Engine Wiring Harness: Everything You Need to Know

Often, automobiles, airplanes, and machinery manufacturers will install dozens, hundreds, or even thousands of wires running connecting one component to another. If all these wires remained loose, it would not only increases the risk of electrical problems but also impedes the ability to perform routine repair and maintenance. Don’t worry. Engine wiring harness is here to help.

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1. What is an Engine Wiring Harness?

An engine wiring harness is constricted wires, cables, terminals, and connectors controlling a vehicle’s electrical system. It relays electrical power and control information to components such as batteries, alternators, fuel injectors, fuses, audio systems, blower motors, fuel injectors, and computers. Wire harness companies customize it for every location, application, and design it interconnects. 

Also known as a cable harness, it is more than just a collection of multiple wires put together. 

The harness also reduces the likelihood of electrical problems that would otherwise affect the vehicle or equipment. Flame retardant overmolding minimizes the risk of electrical fires. Also, It shields cables and wires from moisture, thus preventing corrosion. In addition, by installing the harness on the vehicle or machinery’s frame or chassis, there is a lower likelihood of damage from abrasions and vibration.

Major Types of Engine Wiring Harness

There are different types of vehicle wiring harnesses, including:

  • Airbag Harness – Controls the opening of the airbag in the vehicle.
  • Main Harness – Controls instruments, control panel, guidance system, multimedia, and electronic equipment. Forms part of the chassis wiring harness together with the front and rear harnesses.
  • Engine Harness – Manages the key functions around the engine including engine controller, oil controller gate, transmission plug housing, air pipe sensors, pressure sensors, and water temperature sensors.
  • Body Wiring Harness (or Body Harness or Floor Harness) – Keeps the air conditioning system, back sonar, navigation, dimmer switches, and speakers functioning.
  • Battery Harness – Delivers the main power to the other units. Serves as a connecting bridge between the engine and other vehicle components.
  • Door Wiring Harness – Found in car doors. Controls door and window lock, door and window power, and the speaker.
  • Roof Harness – Located in the car’s roof and controls roof features, including the sunroof.
  • Front Wiring Harness – Forms part of the chassis wiring harness together with the main and rear harnesses. Controls air conditioner fan, cooler fan, power sensor, temperature sensor, pedal sensor, speed limit sensor, oxygen sensor, and more.
Engine Wiring Harness

2. Diagnosing a Broken Engine

The device served by the broken or damaged wire may stop working. So, you must diagnose it with proper tools.

Engine Wiring Harness

Identify Relevant Relay or Fuse Box

Use the fuse box guide or repair manual to identify the relay or fuse associated with the non-working component. If more than one accessory is not working, use the wiring diagram to locate shared points in the circuit. With that, you will narrow down the areas you need to inspect and test.

Inspect and Test the Fuse

Inspect and test the fuse to confirm it has not burned out and is thus not the cause of the problem. If you have a bad fuse, replace it with one having the same amp rating.

Expose and Inspect Component and Wires

Expose the component that is no longer working to see where the wires connect to it. This may call for the removal of interior trim panels. In that case, remove the panels carefully since they tend to grow brittle and delicate with age. 

If the wires are in a plastic tube, remove them so you can touch and see the wires. Go wire by wire as you examine spots where the insulation has worn off, thus exposing the copper cable. Repair or replace damaged cables.

Engine Wiring Harness

Test Voltage Drop

Get a multimeter and power on the problem accessory to check for voltage. Touch the multimeter’s negative leads to a good ground and the positive on the non-working component’s positive wire or terminal. This voltage drop test will help you find shorts and opens. 

Split the circuit into thirds to narrow down on the problem. If no electricity is flowing, then you may have a faulty switch or wiring harness. If the course seems to work just fine, then the problem could be a grounding issue. 

To resolve a ground issue, check for corrosion on the ground bolt, loose connection between the bolt and the wire, or the presence of dirt or water. Confirm the existence of a ground issue by getting a jumper wire from the load to a good confirmed ground.

Troubleshoot the Wiring Harness

If the grounding is fine, troubleshoot the wiring harness. Remove the wires from the accessory and trace them back to the first point of connection you encounter. You may need to remove wiring from the protective plastic tube. 

Disconnect the wires at the first point of connection you encounter. Now, test this stretch of wiring harness for continuity and electricity flow. Place multimeter probes on either end of the wire. If resistance reads infinite or out of limits, the wire is likely damaged. Otherwise, a functioning wire should have a resistance of 0.3 to 0.5 ohms. 

