When diagnosing the relay fault of the Fuel Pump, it is necessary to measure the voltage at its control terminal first. The normal value should be 90%-95% of the battery voltage (for example, 11.4-11.8V in a 12V system). According to the SAE J1455 standard, the resistance of the relay coil should be within the range of 50-120Ω. If the measured resistance value exceeds 200Ω (such as reaching 235Ω in a certain case of an American pickup truck), it indicates that the coil aging has led to a 63% decrease in the magnetic attraction force and is unable to effectively close the contacts. According to the 2022 data from the North American Automobile Maintenance Association, 23% of fuel pump power supply failures result from the oxidation of relay contacts. When the contact resistance rises from the standard value < 0.5Ω to > 2Ω, the voltage drop will increase from 0.1V to 0.8V, causing the input power of the fuel pump motor to decrease by 18% (for example, from 35W to 28.7W).
Using an infrared thermal imager to detect the temperature of the relay housing can lead to abnormalities. During normal operation, the contact temperature should be ≤55°C (at an ambient temperature of 25°C). If a local hot spot is detected to exceed 85°C (for example, a certain German model is measured to reach 92°C), it indicates that the contact area of the contacts has decreased by more than 40%. Laboratory tests show that when the contact pressure drops from the designed value of 4.5N to 2.8N, the contact resistance increases exponentially, and for every 0.5N decrease, the resistance increases by 50%. Referring to Honda’s technical announcement in 2021, due to the coating thickness of the relay contacts not meeting the standard (reduced from the standard 8μm to 3μm), the fluctuation rate of the working current of the Fuel Pump expanded from ±5% to ±15%, causing the ECU to misjudge the air-fuel ratio deviation to reach 12%.
The PID data stream of the fuel pump control module is read through OBD-II. The normal fuel pressure should fluctuate between 270 and 310kPa. If the relay failure causes abnormal duty cycle control, the pressure fluctuation amplitude may exceed ±50kPa (such as a sudden drop from 280kPa to 180kPa). Statistics show that the adhesion of relay contacts can cause the fuel pump to remain powered on continuously, causing the motor temperature rise rate to soar from the normal value of 0.5°C/min to 2°C/min. After exceeding the temperature limit of insulating materials (usually 130°C), the probability of motor winding short circuit increases by 80%. A third-party laboratory’s disassembly and analysis of 500 faulty relays found that 67% of them had sulfidation of silver alloy contacts. When the thickness of the sulfide layer was greater than 10μm, the conduction capacity dropped to 32% of the initial value.
Industry practice shows that when using alternative relays, the load capacity needs to be verified. The rated current of the original fuel pump relay contacts is usually 30A (peak 50A), while inferior parts may only be able to carry a continuous current of 20A. According to the actual measurement data of a certain auto forum user in 2023, after replacing the non-standard relay, the working current waveform of the fuel pump showed a 12% harmonic distortion, causing the brush spark frequency to increase from the normal 400 times per minute to 1200 times per minute, and accelerating the wear rate of the motor commutator by 300%. The recall incident of Toyota in 2020 revealed that the insulation material of a specific batch of relays failed to meet the arc resistance standards. After 10^4 on-off cycles (the design life should be 10^5 times), the insulation resistance dropped from 10^8Ω to 10^5Ω, increasing the risk of power supply interruption to the fuel pump by 15 times.
Suggestions for optimizing the diagnostic process include: using an oscilloscope to capture the rise time of the relay control signal. The normal value should be less than 5ms (for a 12V system). If the delay is greater than 10ms, it will cause a fuel pump start lag of 0.3-0.5 seconds, resulting in a ±8% deviation in the mixed gas concentration during cold start. According to the ISO 16750-2 standard, the fuel pump relay is required to operate continuously for 500 hours at an ambient temperature of 85°C without performance degradation. However, a market spot check found that 15% of the non-manufactured parts showed a contact displacement of more than 0.2mm after a 200-hour test, resulting in a 45% decrease in contact pressure. Maintenance data shows that standardized relay diagnosis can reduce the time for troubleshooting fuel system faults from an average of 2.5 hours to 0.8 hours, lowering the cost of misreplacement parts by 60% (saving 80-120 per maintenance).