Precise measurement requires the use of a digital clamp meter with a resolution of 0.01A for series detection at the positive terminal of the Fuel Pump power supply circuit. Under a typical 12V system, the working current of a fuel pump in a regular car is between 3 and 8A (idle state), and it jumps to a peak of 9 to 15A during sudden acceleration. For example, the no-load current of the Bosch 044 pump is 4.2A under a voltage of 13.5V and an oil pressure of 3Bar, and it can reach 8.3A under load (with an increase of 98%). The SAE J1349 standard requires that the test environment temperature be controlled at 23±5℃. Because for every 10℃ increase in temperature, the resistance of the copper winding increases by 4%, resulting in a current fluctuation of ±0.3A.
The pressure condition significantly affects the current variation. The experimental data show that when the oil pressure rises from 2.5Bar to 4.0Bar, the working current of the same fuel pump increases from 5.1A to 7.8A (with a growth rate of 53%). When diagnosing, A mechanical oil pressure gauge should be connected for synchronous recording. For every 0.5Bar increase in pressure, the corresponding current increases by 0.6-1.2A (specifically depending on the power density of the pump). The 2023 Decco Laboratory report indicates that 85% of fault diagnosis errors stem from the failure to simulate actual oil pressure (such as only measuring no-load current), resulting in deviations as high as ±28%.
Dynamic characteristic analysis needs to pay attention to the startup shock. The inrush current at the moment the motor is powered on can reach 300% of the steady-state value (for example, the start-up peak of a rated 5A pump is 15A), and the duration is 80 to 200 milliseconds. If a common multimeter with a response speed greater than 500ms is used, this crucial data will be missed. It is recommended to capture the waveform with an oscilloscope with A sampling rate above 1kHz. In A certain case of Cadillac CTS, the starting peak was measured at 14.3A (maintained for 160ms and then dropped to the steady state of 5.2A). At this point, if the vehicle’s wiring ages (the wire resistance is greater than 0.2Ω), the voltage drop will expand to more than 3V, causing difficulties in starting.
The correlation of system load cannot be ignored. When the oil level drops to 10%, the cooling effect of the fuel weakens. The winding temperature rises from 70℃ to 110℃. The current decreases by 0.4A due to the increase in resistance, but the power consumption rises by 18% instead (P=I²R effect). When the clogging degree of the fuel filter reaches 70%, the pump body needs to increase its power by 32% to maintain the flow rate, and the current should be increased by 1.8-2.5A accordingly. Industry warning: Overheat protection will be triggered when the abnormal increase in current is ≥15% for 30 seconds (in accordance with ISO 19453 standard).
Fault diagnosis requires comparison with the reference value. The original factory maintenance manual usually indicates the rated current and allowable tolerance (for example, the standard value of the engine pump of Toyota Camry 2AR is 4.7A±0.3A). When replacing after sale, it needs to be verified that when the 340LPH high-flow pump is connected to the original vehicle system, if the current exceeds the original design by 150%, the 16AWG wire diameter must be upgraded (the original vehicle’s 18AWG wire diameter is 0.038Ω/m, with a load limit of 10A). The 2022 IATF Technical Bulletin shows that 23% of vehicle self-ignition accidents are caused by the overcurrent ignition of the fuel pump’s wiring harness (the temperature rise of the wire reaches 120℃/min when the peak is greater than 20A). Safety threshold formula: Fuse rating ≥ measured peak current ×1.5 times redundancy coefficient.