How To Test Car Wire Harness: Ground, Voltage, and Continuity Wire Testing

Here’s how to test car wire harness if your car has electrical issues.

Car wire harnesses are like the arteries and veins that circulate blood (electrical current) to and from vehicle parts. So when something is off, your car will misbehave. 

You can take your vehicle to an electrician, but it is cheaper to check it. And since the electricals have no mechanical parts, taking them apart and reassembling them is easy. 

Read on to learn more about how to test car wire harnesses.

Table of Contents

What Is a Car Wire Harness?

A wiring harness consists of electric cables that distribute power and relay signals to all electric and electronic car systems. The complex wiring system can be as long as four kilometers in modern vehicles and connects systems like:

  • Batteries
  • Alternators
  • Sensors
  • Ignition systems (starters)
  • Actuators
  • Fuel injection systems
  • Lighting
  • Electronic control units
  • Audio systems

Each has separate wiring, so you should test the specific part wires in case of malfunctions.

A car audio wiring harness

A car audio wiring harness

What Can Go Wrong With Wire Harnesses?

Cars are not static objects. As you drive, the vibrations and environmental conditions like heat, snow, and rain can affect the integrity of the wiring system. The wires also age naturally with time.

So the connectors can break, get corroded, or loosen. And the wire insulation can come off, leaving the bare wire exposed. Such wear can cause issues like short circuits that pop up error codes on the dashboard.

Complex car wiring harness

Complex car wiring harness

Typical Car Wire Harness Issues

Electrical system problems can include the following.

Short Circuits

Short circuits are unwanted connections between two circuit points operating at different voltages. This issue can occur due to loose wires, faulty connections, and damaged components.

Open Circuits

Open circuits break wire continuity, and causes can be cut wires, blown fuses, or disconnected electrical components.

A car wire chewed by rodents, leaving an open circuit

A car wire chewed by rodents, leaving an open circuit

Voltage Issues

This issue occurs if the voltage in the circuit is not within the required range (can be over or under). And typical causes are damaged batteries, faulty voltage regulators, or loose connections.

Ground Issues

Since cars use batteries, the ground connection is the negative connection to the battery, which connects to the vehicle body.

Ground issues occur when positive wires make unwanted connections with the ground (0V). The most typical causes are damaged components and loose wires.

Corroded/Dirty Connections

Dirt, debris, and corrosion can make connectors loose or cause arcing on joints that affect the electrical connection.

Car wiring harness connectors

Car wiring harness connectors

Damaged Circuit Components

Components might be the issue, and a simple repair or replacement can solve it.

How To Test Car Wire Harness?

Before getting down to the task, disconnect the battery to prevent shorting, ground connections, or damage to the electrical system.

And you must have these tools.

  • Digital multimeter (You can use an analog multimeter, but the digital one is better)
  • Jumper wires
  • Screwdriver set
  • Automotive test light (optional)
An automotive test light

An automotive test light

The next step is to identify the test points, and it would help if you have your vehicle’s wiring diagram. If you don’t have this diagram, locate the fuse box, which houses fuses, diodes, and relays for car components.

This location is a great place to start if you suspect blown fuses.

If the issue is not in the fuse box, follow the wire harness from the misbehaving part to the battery, fuse box, or switch.

The process is time-consuming, but it can highlight other potential issues that can affect the circuit in the future.

Check for signs like:

  • Blackened/Burned Wires: Excessive current flow in wires causes overheating that burns the insulation. When burned, wire insulations blacken and can have soot on the surface. Such cables can cause short-circuiting or ground connections if the insulation falls off.
Burned wires caused by short-circuiting

Burned wires caused by short-circuiting

  • Cracked Insulation: Burning and aging can break wire insulations, exposing the wire to shorting and ground connections.
  • Corroded Connections: These connections can degrade electrical and signal transmission, which affects performance.
  • Loose Connections: Like corroded connections, loose ones degrade the signal and electrical connectivity, affecting performance.
  • Exposed Wires: These wires are prime agents for short-circuiting.

Once you’ve spotted these points, test using either of these three processes.

Continuity Testing

A circuit has continuity if it features a low-resistance electrical connection between two points. So as the name suggests, continuity refers to a continuous path between two circuit points.

You can use a self-powered test light to check for continuity. The device has a tiny battery with two test leads. Connect these probes to the circuit points and check the light.

A continuous connection will complete the circuit and turn the bulb on. But with no continuity, the bulb will remain off.

If using a multimeter, set it to continuity first. Next, connect the multimeter probes to the two circuit points you want to test. And ensure the black probe is in the COM port. The red one should be in the VΩ (mAVΩ) port.

A multimeter set to continuity mode

A multimeter set to continuity mode

If the multimeter beeps, there is continuity. Otherwise, you have an open circuit.

Voltage Testing

When testing for voltage, you must plug in the battery. And you can only use a multimeter because there will be power in the car wire harness.

Begin by setting the multimeter dial to 20V DC because the car battery supplies about 11-12V DC to the electrical system.

Connect the probes to the two test points and check the multimeter reading. If the voltage is between 11 and 12V, the circuit is okay. Otherwise, the battery or voltage regulator might be faulty.

Ground Testing

Disconnect the car battery for ground testing. If using a self-powered test light, connect one lead to the battery’s negative (-) post and the other end to the test point. The light will illuminate if there is a good ground connection. If it remains off, you have poor circuit ground.

An electrician repairing car wire harnesses from under the vehicle

An electrician repairing car wire harnesses from under the vehicle

You can also use a multimeter for this test. Set it to the Ohm setting, then connect the black probe to the negative (-) battery terminal or car chassis. Contact the red multimeter probe to the test point.

If the multimeter beeps or reads zero, the ground wire is good. But if it does not beep or gives a reading that is not zero, replace the bad ground wire.

How To Test Car Sensors

Car sensors are critical components of the electrical system, and you can test them using these steps when checking the wire harnesses.

Car sensors

Car sensors

  • Identify the faulty sensor
  • Check the sensor’s resistance value and operating voltage
  • Set the multimeter to have the appropriate settings (should match the sensor resistance and voltage)
  • Unplug the sensor from the vehicle’s electrical system
  • Test the sensor using the multimeter while comparing the readings to its specifications
  • The device is not faulty if the results are within the sensor’s voltage and resistance ranges. Otherwise, it is defective and needs a replacement
  • Replace or reattach the sensor to the vehicle
  • Clear error codes using a scanner or code reader (connect it to the OBD port)
A vehicle’s OBD2 port

A vehicle’s OBD2 port

Wrap Up

Testing car wire harnesses is relatively straightforward if you know where to check. 

So first identify the faulty area, then use a multimeter or self-powered test light. But we recommend using a digital multimeter because it is more versatile. 

That’s it for this article. Get right to testing, and let us know if you have any challenges. We’ll be in touch to help.