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How to Test a Fuel Pump Relay with a Multimeter or Test Light

  • Read the Codes with a Scan Tool

    With an advanced scan tool, you can pick your vehicle and get a report to find codes. In this example we found two fault codes for a fuel pump relay control circuit and fuel trim cylinder balance.

    Our car would crank and sputter but wouldn’t start, and reading for these codes indicates there could be a problem with the fuel pump relay.

  • Search Online for Information About This Code, Like a Technical Service Bulletin (TSB)

    Go on the computer and look for any service information regarding these codes, like a technical service bulletin. This can give you information from the manufacturer about problems specific to your vehicle.

    We found a TSB for the Chevy Cobalt in this example that said to check if there are aftermarket devices in the vehicle that could cause the code, like an aftermarket alarm or remote start that could shut off the fuel pump and cause the cranking and sputtering but no starting. While this applies to the code being stored in the PCM as history, our code is current, so it doesn’t apply.

    The description of the code might give you more information, like how to test the circuit. This TSB recommended using a test light, but in this example we use a test light and multimeter to demonstrate how to check the fuel pump relay.

  • Find a Wiring Diagram of the Circuit

    A wiring diagram of the circuit can help you. You can find some of these from public sources like a library or a repair manual. You’ll test the ground side and power with a multimeter, test light, an assistant, and/or an advanced scan tool.

  • Find a Relay Tester, Multimeter, and/or Test Light

    A relay tester will prevent probes from spreading terminals. You’ll be connecting a multimeter and/or test light to the relay tester to test the circuit.

  • Look Under the Hood and in the Fuse Box for Obvious Problems

    Look under the hood for obvious problems or damage. Remove the fuse box cover and look for any signs of damage, like from a rodent making a nest, as that could be the cause. Then find the fuel pump relay.

  • With the Vehicle Off, Remove the Fuel Pump Relay

    Remove the relay with the vehicle turned off. The different terminal numbers are located on the bottom of the relay, and there’s a key on the side of the relay. We’re looking for 85 or 86, and confirm the numbers match the relay tester.

  • Install the Relay Tester to the Fuel Pump Relay Location

    With the fuel pump relay removed, install the fuel pump relay tester.

  • With the Multimeter Prepared, Test the Circuit

    Take the multimeter and attach it to a ground. Put the voltage on for 12V to test the ground, and then go back to resistance. There should be less than 5 ohms for this test.

    Attach the probe to the tester and check the reading on the multimeter. We had less than 5 ohms for the 86 terminal on the fuse, so the ground is good.

    More on how to use a multimeter

  • With the Test Light Prepared, Test the Circuit

    Multimeters can sometimes give a false reading, and the TSB instructed testing this circuit with a test light.

    Attach the end of the test light to ground and test it to confirm it’s working. Press the test light to the fuse tester and see if it lights up, which indicates the ground wire is good.

    Try the next test, which is the wire commanded by the computer found on the 85 terminal. This sends power through the relay to actuate the relay and then ground.

    There are a few ways you can test this terminal.

    1. With the test light pressed to the relay tester, have an assistant turn the key to accessory mode and see if the light illuminates
    2. Attach the ground wire from the multimeter to the terminal, connect the test light to the wire, and turn the key to accessory mode yourself
    3. With the test light pressed to the relay tester, turn on the fuel pump with a scan tool that’s capable

    If the test light lights up, the circuit is good. This means there is a problem with the relay and not the circuit.

  • Swap the Fuel Pump Relay with a Relay that Matches

    You could also try swapping the relay with another relay that has a matching number as the one on the fuel pump relay. In our example, we swapped the AC relay with the fuel pump relay.

    If the vehicle starts and runs, this indicates you need a new fuel pump relay.

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