As the owner of EFI Connection, engine wire harness manufacturing is my specialty. In the early days of my career, I took on used harness rework for 1985–1992 Camaro and Firebird owners who wanted to add a TPI system to their car. I’ve also revised newer LS harnesses for customers who have relocated components in the engine bay. Today, however, I decline nearly all used harness rework. With few exceptions, I recommend new engine wire harness builds whenever possible.
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Used Wire Harnesses
The reality of any used engine wire harness is that it was removed from an old, and likely salvaged, vehicle. These harnesses may have damage due to heat, chaffing, or improper repair. Proceed with caution before reusing an old wire harness for your high-dollar retrofit.
“Let sleeping dogs lie.”
You know that old adage. If it works, leave it alone. If you start messing around, things may get ugly. Some engine bays are left untouched except for oil changes and adding washer fluid; other engine bays have additional wiring for car stereo, alarm, nitrous solenoids, fuel pump relay upgrade, electric water pump, and the list goes on. These add-ons require power, ground, and maybe even sensor signals.
Have you seen the uninsulated butt splice in a vehicle’s wire harness? You know, the red, blue, or yellow plastic barrel that has been crimped on each end? Or maybe you’ve seen an uninsulated blue wire tap? Yes, it’s the handiwork of someone who was in a rush to tie into or repair the vehicle’s wire harness.
Alternations to a vehicle’s harness often create future points of failure. Even the improper handling of a used wire harness from time of removal to your garage floor may have introduced points of failure that may make you wish you had a custom-built new harness for your project
Let me pause here to tell you about a local street rod project. The early Ford pickup was fitted with an LT1 engine and 4L60-E transmission.
The shop that took on the restoration received the project with a used, reworked engine wire harness. Part of the rework service had included the addition of gear select switch wiring to interface with the LT1 PCM and a custom gear indicator panel mounted on the dash.
I was called after the vehicle was finished because the electric fan did not turn on when the A/C system was on. I also found the used gear select switch connectors were damaged and a previous repair attempt involved forcing wire too large into the connector cavity using a terminal not designed to accept such a large wire size.
Had I been involved with this restoration from the start, the vehicle would have had a brand-new wire harness with all necessary provisions for the vehicle. There would have been no need for a mechanic to add the gear select switch and A/C wiring using the wrong type of insulated wire, butt splice connectors, and generic crimp tool.
This restoration project exceeded $100,000 and, sadly, still has a used engine wire harness with surprises yet to be seen under the black electrical tape.
I’ve heard stories of used engine harness reworks where unused sensor connectors were cut and taped, causing a short as the PCM reference voltage circuit made contact with the PCM low-reference circuit. The methods used to rework used wire harnesses often introduce points of failure.
“Beauty is only skin deep, but ugly goes clear to the bone.”
Going back to that LT1 Ford street rod restoration, I found other surprises as I made repairs. The used harness rework was completed with the application of black electrical tape, which oozes adhesive when the wire harness experiences engine bay heat and leaves a sticky residue as it is removed.
As I unwrapped the sticky electrical tape, I found segments of wire spliced and/or crimped to the harness so that the desired length was achieved. Some “professionals” take the completed engine harness that was designed to fit a specific GM application and shorten or lengthen it by splicing wires to fit the layout of another vehicle. It looks pretty all wrapped up in black tape and split loom. The quality of work within? You roll the dice.
“An ounce of prevention is worth a pound of cure.”
Building a plug-and-play wire harness for any production vehicle requires sourcing connectors and related components (associated secondary locks, dress covers, retaining clips, etc.) that are often specific to the vehicle. These items, if serviceable, are often only available in very large quantities. A harness builder would have to purchase the thousands of connectors, secondary locks, and dress covers and then mark up the price of wire harness builds to reflect these costs.
A vehicle that is not appropriate for a standalone, street rod type of wire harness is the 1985–1992 Camaro and Firebird. Unfortunately, General Motors merged much of the engine bay wiring together in 1989, making a new plug-and-play harness a bit more involved than a typical new LS conversion engine wire harness build.
When a customer requests an LSseries PCM conversion wire harness for a 1989–1992 Camaro or Firebird, my typical approach is to build a brand-new wire harness segment for the LS-series PCM with added length at the sensor ends.
The customer sends me the original engine bay harness. It is carefully disassembled to remove the TPI ECM-related wiring and then inspected for damage. Rather than splice in repair segment(s) for a damaged circuit, such as a cut ring terminal at the starter, I place a brand-new length of 10-gauge purple wire in the wire harness, which is terminated at the starter and at the bulkhead connector for the firewall.
I remove the original damaged circuit. After the repairs, I set the remaining original engine bay harness on top of the new harness and finish it on a vehicle-specific template.
The end result is an OEM-quality engine harness that would have been built by General Motors had it used the LS-series PCM in that car.
A wire harness shop should have the proper terminals for GM engine harnesses, be tooled up to make proper crimps using production tooling, and apply proper techniques for reworking an original engine harness. You typically do get what you pay for as many side businesses offering only used harness rework have not invested in the proper supplies (TXL wire and replacement terminals) or crimp tooling to provide you with a product that matches the quality of a new engine harness.
This equipment is expensive and, as a result, causes the repair costs and new construction costs to exceed any low-price deal you find on eBay.
New Wire Harnesses
A brand-new custom wire harness (using the same production tooling and quality components as an OEM harness) is always superior to a reworked, used engine harness. Wire lengths are appropriate for your application, connectors are new (not dirty, brittle, or broken), and the wire does not suffer from oxidation due to years of use in temperature and moisture extremes.
The recipe for a quality engine harness is rather simple: Use the same components and production tooling as General Motors did. Only a few harness shops stand out from the rest because of the high cost involved in material and tooling.
Written by Mike Noonan and Posted with Permission of CarTechBooks