Katerina Swanson
A fuel pump's current draw is influenced by fuel pressure, with higher fuel pressure resulting in increased injector flow and power requirements. Various factors, such as clogged fuel filters, fuel line restrictions, and mechanical resistance, can lead to higher-than-normal current draw. Voltage drops, caused by resistance in the circuit, can also impact the fuel pump's performance. While some voltage drops are normal and necessary for controlling fuel pump speed, excessive voltage drops may indicate a malfunction. Proper diagnostics using voltage drop tests, ammeters, and inductive amp probes are crucial for identifying issues with the fuel pump's current draw and ensuring its efficient operation.
Characteristics and Values of a Fuel Pump Drawing Too Much Current
| Characteristics | Values |
|---|---|
| Fuel pump current | Can be measured by inserting an ammeter in series with the fuel pump load circuit at the fuse block |
| Fuel pump current (alternative method) | Use an amp clamp on the positive wire of the fuel pump |
| Fuel pump current draw increase | 1 amp for every 10 pounds of fuel pressure |
| Amperage level above specification indicates | Malfunction, if pressure is low |
| Amperage level below specification indicates | Malfunction, if pressure is high |
| Voltage drop test | Can find excessive resistance in high-amperage fuel pump circuits |
| Voltage drop | Occurs when a fuel pump is given multiple speeds with a module connected to a computer with a dedicated signal wire |
| Voltage drop (alternative method) | Occurs with a serial data bus, like on vehicles with a Controller Area Network (CAN) bus-controlled fuel pump module |
| Voltage drop test | Should be conducted with the engine off to avoid affecting the available voltage to the pump |
| Fuel pump current draw increase (high-pressure pumps) | 35 to 45 psi |
| Fuel pump current draw increase (low-pressure pumps) | 9 to 13 psi |
| Fuel pump waveform | Should be uniform; jagged or low-amplitude humps indicate poor contact between brushes and commutator segments |
| Fuel pump current draw increase | Caused by mechanical resistance inside the pump, clogged fuel filter, or fuel line restriction |
| Fuel pump current drop | Caused by electrical resistance in the pump or its circuit, reducing available pump voltage |
| Fuel pressure increase | Results in an increase in injector flow and current draw |
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What You'll Learn
Voltage drops and current draw
Fuel pumps and voltage drops are usually not a good combination, but good voltage drops can occur with fuel pumps that are given multiple speeds with a module that is connected to a computer with a dedicated signal wire that tells the module to go to a lower or higher speed. Good voltage drops also come from a serial data bus, like on vehicles with a Controller Area Network (CAN) bus-controlled fuel pump module.
The speed of a fuel pump can be controlled by reducing the voltage or by turning the fuel pump off and on with a pulse width-modulated output. These types of voltage drops are fine because we want the fuel pump to run at different speeds based on the engine's demand for fuel. However, if it's a system that's supposed to have full charging voltage, such as a 12.5-volt or 14.5-volt system, and the engine is running the fuel pump to make the right amount of fuel to go into the engine with a particular pulse width on the injectors, then it needs to have the correct voltage supplied to it.
Fuel pressure and volume are influenced by voltage, current, and the demand controlled by the regulator. If a fuel pump is not receiving the correct voltage, it cannot spin fast enough. Voltage drop testing can isolate problems that can still remain even if the pump is replaced. Voltage drop testing compares the battery or charging voltage to the voltage at the component. The voltage drop occurs because of resistance in the circuit that supplies the pump. The resistance could be in the connectors, grounds, or harness. A resistor or insulator prevents the flow of electricity in a circuit. Resistance can be caused by corrosion, fretting, and even air.
A voltage drop test is the only effective way to find excessive resistance in high-amperage fuel pump circuits. With the right leads and back-probes, it is possible to perform the test without any disassembly. The results are immediate as to whether you have a good connection or a bad one. Since the fuel pump circuit must be energised, you will need to back-probe the connectors or pierce the wires. Most fuel pump wiring harnesses reside in a harsh environment.
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Testing for excessive resistance
A voltage drop test is the only effective way to find excessive resistance in high-amperage fuel pump circuits. Resistance can be caused by corrosion, fretting, or even air. A voltage drop test can be performed by inserting an ammeter in series with the fuel pump load circuit at the fuse block and measuring the current while the fuel pump is running. Alternatively, an amp clamp can be used on the positive wire of the fuel pump. This method works for vehicles that use a pulse-width modulated voltage supply to the pump.
It is important to note that the fuel pump current draw increases by 1 amp for every 10 pounds of fuel pressure. Therefore, an amperage level above or below the specification indicates a malfunction, especially if the pressure is low and the amperage is above specification. This could be due to a restricted regulator or a fuel return line.