Repeat this process for all other wires in the wiring harness until you find the faulty one. If all cables pass the test, the problem could be in a different saddle section between the power source and the accessory. Once you identify the wrong wire, your options for fixing it include removing the faulty unit and installing a new wire or replacing a section of the wiring harness with a new one.

Engine Wiring Harness

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3. The Cost of Engine Wiring Harness Repair

The cost of repairing your engine wiring harness will depend on the extent of the damage and the specific component of the harness that needs fixing. Ergo, the price could be under $100 or more than $1000. For example, a primary engine harness wiring could go for $40, while the cost of a specialty harness cable may begin at $90.

You can save on repair costs by doing the work on your own as long as you are well acquainted with the procedure. Note that it might be more prudent to contract a professional instead if you are unsure of your ability to do the work satisfactorily. If you do not do the repair works correctly, you can expect the total cost to go up due to the subsequent remedial works.

Remember, cost concerns should not supersede safety considerations. You would rather spend as much as is needed to minimize the risk of fire and accidents due to incorrect repairs.

Engine Wiring Harness

4. Buying an Engine Wiring Harness

You can either buy an aftermarket wiring harness or order a custom one from reputable manufacturers such as Cloom.

Aftermarket Engine Wiring Harness

Order the aftermarket wiring harness online or from a physical auto shop. The harness you buy will primarily depend on your vehicle or machinery’s make and model.

Wide Custom Wire Harness Options at Cloom

An aftermarket harness may not always be the right fit for your needs. The installer may have to cut and splice it to make it work. This could immediately degrade its quality. In this regard, a custom engine wiring harness has a distinct advantage. It is plug-and-play ready. 

Visit Cloom’s website to see various types of wire harness covering options and then follow the process needed to place an order for a custom engine wiring harness. You will need to provide specifications of the vehicle or machinery where the saddle is going, so the manufacturer will build it to fit.

5. Replacing an Engine Wiring Harness

Replacing an engine wiring harness does not require extensive training. Just follow the process step-by-step. 

Photograph Current Wiring

Take detailed and extensive photos of the current engine wiring harness, connections, and routing. Use masking tape to label each relationship for future reference. Get a shop manual, then study the engine wiring diagram. Take note of the wires’ color codes. Over time, the wire colors will fade, and the stripes grow increasingly illegible.

Disconnect and Remove the Battery

Use a socket wrench to get rid of terminal clamps. Begin with the negative, then move to the positive. Hold down the brackets that hold the battery in position. Steer clear of stray sparks by setting the battery outside while placed at a significant distance from the car.

Systematically Remove the Current Harness

Work through the individual connections of the harness. Get rid of the primary bulkhead connection situated at the firewall. This is where the harness plugs onto the vehicle to connect to the car’s central computer. 

The majority of the engine connections are snap-in plugs. Still, use a tiny screwdriver and socket to eliminate assorted retainers and clips that hold the harness in position and unbolt the starter wiring and alternator. The knock sensors, crank sensors, and other hidden sensors may be difficult to reach and easily overlooked. So be thorough and work methodically.

Jack Up the Vehicle

Lower the vehicle onto the stands. Remove the wire connections underneath the engine. Separate harnesses such as the engine fan harness and lights harness will be intertwined with the engine harness. Separate and isolate these as it may not be necessary to remove them.

Label New Connections

Use masking tape to label new harness connections based on the labels, photographs, and documentation of the removed harness. This will save you time and planning. Sort any tangled wires, understand their installation and routing before you can proceed. Once you are done, you can then install the new cables.

Install New Wiring

Install new wiring, connect plugs, then tuck back the wires into their respective conduit or secure them to the engine or chassis. Note that wiring harnesses can be deceptive in terms of split points and wire lengths.

If you finalize wire routing before you connect anything, you may discover that you have too little or too much wire for making connections or snaking around obstacles. Make sure the cables are routed away from exhaust pipes and moving parts. Lower the vehicle, reinstall the battery, start the car and then inspect the installation.


With the high-performance demands of the original equipment manufacturing (OEM) marketplace, there is a relentless need to reach a consistent wire harness performance. 

Choosing a product that aligns with your engine wire harness requirements will facilitate the correct relay of electrical and information signals as needed. For help with your cable assemblies, get in touch with us.

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