Why does the transmission wiring harness need to be replaced?

⚡ 1. Physical Damage and Aging
Wear and Tear: Vibration, high temperatures, and component friction in the engine compartment may cause the outer sheath of wiring harnesses to crack and internal metal wires to break.

Connector Corrosion/Loosening: Transmission fluid leaks, moisture, and salt can corrode and oxidize connector pins, leading to signal interruptions or distortions (e.g., triggering fault code P0727 for invalid engine speed signal).

Rodent Damage: Rodents may chew through wiring harness insulation.

🌡️ 2. High Temperature and Fluid Corrosion
Heat-Induced Brittleness: Harnesses adjacent to engines and transmissions endure prolonged exposure to 120°C+ environments, causing rubber sheathing to harden, crack, and lose protective function.

Fluid immersion: Transmission fluid (ATF) is corrosive. Prolonged exposure causes rubber expansion and dissolution, leading to:

Insulation failure: Triggers short circuits, burning out the TCM or solenoid valves.

Resistance changes: Causes signal transmission errors, resulting in shift shock or lock-up.

🔧 3. Manifestation of Electrical Fault Symptoms
When specific fault phenomena occur, wiring harnesses are typically the primary suspect:

Intermittent faults: Warning lights flash during vehicle vibrations and disappear when driving smoothly.

Multiple unrelated fault codes: Simultaneous reports of faults in unrelated components like solenoids and sensors (e.g., P0963 Pressure Control Solenoid A Circuit + P0717 Input Shaft Speed Sensor No Signal).

Abnormal data stream: Sensor values fluctuate, distort, or read as zero when read by a diagnostic tool.

Solenoid valve test failure: Normal resistance but no operation when energized, possibly indicating a power supply circuit issue.

🛡️ 4. Preventive Maintenance and Performance Upgrades
Age/Mileage: Vehicles exceeding 150,000 km (93,000 miles) or 8+ years old, even without current faults, have wiring harnesses entering the aging phase.

Modification requirements: Electrical system reinforcement for high-power engines or high-performance transmissions.