Fuel Pump Voltage Problems Testing The first thing to keep in mind about the fuel pump is that just like any other electric motor, the electric supply to the fuel pump is critical in ensuring that the fuel pump operates normally. Most fuel pumps in an automobile runs at 12 volts which is provided by the car battery as controlled by the fuel pump relay and fuse. Over the years, a study by the Automotive Electrical Systems Institute (AESI) found that nearly 20% of all fuel pump returns were related to insufficient voltage or electrical problems. You are testing voltage, and the first thing is checking the fuel pump relay, which sends power to the pump. A broken relay or wiring will cause intermittent voltage jumping into the fuel pump. The cost to replace a fuel pump relay typically ranges from $75 to $150, depending on the make and model of the car.
Usually, a multimeter is used to measure the voltage present on the plug of the fuel pump. To conduct the test, a technician attaches the positive lead of the multimeter to the power terminal of the fuel pump and the negative lead to a good ground. While the engine is, on but not running, the multimeter should show a voltage of around 12 volts. A voltage in the low-end was suspiciously low from twelve-volts, indicating a problem in the electrical system, possibly a failing system relay or wiring problem. A report by AutoTech Solutions in 2022 revealed that they found wiring faults caused 30% of diagnosed voltage problems on fuel pumps, particularly in older vehicles where installation may have suffered from degradation over the years.
If the voltage is correct, the next step is to check the current draw of the fuel pump. But if the fuel pump is struggling with wear or contamination, it’s going to draw more current, and when the current draw rises, anything can happen. On average, a fuel pump pulls between 4 and 8 amps, depending on the make and model of the vehicle. If the amps are higher that this range, the pump is likely blocked or defective and will require replacement. A NASTF study done in 2021 found that 15% of fuel pump failures were caused by excessive current draw, leading to overheating and pump failure.
Testing for voltage problems is also checking the ground connection. Insufficient power delivery can come from fluctuations of the fuel pump due to poor grounding. This can be verified by measuring the voltage across the fuel pump negative terminal and vehicle chassis. Excessive voltage difference (≥0.5V) can suggest a poor ground connection that needs to be cleaned or repaired. Sam Peterson of CarTech Solutions, an automotive performance specialist, said, “People often overlook the ground, and the ground is as much as a key, if not more than the positive side of the circuit when it comes down to proper functionality of the fuel pump.
Now that voltage and current are confirmed, and you’ve ruled out relay, wiring, or ground issues, it’s time to check the fuel pump itself. If the voltage supply is both stable and correct, and the pump is still not running, this means there is likely an issue with one of the internal components of the pump. In this case, the pump would probably have to be replaced. The average cost will be anywhere from $300 to $1,000 or even more, all depending on what make/model car they are replacing, and how difficult the replacement job is. When fuel pumps are located inside the gas tank, labor costs are usually higher because the tank must be removed.
To confirm adequate fuel pump pressure, mechanics use specialised diagnostic equipment, including a fuel pressure tester. If pressure is low, either the fuel pump is faulty, or the fuel filter is building enough debris to clog the intake port. Fuel pressure tests in such scenarios help determine the actual issue. In fact, a proper fuel pressure test, which typically runs about $100 to $250 for parts and labor, can tell you with certainty if the fuel pump is failing.
For more information on how your fuel pump works — and how to keep it in tip-top shape, or replace it if need be — go to Fuel Pump.