To troubleshoot the circuit faults of the Fuel Pump, it is necessary to start from the voltage test. Use a digital multimeter to measure the voltage at the power supply terminal of the pump body. The normal value should be 12-14V (ignition ON position). Ford F-150 maintenance case shows that under the condition of voltage below 10V, the relay contact resistance should be checked – if the test result is more than 0.5Ω, it is determined as failed (normal value ≤0.1Ω). In this instance, the voltage recovery rate can be restored by 92% with the replacement of the relay. Volkswagen MQB platform car’s common wiring harness corrosion results in voltage drop. During measurement, if the potential difference between the ignition switch and the pump body exceeds 1.5V, attention must be paid to check the wiring harness connector under the rear seat (TSB 2014479 specifies that the failure rate at this location accounts for 63%).
Current tests show hidden faults. The Fuel Pump working current is typically 4 to 8A (model-related). When the current fluctuates by more than ±0.3A or exceeds the nominal value by 15% upon measurement using a clamp meter, this can indicate that the carbon brushes in the motor are worn. In the case of the Toyota Camry 2AR-FE engine, the current suddenly rose from 5.2A to 7.8A. Tear-down revealed that the impeller was clogged, which caused the overloading of the motor. After replacing the pump body, the current returned to the normal range. As per Bosch’s technical bulletin, if the current is below 3A, then 80% of the chance of having an open circuit in the line or short circuit in the armature winding.
To measure the line resistance to identify poor contact. Measure the overall value of the grounding loop resistance by using a micro-ohmmeter. The requirement is less than 0.2Ω. The owner of the Chrysler 300C discovered that the resistance between the grounding terminal and the negative battery terminal was 0.8Ω. It went down to 0.1Ω after cleaning the grounding point on the vehicle body, and the operating noise of the pump body was reduced by 12dB(A). For power wires, measure in sections: Resistance between the relay output terminal and the pump body plug should be less than 0.3Ω. When a section measures 1.2Ω (such as Chevrolet Silverado’s fuel tank wiring harness), the oxidized terminal or wiring harness should be replaced.
Insulation test to prevent short circuit hazard. Use a 500V megohmmeter to measure the insulation resistance between the Fuel Pump power cord and the vehicle body. The value of the specification is > 20MΩ. In the BMW N55 engine, the fuel vapor caused the insulation layer of the wiring harness to age, and the insulation resistance dropped to 2.3MΩ, causing an intermittent fuse. The insulation value was restored to 50MΩ after it was repaired, and the elimination rate of fault code P0087 was 100%. In humid conditions (humidity > 80%), this test needs to be done three times to take the intermediate value in order to avoid incorrect judgment.
Signal diagnosis is under the control of the intelligent control system. The new Fuel Pump module sends ECU signals via PWM signals (frequency 100-500Hz, duty cycle 10-90%). When measuring the signal waveform with an oscilloscope, if the deviation of the duty cycle is more than 5% (e.g., the nominal 50% but actual 45%), then abnormal oil pressure regulation can occur. The case of Mercedes-Benz M276 engine shows that CAN bus interference causes the signal noise to be greater than 200mV (allowable value < 50mV), and after the installation of the shielded wiring harness, the fluctuation rate of fuel pressure is reduced from ±1.2Bar to ±0.3Bar.
Thermal imaging technology assists in locating hidden hotspots. Poor contact points can be found using the FLIR E8 infrared camera – the typical temperature difference between the joint and the surroundings is less than 5℃. When up to 15℃ (for example, oxidation of Ford Explorer wiring harness joint), the loss power due to resistance is up to 7W (normal < 1W). SAE research proves that the method increases the efficiency of line fault location by 60% and reduces the rate of misreplacement parts by 45%. When voltage drop testing is combined with thermal imaging, it is possible to locate 98% of line contact faults accurately.