How to Test Fuel Pump with Multimeter (6-Step Guide)

The fuel pump is a critical component that is essential for a car’s entire operation. When the fuel pump fails, it implies the engine’s fuel supply is insufficient, or there is none. This will be true if the engine fails to start or stall. To help you diagnose this issue with a multimeter we’ve outlined some steps below.

In general, there are 3 main steps to test a fuel pump with a multimeter:

Look for open or closed loops. Then start the engine.
Separate the positive and negative circuits. Now, connect the multimeter.
Locate your pump relay and take out the fuel pump.

If you want to go into depth into how these steps work, I highly suggest you read through to the end of this guide. Here, I will talk about how to test the fuel pump relay with a multimeter in detail.

How to Test Fuel Pump Relay with Multimeter

Step 1: Look for Open or Closed Loops

Connect your digital multimeter to its positive and negative stands on the test fuel pump to determine whether closed or open loops exist between the fuel pump and the relay. This should be a closed-loop circuit with no voltage loss between the terminals.
The reading should be the same when the multimeter is not attached to the gasoline pump. If the result is negative, you can proceed to the ignition switch.

Step 2: Start the Engine

Turn the ignition on after replacing the fuel pump probes without starting the vehicle. The test fuel pump system should emit a quiet sound to signal that it is ready to provide gasoline to the engine. (1)
If the sound is inaudible, the switch may be defective, or there may be an open circuit between the battery and the switch. If the sound can be heard, the pump is not broken.
A voltage drop test is required to determine if the power supply is efficient or not.

Step 3: Separate the Positive and Negative Circuits

mechanic in black gloves holding red and black multimeter probe

To prevent incorrect readings on the multimeter when connected to the gasoline pump, you’ll need to divide the positive and negative circuits.

Place the ground on one side and the live phase on the other to separate the connections.
Connect the digital multimeter to the ground circuit. One socket should be directed at the gasoline pump, while the other should be directed toward the battery.
On the negative circuit, the multimeter should show a reading of less than 0.1. If the value is larger than 0.1, the circuit’s resistance is high and might be the high resistance source. Only a bad connection on the harness or poor wiring can cause this.
If there are no instances of excessive resistance, go to the positive phase to evaluate the power decrease.

Step 4: Connect the Multimeter

Connect one of the multimeter’s sides to the battery and the other side to the test fuel pump connector.
Make sure you’re not testing the wrong circuit. The value on the multimeter’s screen should be 0.1.
The resistance in the circuit is likely significant and might be the cause of the voltage decrease if it is other than 0.1 unless the battery is depleted or malfunctioning.
Poor wiring or a broken connection might be the root of the problem. Continue to test the relay that supplies the pump if the findings are positive.

Step 5: Locate your Pump Relay

The pump relay is located in the switch box, in a dashboard compartment, or next to the battery.

To connect electricity from the battery, plug in the relay, turn on the ignition, but don’t start the engine.
Check the voltage at the relay’s connection using the multimeter. The battery’s voltage will be most likely 12 volts.
Replace the relay after turning on the engine.
Check the voltage on the connector adjacent to the test fuel pump by turning on the ignition again without running the engine.
The voltage should be somewhat below 12 volts but not below 5 volts.

Step 6: Take Out the Fuel Pump

You’ll need to disconnect the gasoline pump from the fuel tank at this point. You need to remove the gasoline tank. This is the preferred method.
If you can, you may still place the batteries near the gasoline pump. In this situation, exercise greater caution to avoid igniting.
Before the socket, connect the batteries directly to the probes.

The voltage on the multimeter should be the same as the voltage on the battery after the socket links the multimeter to the test fuel pump. If the voltage on the multimeter is lower, the issue might be that the probes are attached loosely to the socket. We highly recommend replacing the socket rather than connecting the electricity directly to the pump for your safety.

FAQs

What are the causes of bad fuel pumps?

Check the fuel pump’s cooling and lubricating if the inadequate flow hinders it. Running the gasoline tank low is a common cause of fuel pump failure. This is especially important in late-model automobiles that don’t have a fuel pressure return mechanism. In such a vehicle, running out of gas might severely harm the test fuel pump.

What happens when the engine’s fuel pressure is too high?

Over-fueling of the engine might occur if the pressure is too high. The engine may run roughly, have poor fuel mileage, and emit black smoke from the exhaust. If your regulator fails, your vehicle may exhibit a variety of symptoms.

Will a bad battery affect a fuel pump?

Yes, it can. If the battery is low, the fuel pump relay will slow down. This will cause low fuel pressure and a low fuel condition. Low battery levels can prevent one or more injectors from opening, resulting in a lean misfire and/or difficult starting. (2)

Take a look at some of our related articles below.

References
(1) gasoline – https://www.eia.gov/energyexplained/gasoline/
(2) misfire – https://www.jdpower.com/cars/shopping-guides/what-happens-when-a-car-misfires

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