Story Overview
- Learn the basics of automotive wiring harness repairs
- Remove circuits you don’t need to unclutter the wiring harness
- See some of the basic tools and supplies needed for wiring harness repair
- Learn how to rewrap a wiring harness and protect wire terminals
There are two basic approaches to deal with a failing OEM wiring harness assembly: Fix it or replace it. You might be able to salvage some of the original harness assemblies and replace some of the others. It all depends upon the overall condition of the harness, especially if some ham-handed ‘electrician’ has done their thing on it.
Another tactic is modifying a wiring harness to remove circuits you don’t need. This is common with engine swaps. It allows you to eliminate a lot of bulky wiring, remove stray unconnected wires, and clean up the engine compartment. Removing an unneeded circuit can’t cause you any grief.
Learn more about right-sizing a wiring harness in The Simple Wiring Life article!
I’m going to cover some basics of harness repair and circuit removal using my 1971 Corvette as the guinea pig. Wish me luck.
Where to Start
The first thing to do is lay out the wiring harness on your shop floor and photograph it. This will give you visual references for the harness layout. They will be especially helpful when it comes time to rewrap it.
Once you have your photos, you can unwrap the harness. A good pair of scissors is essential for cutting the end of the wrap; I use Klein Tools electrician’s scissors. Once cut, you can simply unravel the harness.
If you’re removing a circuit or two, start the unravelling job at the firewall connector if the car has one. Trace the wire or circuit you’re removing. It’s a good idea to confirm you have the correct wires by referring to a factory wiring diagram. You can often download these from the Internet. An OEM service manual will also include wiring diagrams.
Next, follow the wires back to the firewall connector. In a GM application the wire terminal can be released from the connector as follows:
- Push the terminal forward into the firewall connector. This will move the terminal slightly
- Close the tab on the terminal with a pair of needle nose pliers and pull back on the wire
- If you’ve closed the tab sufficiently, you’ll be able to remove the wire and terminal from the back side of the connector
At this point you should be able to remove the wires (and in many cases, the entire circuit) out of the harness. In the accompanying photos you’ll see how many wires and how much bulk I managed to remove with one simple terminal disconnect at the firewall harness connector.
Repairing
We’re now ready to make repairs. The type of repair usually depends upon whether or not the harness has been modified. Some of the “improvements” in my Corvette harnesses included wires twisted together and wrapped with tape; bare wires; wires with no terminals attached; and wires tapped into other wires with ‘scotch lock’ type tap-in connectors.
OAC Editor: I interrupt this article to tell you not to use scotch lock-style tap-in connectors for your automotive wiring projects. They’re easy to use but will cause you nothing but grief.
Tap-ins allow you to piggyback a new circuit to an existing wire in the harness. The two wires are placed inside the connector. When you crimp it closed, the steel blade or blades cut through the wire insulation and into the wire itself, making the electrical connection. Here are the main reasons why that’s a bad system:
- There is little to no mechanical support for the connection, which can cause poor or intermittent electrical contact due to vibration
- Tap-ins provide no insulation, leaving the wire exposed to air and moisture. That makes the connection vulnerable to corrosion. The corrosion causes resistance in the wire, which can lead to melted wires or a fire
- Cutting strands of wire decreases the amperage load that wire can handle. That can cause circuit or component failure
- Tap-ins are not designed to handle the high amperage loads found in automotive electrical systems. For example, using a tap-in connector rated to handle 7 amps on a 20-amp circuit will create a guaranteed failure point
Now that you know why tap-ins are bad juju, read The Basics of DIY Automotive Wiring Harness Repairs to learn the right ways to make electrical connections.
Back to our story.
I prefer to do soldered connections. I coat the terminal connection with liquid tape and cover it with heat shrink. When I use non-insulated terminals, I cover the terminal wire end with heat shrink. Some say that vibration can break a soldered joint. That is true to some degree, but poor soldering techniques or using soldered connectors in areas exposed to moisture or corrosive chemicals are more likely culprits.
Stripping and Crimping Wires
I spent many years using basic hand strippers and crimpers. Then I discovered the pleasure of using high-quality tools like the Knipex Automatic Wire Stripper, Klein Tools Automatic Wire Stripper, and the Astro Pneumatic Ratcheting Crimper Tool Set. The Knipex and Klein Tools strippers made the cleanest cuts and nicest strips I’ve ever done. The Astro Pneumatic crimpers come with jaw inserts for various types of terminals.
Rewrapping the Wiring Harness
Once the repairs and modifications are done, it’s time to rewrap the harness. In the case of many vehicles from the 1960s and 1970s, the factory used vinyl wrap. It looks like electrical tape but it has no glue—it clings to itself as you wrap.
