When the Fuel supply pressure of the Fuel Pump fluctuates by more than ±0.3bar, it will cause the standard deviation of the engine torque output to increase by 42%, directly affecting the shifting logic of the automatic transmission TCU. The test data of ZF Friedrichshafen in 2023 shows that when the Fuel Pump pressure drops from 4.0bar to 3.2bar, the 2-3 gear shift time of the 8HP75 transmission extends by 83ms, and the peak shift impact force increases from 12.6G to 18.3G. The user report of the BMW B58TU engine indicates that after replacing the aging fuel pump with a 15% flow attenuation, the shift speed error of the ZF 8AT in Sport mode has expanded from ±50rpm to ±180rpm.
Abnormal voltage of the fuel pump will interfere with the TCU signal through the CAN bus. Experiments show that when the control line voltage of the Fuel Pump drops to 10.8V, the communication bit error rate between the ECU and the TCU surges from 0.01% to 1.2%, causing delays or errors in shift instructions. In the 2021 Ford Mustang GT recall incident, due to poor contact of the fuel pump relay, the transmission had a 27% probability of gear skipping when shifting at 5500rpm. The repair plan included upgrading the fuel pump wiring harness (cost $240) and refreshing the TCU software to restore the gear shifting accuracy to 98.7%.
In turbocharged models, insufficient flow of the Fuel Pump can cause abnormal fluctuations in the air-fuel ratio. When the fuel supply is 12% lower than the demand value, the torque drop of the engine at the moment of gear shifting can reach 35%, triggering the protective locking of the transmission. The technical manual of the Porsche PDK transmission indicates that in the launch start-up condition, if the fuel pump pressure cannot be increased from 3.5bar to 6.2bar within 500ms, the engagement time of the dual clutches will be forced to be extended by 120ms, resulting in an increase of 0.4 seconds in the 0-100km/h acceleration time. The actual measurement data of the Audi RS6 owner shows that after upgrading the 850L/h high-pressure fuel pump, the torque fluctuation range at the wheel end during gear shifting narrowed from ±210N·m to ±45N·m.
When the pulsation frequency of the fuel pump resonates with the valve body of the transmission, it will interfere with the hydraulic control system. When the 240Hz working frequency of the Fuel Pump coincides with the natural frequency of the transmission solenoid valve (typically 200-260Hz), the adjustment error of the main oil pressure can reach ±0.8bar. General Motors’ TECHLink report shows that in the 10-speed automatic transmission shift errors of the Cadillac CT5-V Blackwing in track mode, 38% of the cases were caused by the misjudgment of the valve body oil pressure sensor due to the high-frequency vibration of the fuel pump. The solutions include installing a fuel pulsation damper ($165) and modifying the PWM control frequency.
The energy management strategy of hybrid vehicle models will amplify the impact of the Fuel Pump. When the Toyota THS-IV system switches between EV-HEV modes, if the start-up delay of the fuel pump exceeds 50ms, the torque shock at the moment of engine ignition will subject the eCVT planetary gear set to an additional 17% stress. In the 2022 Prius owners’ class action lawsuit, it was revealed that the defective fuel pump caused the abnormal noise rate during the mode switching of the hybrid system to reach 63%. After replacing the improved electric drive fuel pump (DENSO 95020-47010), the gear impact noise was reduced by 22dB and the gear shifting smoothness index improved by 41%. The measured data shows that the new pump body reduces the fuel pressure establishment time from 380ms to 220ms, and improves the speed difference control accuracy when the engine is activated to ±3rpm.