The feasibility of wire harness multiplexing primarily depends on the matching degree of current parameters. If the peak working current of the new Fuel Pump exceeds the original design value by 30%, the original wire diameter will not be able to withstand it. For instance, the original vehicle used 18AWG wires (with a resistance of 0.021Ω/m and a current carrying limit of 10A), but when the starting current of the 340LPH pump was upgraded to 14A, the temperature rise rate of the wires reached 15℃ per minute, exceeding the heat resistance limit of 105℃ for the insulation layer. The SAE J1128 standard clearly stipulates that if the current exceeds the initial value by 20%, the corresponding wire diameter must be replaced; otherwise, the probability of short circuit risk increases by 4.2 times. The Bosch recall incident in 2019 confirmed that the failure to upgrade the wiring harness along with the high-pressure oil pump led to the risk of wire melting in 57,000 vehicles.
The aging problem of connectors directly affects the conductivity efficiency. After the vehicle has traveled 100,000 kilometers, the Fuel Pump plug-in terminal generates a contact resistance of 0.5-1.2Ω due to fretting wear (new product standard < 0.1Ω). Calculated under the 12V/8A working condition, the additional heat loss amounts to 4-10W (Joule’s Law Q=I²R). Thermal imaging detection shows that the local temperature of such deteriorated interfaces reaches 98℃ (ambient temperature 25℃), accelerating the embrittlement of the insulation layer. Industry statistics indicate that after reusing old wiring harnesses over 8 years old, the incidence of poor contact faults reaches 31%, which is equivalent to one rework for every three repairs on average.
The degradation of insulation performance poses a potential risk. The engine compartment wiring harness is exposed to a high-temperature environment of 110℃ for a long time, and the tensile strength of the PVC insulating material decays at an average annual rate of 7%. When the crack length exceeds 2mm or the insulation resistance value drops below 50MΩ (new product standard > 500MΩ), it may induce a leakage current > 5mA in an environment with 85% humidity. Ford Laboratory simulations have confirmed that such wiring harnesses may cause arc discharge in a 48V mild hybrid system, with energy release reaching up to 15mJ (sufficient to ignite gasoline vapor). Authoritative advice: Any wiring harness with hardened insulation or visible cracks must be forcibly replaced.
Electromagnetic compatibility risks are often overlooked. The PWM control frequency of the new fuel pump has generally been upgraded to 500Hz (the old model was 200Hz). When using the old wiring harness, the signal distortion rate reached 15% due to the increase in distributed capacitance (about 120pF/m). Oscilloscope observations show that such interference causes the duty cycle control error to exceed ±8%, resulting in an oil pressure fluctuation of ±0.3Bar. The BMW technical notice particularly emphasizes that after 2018, when upgrading the electronic Fuel Pump of the model, the shielded wiring harness must be replaced simultaneously (the shielding coverage rate must reach 95%), otherwise the CAN bus bit error rate will rise to 10⁻⁴ (the upper limit allowed is 10⁻⁶).
Cost-benefit analysis reveals potential losses. On the surface, reusing the old wiring harness may save about 120 yuan in new parts costs, but the actual risks may lead to: ① Repair costs for 650 ECU burnout (with a 12% probability of overvoltage impact); ②280 towing and rescue fees (231,500 cost for handling the aftermath of a wire harness fire). Actuarial data of insurance shows that the claim amount for faults in the electrical system of vehicles using aged wiring harnesses is 2.3 times that of standardized maintenance. The only safe multiplexing condition must be met simultaneously: ① The peak current of the new pump is less than 10% of the original design value; ② The service life of the wiring harness shall be no more than 5 years; ③ The insulation resistance value measured by a 500V megohmmeter is greater than 100MΩ. ④ The thickness of the gold plating layer on the plug-in terminal should be ≥0.8μm (ensuring that the contact resistance is < 0.1Ω).