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.
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.
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.
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.
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.
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.