The Art of Rewiring

Rewiring an old car, motorcycle, or building a race car’s electrical system from scratch might seem like a daunting task, especially when staring at a tangled mess of colored wires. But in reality, it’s more straightforward than it looks—if you approach it with the right tools, a solid plan, and a bit of patience.

Matt Brown, an automotive engineer with experience at Tesla, Apple, and NASCAR, dives into the process of wiring vehicles with a blend of expertise and practicality. Let’s unpack his method to simplify rewiring into a manageable, rewarding project.

The Case for Starting Over

Old car wiring can be a chaotic mess, sometimes literally hosting rat nests. If you’re working on a vehicle from the 1990s or earlier, or creating a dedicated race or off-road machine, it’s often easier and more efficient to rip out the old wiring and start fresh. However, if your vehicle relies on a body controller or is as complex as a Tesla, this guide isn’t for you.

For the rest of us, tackling a wiring job involves understanding the circuits, planning the layout, and using the right materials to ensure the new system is safe, durable, and functional.

Step 1: Break It Down by Circuits

A wiring harness may look like an intimidating jungle, but most circuits in a vehicle are separate and don’t interact. Start by focusing on one system at a time—such as headlights, tail lights, or the horn. Complete each circuit before moving to the next. This piecemeal approach keeps the task manageable.

The engine control unit (ECU) is an exception, as it often has more wires than the rest of the car combined. For race cars with standalone management systems, pre-made ECU wiring harnesses are typically available.

Step 2: Plan Your Wiring Layout

A wiring diagram is your best friend in this process. Don’t be discouraged if it looks chaotic at first glance. Start by listing everything in the vehicle that needs power: lights, horn, ECU, and so on. Determine the fuse size and whether a relay is needed for each. Err on the side of slightly larger fuses if unsure.

Using software like PowerPoint or Keynote to draw your diagram allows for easy updates. Color-coded lines and boxes can visually represent your circuits. Every component will need at least two wires—a power and a ground—so plan accordingly.

Step 3: Choose the Right Components

Fuse and Relay Blocks

Pre-made wiring kits exist, but they rarely fit your exact needs. Opt for a customizable fuse and relay block, like the Bussmann series, which accommodates different configurations. Select a block with enough fuses and relays for your system—always plan for a bit of future expansion.

Relays

Relays are essential for handling high-current circuits. A good rule of thumb is to use relays for circuits pulling significant current, such as headlights or fuel pumps. Relays prevent overloading switches and protect sensitive electronics like the ECU.

Connectors

Avoid cheap connectors that fail under stress. Delphi (now Aptiv) connectors strike a balance between cost and quality. Proper crimping tools are essential for a secure connection, and heat-shrink tubing protects connections from environmental damage.

Step 4: Wire Selection and Labeling

Not all wire is created equal. Automotive environments demand wires resistant to heat, abrasion, and chemicals. SXL wire, with its durable cross-linked polyethylene insulation, is a reliable choice. Use only a few wire colors to avoid confusion and label everything clearly—heat-shrink labels are ideal for a professional finish.

Step 5: Protect Your Wiring

Protect your wires with appropriate coverings. Expandable braided sleeving offers flexibility and durability, while fabric Tessa tape is excellent for bundling wires in tight spaces. Avoid using rigid plastic tubing unless necessary.

Step 6: Test and Troubleshoot

As you connect circuits, replace the main fuse with a lower amperage one, such as a 5-amp fuse, to protect against shorts. Test each circuit individually before moving on. If something doesn’t work, trace the circuit to locate the issue—usually a loose connection or a missed ground.

Practical Tips for Success

Grounding: Use a grounding bar for centralized grounding and run separate grounds for high-current components like fuel pumps and headlights. Always verify continuity with a multimeter.

Relays with Diodes: Diodes protect against voltage spikes when relays disengage, making them safer for sensitive circuits.

Wire Routing: Plan your wire paths to avoid high-heat areas like exhaust manifolds. Secure wires with zip ties and ensure there’s slack to accommodate movement.

Why Rewiring Is Worth It

Rewiring isn’t just about functionality—it’s about reliability and customization. With a clean, well-organized wiring system, your vehicle will be easier to diagnose, repair, and upgrade in the future.

For an in-depth visual guide, including specific part numbers and more tips, watch the full video from Matt Brown on his channel, SuperfastMatt. You’ll gain confidence and clarity as you tackle your wiring project with expert insights and a bit of humor to keep things grounded.