You can begin a rewrap by tying a knot in it at the beginning of the harness assembly—the firewall connector, for example—and winding it tightly around the harness in an angled pattern. Wrapping any wiring branches before you start the main loom will yield better results. When you reach a branch, simply wind the wrap tightly around the joint several times. When you come to the end of the harness, you can tie the wrap or cover the end with high-quality electrical tape. I recommend 3M’s Super 88 tape.
As you can see, repairing a wiring harness isn’t difficult, but it is time-consuming. For example, it takes me a couple of days to reconfigure a simple engine harness. Some lighting harnesses and dash harnesses in particular are much more complex and can take more time. Bottom line, if the harness isn’t butchered too severely, you can fix it.
Here’s one of the harness assemblies from my 1971 Corvette. It’s a good idea to lay the harness on the floor and take photos of it. You can position the various branches as shown here. The photos will be a great reference when rewrapping. The white masking tape is a reminder to remove this circuit. (Image/Wayne Scraba)
If you’re having difficulty tracing a circuit, you can sometimes download free wiring diagrams from the Internet. One word of caution—the diagrams are not always correct. That’s because manufacturers made running changes and often neglected to include them in service publications. (Image/Wayne Scraba)
Someone decided to “improve” this harness. A couple of wires wrapped together and loosely covered with cheap hardware store electrical tape isn’t the best way to fix an electrical issue. (Image/Wayne Scraba)
More evidence of hackery. There were a number of bare wires like this in my harness searching for a spot to ground themselves. (Image/Wayne Scraba)
Carefully unwrap the harness. I used really sharp Klein Tools electrician’s scissors to cut through the wrap. Once you get it started, it’s easy to unravel the wrap because it doesn’t have an adhesive backing. (Image/Wayne Scraba)
I wanted to remove the transmission control spark system from the car, so this branch of the wiring harness had to go. These two switches are part of that system. Not shown is the idle stop solenoid that operates the carburetor. (Image/Wayne Scraba)
When removing a circuit, follow the wires back to the firewall connector. Releasing the wire appears next to impossible, but it’s not. (Image/Wayne Scraba)
This is the wire terminal I had to remove. I pushed the wire and terminal forward into the firewall connector. I then closed the tap on the terminal with small needle nose pliers. A small flat screwdriver also works. With the terminal tap closed, pull back on the wire. It should release out the back. At this point the circuit can be removed. (Image/Wayne Scraba)
To release a more conventional connector such as this alternator plug, insert a small flat blade screwdriver or a terminal tool into the front side. Press down on the terminal to release the lock, then pull the wire out the back. (Image/Wayne Scraba)
If you have to fix an issue like this bare wire or need to terminate a wire inside the harness, there are a couple of ways to do it instead of wrapping it with electrician’s tape and calling it a day. Using liquid electrical tape as shown here is one method. (Image/Wayne Scraba)
Another option is to cover the wire with heat shrink. My overkill method is to apply liquid tape and then cover with heat shrink. You can cut off the excess or squash the end of the heat shrink with a pair of pliers. (Image/Wayne Scraba)
If you need to use electrical tape, Scotch Super 88 is highly recommended. It costs more than other electrical tape, but it has a broader temperature range, is slightly thicker, and incredibly easy to work with. (Image/Wayne Scraba)
When rewrapping a harness, I find it works better to tape the bundle in small increments. This keeps the harness taut and more manageable. (Image/Wayne Scraba)
Wrapping the wires with vinyl harness wrap is simple. The factory often tied one end in a knot—at the firewall connector, for example. Keep in mind that vinyl wrap is not adhesive backed. Start at the knot and wrap it tightly in an angled pattern. (Image/Wayne Scraba)
Sometimes, harness assemblies are fastened to the vehicle with push-in fasteners called “Christmas Trees”. They have legs to make it easier to tape them into place on the harness. (Image/Wayne Scraba)
This is all the stuff I removed from the Corvette’s engine harness. In some cars I’ve removed even more. (Image/Wayne Scraba)
Here is the harness that will remain in the car. It is definitely less complex than the factory harness. (Image/Wayne Scraba)
This little wire connector will cause you all sorts of grief. Known as a scotch lock or tap-in connector, it allows you to tap an existing wire into the harness to power another circuit. The two wires are placed inside the connector. When you crimp it closed, the steel blade or blades cut through the wire insulation and into the wire itself, making the electrical connection. Tap-ins are easy to use but are guaranteed to cause wiring failures due to vibration, lack of insulation, and inability to handle high amperage loads. (Image/Summit Racing)
