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HomeTips & GuidesEngine Harness – Definition, Fault Diagnosis, and Replacement Steps

Engine Harness – Definition, Fault Diagnosis, and Replacement Steps

About Engine Harness: Every vehicle, boat, plane, or any other machinery needs a wiring harness in its system. This vital set of wires and cables connects various electrical systems in these machines to help relay information and power. 

Today’s article discusses the engine harness, what it is, and how to diagnose its faults and replace it. 

Doing so will help prevent failure in your car systems and ensure optimum functionality.

What Is an Engine Wiring Harness?

An engine wiring harness refers to an engine’s set of cables and wires, plus their terminals and connectors.

Typically, the cables are in casings organized to control the electrical system of a vehicle.

 They do this by relaying electrical power and information to various components of the car system. For example, they transmit power and sound to the car’s audio system.

 They also connect components such as the alternator, battery, injectors, blowers, motors, and computers.

 As you may expect, wiring harnesses are not the same as manufacturers design and customize each for its application.

Unlike wires, a wiring harness features fire-resistant over-molding to reduce fire risk and the resulting electrical problems.

 In addition, they come in shielding that keeps oil and moisture away, hence preventing corrosion.

 Lastly, the manufacturer installs wiring harnesses in the machine frame and vehicle chassis to prevent abrasion.

Old car engine wiring harness

Old car engine wiring harness

How to Replace an Engine Wiring Harness

Now that you know what an engine wiring harness is, let’s see how you can replace it. But first, we discuss how to detect its faults to know if it’s worth replacing.

Detection of Fault

It’s obvious that the gadget that the fault wiring harness serves becomes dysfunctional or functions irregularly. 

Now, you will need the right tools and know how to inspect the functionality of the harness. Here are some of the steps to use:

Identify the fuse box or the relay associated with the nonworking system

The first step is to know which system is not working and the associated fuse or relay. 

You can easily do this by checking your vehicle’s fuse box guide or repair manual.

 If you find many accessories that stopped working, I recommend using a wiring diagram to know where they share the relay or fuse. 

Once done, you can know the specific fuse areas that need inspection.

Remove and Check the Fuse

A fuse is a device that allows only predetermined electrical current to pass through.

 If this current is beyond the fuse capacity, the fuse breaks down to prevent the connected accessory from electrical destruction. 

So, you need to open each fuse in the areas you identified above. If any is broken, replace it with another fuse of the same amperage. 

Check dysfunctional components and wires.

Once the fuse and relays are okay, it’s time to check the non-working components and the wires connecting them.

 Here, you may need to carefully remove the interior trim panels and be sure not to break them. 

Then, remove the wires from their plastic tubes and check them physically. Inspect each wire for signs of damage, such as insulting burning, and replace or repair where necessary.

Voltage drop test

Voltage drop involves testing an open circuit for disconnected or broken wires that may hinder current flow. 

For this test, you will need a multimeter to test the non-working component in your system. Now, ground the negative terminal of the multimeter to a good grounding. 

Then, connect the positive terminal with the non-working accessory positive terminal. 

You can also split the circuit into various parts to help narrow it down to a specific problem area. 

Check to see if there is current flowing in each check. Your wiring harness or switch may be the problem if it does not flow. 

If the flow is normal, maybe your grounding has an issue. Inspect the grounding bolt for loose connection, dirt, corrosion, or moisture and act accordingly.

Using a digital multimeter

Using a digital multimeter

Troubleshoot the wiring harness.

Once you ensure the grounding works fine, the next step is the check connected wires. 

Here, you will remove the first wire you encounter from its plastic casing and trace it back the the first connection. Then, use the multimeter to check for continuity. 

You do this by connecting the terminals of the multimeter to that of the cable. Then, read a normal wire’s resistance value of 0.3 to 0.5 ohms.

 If the value is infinite or unlimited, then the wire has a problem. Inspect all the cables and replace the one with a problem.

Car electrical wiring harness

Car electrical wiring harness

Replacement Steps

Now that you have identified the cable with the issue, let’s discuss how to replace it. The good news is that it’s a simple process that does not require much training:

Take photos of the current wiring

You need detailed photos of the current system to start your installation journey. Use your camera to photograph your engine harness and label the connections using masking tape. 

Now, study the engine wiring and connection manual and take note of the color codes and stripes. Remember, these colors may fade with time.

Disconnect the battery

Here, you will need a socket wrench to open the terminal clamps, starting with negative and then positive. 

Then, please remove the battery and place it somewhere to prevent stray sparks.

Carefully remove the current harness.

Now, working with each connection individually, disconnect the main connection. 

You will find this at the firewall, where the harness connects to the vehicle’s central computer.

 Utilize a screwdriver to snap all clips and retainers holding the harness in place. 

Then, open the starter and alternator wiring bolts and remove the connected plug. 

Be careful to remove other hidden harnesses, such as sensors that can be difficult to reach.

Jack the car

Now, lift your car using a floor jack and hand crank. Lower it back on the jack stand and wiggle it to ensure stability. 

Remove the harness under the engine once the car is on the stand. You may find different harnesses, such as fans and lights twined with the engine harness. Ensure you untwine them to aid their isolation and removal.

Label new connection

Now, use the photos of the harness you removed to label the new one using masking tape to save time. 

Now, isolate any tangled wires and understand their routing and installation.

An isolated engine wiring harness

An isolated engine wiring harness


Now install the wires in their various place as labeled above. Ensure to put the harnesses in their respective conduits and sure them properly. 

Remember, the lengths of these cables are sometimes tricky. Therefore, I recommend connection first, then route later. 

When routing, avoid doing so on the exhaust or moving parts. Finally, you can now lower your car, install the battery, and then start it to check the installation.


Now, you know how to diagnose engine wiring harnesses using various methods. You also know how to replace the harness armed with simple tools and using the steps above.

Hi I am Christa, sales manager of Cloom.

I have extensive expertise and experience in wiring harnesses and I believe I can help you.

And we have a very professional technical team who can clearly understand the customer’s needs and give professional suggestions and solutions after receiving the drawings.

If you also have wiring harness needs, please send me the drawing so that we can give you our quote and start our business.

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