine whether mass air or speed density, to be retrofitted into a variety of applications. In our particular case we ordered part number Cobra-75SD.

This harness is designed to put a speed density 5.0 into a Factory Five MK 5 roadster . The harness offers enough length to fit a Fox-body. The harness eliminates a few circuits that the factory harness includes. Keep this in mind before you go out and order a new harness.

Left: All of the wires in the RFW feature screen printed insulation for easy idnetification. Center: A new relay and fuse block are included for fuel, EEC, and other systems tied into this harness. Right: New OE style replacement connectors ensure an exact fit.

Proper Connections

The RFW harness requires connecting a few wires to get everything working properly. We had previously installed a power junction box under the hood, which we used as our connection point for the constant power feed. For our ignition key on power/running, we went to the ignition switch, where we tapped into the black/purple wire. For cranking, we also went to the ignition switch and tapped the white/pink wire.

Circuits eliminated include EGR, purge canister, AC compressor, oil pressure sender, water temperature sender.  The reason for the differences is because the harness was originally designed to go into a new build like the FFR MK5. Rehab is not subject to emissions testing in it’s home state, and we are also preparing to upgrade the instrument cluster as well. 

Why Replace The EFI Harness

Time, and conditions inevitably take their toll on a vehicle’s wiring. We spoke with Scott Bowers of Ron Francis Wiring, and he gave us several reasons for replacing an aging harness, “There are numerous reasons to replace an old harness, with damage being the primary reason. Broken connector locks, insulation degradation, inexperienced repairs, and even engine fires or rodent damage take a toll on a vehicle’s wiring.”

Having worked on the engine on several occasions we can attest to the condition of Rehab’s wiring insulation. Most of the connector locks were also extremely brittle, with many breaking as soon as we pried them, and many others were already broken when we bought the car. There were signs as well of poor repairs, we’ve even found wires twisted together and wrapped in masking tape.

Broken connector locks, improper repairs, dirt, contamination, and other damage are all reasons to replace an aging harness.

After removing the old harness our first step was to feed the computer connector through the firewall and secure the grommet.

Ford used striped wire throughout the original harness, all our wires are clearly screen printed, and solid color so that there’s no confusion. -Scott Bowers, Ron Francis Wiring

RFW manufactures their harnesses to fit a variety of applications, they use high quality automotive grade cross-linked SAE wire. A hallmark of their harnesses is that all the wires are screen-printed. This makes routing, and any potential trouble shooting easier. “Ford used striped wire throughout the original harness, all our wires are clearly screen printed, and solid color so that there’s no confusion,” says Bowers.

Special attention was also paid to the TFI (ignition) circuit wiring. This wiring is carefully shielded to avoid interference. Bowers tells us, “Any noise or interference in this circuit can cause ignition system problems,” those problems can be difficult to troubleshoot, especially if the problem is in the harness itself.

Top Row: Left: We laid out the harness and started by connecting the injectors in order 1-8. Center: RFW includes plenty of loom and wrap to protect the wires, so don't spare any of it. Right: Our previously installed power junction is where we sourced our constant 12-volt power. Bottom Row: Left: We mounted the relays and fuses between the passenger's strut tower and the firewall. Center: There are two pink/black wires under the driver's seat. We used a jumper wire to determine which supplied power to the fuel pump and connected the pink inertia switch wire from the RFW harness to it. Right: The green alternator signal wire in the RFW harness is not used. In our car the alternator is triggered through the body harness .

Hooking Up

We spent around eight hours replacing our harness. This job does require removing the intake manifold plenum to gain access to all the fuel injectors. It also requires some knowledge of basic wiring, and wiring skills, including soldering.

We had the old harness out in about 20 minutes with no cutting. The majority of our time was spent routing specific wires, and verifying specific circuits. RFW provides instructions that list what each wire in the harness is for, and the best way to tackle the installation.

With everything connected and verified we plugged in the computer, reconnected the battery and twisted the key in the ignition, success on the first attempt meant that not only had we done everything correctly but we had installed a quality product.

According to Bowers Ron Francis Wiring is committed to developing high-quality replacement wiring products for the Fox-body Mustang, and has several direct fit harnesses already. As Fox-bodies continue to age and be restored, we look to see more RFW products make their way into this popular platform.

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About the author

Don Creason

Don Creason is an automotive journalist with passions that lie from everything classic, all the way to modern muscle. Experienced tech writer, and all around car aficionado, Don's love for both cars and writing makes him the perfect addition to the Power Automedia team of experts.
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