Having a car that starts right up and runs smoothly is important for reliable transportation. However, as a vehicle ages, the electrical system can degrade leading to slower cranking, uneven idling, dim lights, and weak accessories. Fortunately, I can significantly improve performance by rewiring the electrical system. In this comprehensive guide, I will cover everything you need to know to rewire your car's electrical system for faster starts and smoother operation.

Reasons to Rewire the Electrical System

There are several key reasons why rewiring the electrical system can provide major benefits:

Corroded Connections

Over time, vibration, moisture, and temperature changes cause electrical connections to corrode leading to increased resistance. This results in voltage drops that affect starting, charging, and electrical component operation. Rewiring restores clean connections.

Faulty or Undersized Wiring

Sometimes wiring is incorrectly installed or unable to handle the load. Upgrading to proper gauge wires ensures adequate current flow. I'll cover how to select appropriate wire sizes.

Unreliable Grounds

Faulty ground connections are a common cause of electrical gremlins. Properly rewiring grounds eliminates these issues.

Added Electrical Loads

As accessories are added over the years, the original wiring becomes overloaded. A rewire provides circuits capable of handling added electrical loads.

Safety Hazards

Fraying insulation and improper connections can create fire and shock hazards. A complete rewire makes the system safe and reliable.

Electrical System Basics

Before getting into the specifics of rewiring your car, it helps to understand some fundamental electrical system basics:

Power Source

The battery provides the main power source. It supplies electricity to start the engine and run electrical loads when the engine is off. The charging system replenishes the battery when the engine is running.


Most modern automotive electrical systems run at 12 volts DC. This is the force that pushes current through the circuits.


A ground connection provides the return path back to the negative terminal of the battery to complete the circuit. Proper grounding is essential for the system to work correctly.


Fuses or circuit breakers protect the wiring from excessive current flow. If a circuit shorts or overloads, the fuse blows opening the circuit.

Relays and Solenoids

These allow a low current circuit to control a high current circuit. For example, a small trigger current engages the starter solenoid which then closes the high current circuit to the starter motor.

Wiring Size

Wires must be sized to carry adequate current for the load. Undersized wires cause excessive voltage drop. Oversized wires waste money and space.

Preparing for the Rewire Job

Careful planning and preparation will ensure a successful rewiring project. Here are some key steps to take:

Create Wiring Diagrams

Diagram the existing wiring and plan the new wiring. This provides a roadmap for the required connections. Pay attention to wire gauges and circuit protection.

Select Wiring Components

Choose the right wires, terminals, connectors, tape, tubes, junction blocks, etc. Using high quality components prevents future problems.

Safety Gear

Be sure to have insulated tools, a fire extinguisher, eye protection, gloves, and sturdy jack stands. Take precautions when working around batteries.

Allow Sufficient Time

Plan to take several days for the job. Rushing increases the chance of mistakes. Taking breaks maintains concentration.

Have Backup Transportation

Rewiring means the vehicle will be out of service. Have alternate transportation arranged so you don't feel rushed.

Pick a Clean Area

Choose a dry, well lit, comfortable space with good ventilation. Avoid dirt and humidity. Lay down a tarp to protect the floor.

Removing Old Wiring

Taking out the old wiring is the first step in the rewiring process:

Disconnect the Battery

Always disconnect the negative battery cable first to avoid shorts. Remove the positive cable next. Tape the cable ends to prevent contact.

Remove Access Panels

Take out kick panels, trim pieces, and underhood covers to access wiring. Label pieces for proper reinstallation.

Take Notes and Pictures

Document wiring locations, routing, and connections before dismantling. This provides a reference for the new wiring.

Extract Wiring and Components

Carefully remove existing wiring from the vehicle. Also extract any electrical components that will be reused like gauges, switches, and control modules.

Clean Surfaces

Thoroughly clean all mounting areas and connection points of dirt, corrosion, and old insulation. This prevents contamination issues in the new wiring.

Inspect Circuits

Check for blown fuses, faulty grounds, corrosion damage, or insulation breakdown indicating problem circuits that must be addressed.

Installing New Wiring

The following tips will help in routing and installing fresh, clean wiring:

Route Wiring Carefully

Run wires to avoid sharp edges, heat sources, and moving parts. Use grommets when passing through sheet metal. Keep wiring away from exhaust components.

Anchor Wiring Securely

Use anchor clips, conduit, and wire ties to prevent chafing and damage from vibration. Don't let wires droop onto hot or moving surfaces.

Apply Dielectric Grease

Coat connections with dielectric grease to prevent corrosion and loss of contact. Avoid using tape which can retain moisture.

Solder or Crimp Properly

Any splice connections must be mechanically and electrically secure. Solder and cover with heat shrink tubing or use properly sized crimp connectors.

Separate High Current Wiring

Route starter cables and battery feed wires away from low current signal wiring to avoid electrical noise issues. Twist positive and negative wires together.

Use Proper Grounding Techniques

Grounds should go to clean, unpainted metal surfaces. Use star washers and bosses to bite through paint if necessary.

Confirm Quality Workmanship

Carefully inspect wiring installation to confirm proper routing, secure mounting, strain relief, and tight connections. This prevents problems down the road.

Replace Removed Panels

Securely reinstall any panels and protective covers. This protects wiring and prevents rattles.

Testing and Verification

Thoroughly testing the vehicle electrical system is mandatory after completing the rewire. Here is the proper process:

Inspect Wiring

Do a visual inspection of all wiring looking for loose, chafed or pinched wires. Also check for unsecured connections and bare conductors.

Reconnect Battery

Reconnect the battery starting with the positive cable first. Make sure terminal connections are clean and tight. Cover terminals with a non-conductive protectant.

Check Fuses

Inspect all fuses to ensure they are the proper type and amp rating for the circuit. Replace any blown fuses starting with the highest rated first.

Verify Voltage and Grounds

Use a multimeter to check voltage at the battery and several points throughout the wiring harness. Voltage should be stable at around 12 volts. Also verify continuity between negative battery terminal and several grounding points.

Operate Switches and Devices

Systematically operate every switch, control, and electrical device on the vehicle. Check for proper operation of headlights, outlets, wipers, power windows, etc.

Start Engine

Crank the engine and check voltage remains above 10 volts during cranking. Let engine run and check for smooth operation without dimming of lights or air speed changes.

Drive Vehicle

Take the vehicle for an extended test drive under varying conditions. Listen for odd noises or look for intermittent operation indicating wiring issues.


Rewiring a vehicle takes planning and attention to detail but pays big dividends in performance and reliability. The improved electrical connections provide faster, smoother operation and can add years of life to an older vehicle. While it takes some effort, with good wiring practices and careful testing, you can turn that unreliable heap into a dependable driver.