Auto Transmission Wire Harness: How It Works and Failure Causes/Symptoms

The auto transmission wire harness is a critical component of a vehicle’s transmission system.

A vehicle’s transmission system sits between the engine and wheels to transmit the optimal torque and horsepower to suit the current driving conditions.

Modern vehicles with sophisticated electricals monitor the performance and condition of their gearbox using sensors, which transmit signals to the computer via a wire harness.

We’ll look at how this transmission wire harness works, plus its symptoms and causes of failure. Read on to learn more!

What Is an Auto Transmission Wire Harness?

The auto transmission wire harness is a cable bundle that links sensors installed in a vehicle’s transmission system to the TCM (Transmission Control Module). This wire distributes power and data to and from the TCM.

How an Auto-Transmission Wire Harness Works

Although the specific design of the transmission module harness varies depending on the vehicle, the functioning is almost similar.

This wire bundle transmits electrical signals between the components of the transmission system. These include actuators, sensors, TCM (TCU), and ECU.

An automatic transmission control unit

Sensors monitor the gearbox performance, providing input to the computer units or modules. These computers then control the performance of the transmission using actuators.

The harness links these parts and consists of transmission connectors, wires, relays, and fuses.

Symptoms of Auto-Transmission Wire Harness Failure

A damaged or malfunctioning auto transmission wire harness can cause a myriad of issues in your vehicle, the most typical being shifting problems. It can even cause transmission failure in the worst-case scenario. Others include the following.

Stalling
Misfires
Rough starts

Most of these issues will make the dashboard warning lights turn on.

The external automatic transmission wiring harness for an external control unit

If you detect any of these issues early on, you can confirm the problem first using an OBD scanner. Once verified, you can replace or repair the wire bundle as a DIY project, provided you have a proper repair kit.

The challenging part might be to access the cable inside the vehicle.

Causes of Auto-Transmission Wire Harness Failure

These transmission wiring-loom failure causes can lead to the symptoms described earlier.

Open Circuit

An open circuit is as simple as a wire cut, which creates a physical separation that disconnects the current or signal flow.

This situation can cut power supply or create an electrical shortage that can damage sensitive transmission components.

Causes like burning caused by arcing are easy to find along the harness. But if you can’t spot any visual indicators, you’ll have to use a multimeter or test light for continuity tests.

If all fails or multiple wires in the bundle have discontinuities, we recommend replacing the entire harness.

An engineer using a multimeter to test a fuse for continuity

Faulty Connectors

Corrosion can damage or loosen transmission connectors on the ends of the wire harness. You can visually inspect these hardware pieces for signs of decay, which can appear as scorch marks, tattered wires near the terminal, or melted plastic.

You can repair such damages if they are mild. Otherwise, you’ll have to replace the entire harness.

But to check for loose connections, wiggle the connector gently when seated on the port. It should be firm. If not, reseat the component and check if it becomes tight. You’ll have to repair this part if it remains loose.

A braided auto wiring harness with multiple plastic connectors

And like open circuit wires, you can test these connectors for continuity if the problem proves challenging to troubleshoot.

Ground Fault

Ground faults occur when a wire that runs at high potential contacts a ground or zero voltage wire. This issue usually occurs when the wire insulation comes off these wires, leaving the bare copper strands to touch and drain the current.

The fault can cause several symptoms that transcend the gearbox to other car systems. For instance, you can note dimming or flickering lights, radio static (via speakers), difficulty starting, and engine stalling or hesitating.

Short-circuiting can also cause similar problems but with reduced severity. But both issues run the risk of causing electric fires, which can cause expensive damage.

An electric fire caused by short-circuiting

Stretched Wires

High tension along the auto transmission wire harness can overextend the cables, leaving a thin conductive copper connection or a complete wire cut.

This issue usually occurs due to improper installation, which gives little to no allowance for the cable to stretch.

The best remedy for this situation is to replace the stretched section or the entire harness. And fit it well to avoid applying tension along the line.

Damaged Insulation

Worn-out insulation exposes the copper wire strands to corrosion, arcing, short-circuiting, and ground faults.

A wire with damaged insulation exposing the copper strands

But this damage does not happen on its own. Issues like pinching, arcing, and stretching can damage the protective cover. So, investigate or troubleshoot further before doing repairs.

Corrosion

The last culprit is corrosion, which can occur due to aging or exposure to moisture. You can diagnose the issue physically by looking for things like brown, green, or white residues on the wires or connectors.

A mechanic loosening the lock nut of the solenoid valve in an automatic transmission

If corrosion is deep in the harness, you’ll have to replace the individual wires or connectors or replace the harness.

Otherwise, you can clean the contamination using baking soda and water, similar to the cleaning done on battery posts when salts form on the surface.

Upgrading and Replacing Auto Transmission Wire Harnesses

The Need for an Upgrade or Replacement

You might have to replace or upgrade your auto transmission wiring harness for these reasons.

Electrical issues (short circuits, ground faults, and open circuits)
Damage/wear
Improper installation
Preventive maintenance
Modifications
Manufacturer recalls

Factors To Consider When Selecting Auto Transmission Wire Harnesses

Consider these seven factors when selecting a vehicle’s transmission cable harness.

Compatibility (fitting, length, vehicle’s make, model, and manufacturing year)
OEM or aftermarket
Transmission type
Quality (wire gauge, conductor, insulation, connectors, etc.)
Cost
Safety and regulatory compliance
Warranty

How To Properly Install a New Wire Harness

Step 1: Cut Off The Power

Use the wrench to loosen and disconnect the negative terminal from the battery.

Step 2: Locate and Remove the Old Harness

Find the existing transmission wiring harness in your vehicle via the wiring diagram. It is usually hidden or routed through several components. Remove it carefully while taking note of its routing and connector positions.

Step 3: Inspect and Route the New Harness

Check the new harness to ensure it matches the old one in color coding, number of connectors and their positioning. If there are differences, transfer the connectors, wires, or other components from the old to the new.

After that, route the new harness through the same path while avoiding hot areas and sharp edges.

Step 4: Connect the Harness Connectors

Link the wire to the components along the route. These include solenoids, switches, sensors, etc. Next, secure the harness using zip ties or cable clamps to avoid sagging or hanging.

Step 5: Reconnect the Power Supply

Fit the negative terminal back to the negative battery post and test the system. The vehicle should shift smoothly and function without throwing error codes.

Guidelines for Prolonging Its Lifespan

Use the following tips to prolong the lifespan of your transmission’s wiring harness.

Insulate exposed parts using heat shrink tubing or electrical tape
Regularly inspect the cable bundle and connectors
Route it through areas with minimal exposure to extreme conditions or the elements
Avoid modifications that overload the harness
Maintain the required fluid levels to prevent overheating that can burn the wires
Secure the harness using cable clamps, zip ties, or clips to prevent sagging

Wrap Up

Auto transmission wire harnesses are critical in the gearbox system, and failures can hamper smooth running or cause fires in the worst-case scenario.

So, try to diagnose either of the above causes if your vehicle exhibits any of the symptoms listed above.

That’s it for now. You can learn more about how the transmission works in this article, and comment your feedback below.