Additionally, when replacing a fuel pump, it is crucial to service the connectors. This involves cleaning the connector with an electrical contact cleaner, removing any seals, and applying a light film of dielectric grease. By performing a thorough voltage drop test and maintaining the fuel pump connectors, you can effectively identify and address excessive resistance in the fuel pump circuit.
Furthermore, on some vehicles, it may be necessary to follow specific test procedures provided by the vehicle manufacturer. These procedures may include loading the engine or activating the pump with a scan tool. By referring to the vehicle manufacturer's recommendations, you can ensure an accurate and safe testing process.
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Fuel pump malfunctions
Fuel pumps are responsible for delivering fuel from the tank to the engine at the right pressure, ensuring a smooth and efficient drive. They are designed to be durable and long-lasting, but they are not immune to wear and tear. A fuel pump malfunction can cause the engine to sputter or stall, especially at high speeds, posing safety risks.
Some common symptoms of a malfunctioning fuel pump include a whining noise coming from the fuel tank area, stalling that restarts after several minutes, trouble starting the car, lack of power, and long cranking time. A weakened fuel pump can cause the vehicle to take more cranks to start than normal, and if the issue is left unaddressed, the vehicle may not start at all.
Several factors can lead to fuel pump malfunction or failure. One of the most common issues is contaminated fuel. Dirt, debris, or water in the fuel can damage the pump over time by increasing friction and causing premature wear of the pump components. Additionally, electrical issues such as faulty wiring, corroded connections, or blown fuses can disrupt the power supply to the pump, causing it to malfunction or stop working.
Another factor that can contribute to fuel pump malfunction is resistance within the fuel pump motor. Resistance can be caused by corrosion, fretting, or even air. This can lead to a voltage drop, resulting in insufficient fuel supply to the engine, especially during acceleration when more volume is required.
To diagnose a fuel pump issue, a voltage drop test can be performed to identify excessive resistance in high-amperage fuel pump circuits. Additionally, the fuel pump current can be measured by inserting an ammeter in series with the fuel pump load circuit or using an amp clamp on the positive wire of the fuel pump. An amperage level above or below the specification indicates a malfunction, especially if the pressure is low and the amperage is high.
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Fuel pump circuit issues
A fuel pump is an essential component of a vehicle, transferring fuel from the tank to the engine at the appropriate pressure. A failing fuel pump can cause various issues, including engine misfires, difficulty starting, stalling or surging, decreased fuel efficiency, and loss of power.
To maintain the fuel pump's health, regular inspections and maintenance are necessary. Replacing the fuel filter as needed and ensuring clean, uncontaminated fuel can prolong the life of the fuel pump and prevent costly repairs. Additionally, servicing the connectors during fuel pump replacement is essential to protect the pump from chemical attacks that can cause increased resistance and voltage drops.
If issues persist after troubleshooting, seeking professional help from a mechanic is recommended. Fuel pump issues can lead to broader engine damage, so early intervention is crucial to prevent more expensive repairs.
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Measuring fuel pump current
Fuel pump current can be measured by inserting an ammeter in series with the fuel pump load circuit at the fuse block and measuring the current while the fuel pump is running. An alternative method is to use an amp clamp on the positive wire of the fuel pump. This method is more suitable for vehicles that use a pulse-width modulated voltage supply to the pump.
It is important to note that the engine and other electrical components must not be running while the fuel pump is being tested, as they may affect the available voltage to the pump, which will impact amperage readings.
When performing a fuel pump current test, it is crucial to ensure that there are no additional components, such as a transfer pump, wired in parallel to the main pump, as these can influence the test results.
For accurate measurements, the fuel pump should be powered on using the same wiring used during normal operation. A properly functioning pump will exhibit a consistent waveform pattern. Deviations from this pattern, such as jagged or low-amplitude humps, may indicate issues with the fuel pump or its circuit.
Additionally, a voltage drop test can be performed to identify excessive resistance in high-amperage fuel pump circuits. This test involves measuring the voltage drop between the battery or charging voltage and the voltage at the component. A larger voltage drop indicates a potential problem, often due to damaged or corroded vehicle wiring or harness connectors.
By combining voltage drop testing with current testing, you can effectively diagnose fuel pump issues and address resistance concerns to ensure comprehensive repairs.
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Frequently asked questions
An amperage level above the specification indicates a malfunction. You can measure the fuel pump current by inserting an ammeter in series with the fuel pump load circuit at the fuse block and measuring the current while the pump is running.
A voltage drop in the bypassed circuit could cause low current at the pump during normal vehicle operation. This could be caused by a corroded ground, a connector that is no longer sealed, or a weak connection due to poor retention.
Clean the connector with an electrical contact cleaner. Remove any seal, clean it, and apply a light film of dielectric grease. It is highly recommended that you replace the connector and weatherproof the connection.
If the fuel pump is drawing too much current, the engine may be starved for fuel when more volume is required, such as when the vehicle is under acceleration.
