How to Test a Fuel Pump Relay with a Multimeter: A Step-by-Step Guide

Experiencing car trouble? A faulty fuel pump relay could be the culprit. The fuel pump relay is a small but crucial component in your vehicle’s electrical system. It acts as a switch, controlling the power supply to your fuel pump. When this relay fails, your fuel pump won’t receive the necessary electricity to operate, leading to starting problems, stalling, or even a complete inability to start your car. Fortunately, testing a fuel pump relay is a relatively straightforward process you can perform at home with a multimeter. This guide will walk you through each step, providing clear instructions and helpful tips to diagnose a potentially bad fuel pump relay.

Why Test a Fuel Pump Relay?

Before diving into the testing procedure, let’s understand why it’s essential to consider the fuel pump relay when troubleshooting fuel delivery issues. Here are some common symptoms that might indicate a failing relay:

* **Engine won’t start:** This is the most common symptom. If the relay isn’t providing power to the fuel pump, the engine won’t receive fuel and therefore won’t start.
* **Engine stalls:** A failing relay can intermittently cut power to the fuel pump, causing the engine to stall unexpectedly.
* **Engine hesitates or lacks power:** If the relay is providing insufficient power, the fuel pump might not be able to deliver enough fuel under load, leading to hesitation or a lack of power during acceleration.
* **Fuel pump doesn’t prime:** When you turn the ignition key to the “on” position (but before starting the engine), you should hear a brief whirring sound from the fuel pump as it primes. If you don’t hear this sound, the relay could be the problem.
* **Clicking sound from the relay:** A faulty relay might produce a rapid clicking sound, indicating that it’s repeatedly trying to engage but failing.

While these symptoms can also be caused by other issues (such as a bad fuel pump, clogged fuel filter, or wiring problems), testing the fuel pump relay is a simple and inexpensive way to rule it out as the source of the problem.

Tools and Materials You’ll Need

Before you begin, gather the necessary tools and materials:

* **Multimeter:** A digital multimeter (DMM) is recommended for its accuracy and ease of use. Make sure it can measure voltage, resistance (ohms), and continuity.
* **Wiring Diagram (optional but recommended):** A wiring diagram for your specific vehicle model can be invaluable in identifying the correct terminals on the relay and understanding the circuit.
* **Jumper Wires (with alligator clips):** These are used to provide power to the relay’s coil during testing. Ensure the gauge of the wire is appropriate for automotive applications (18-gauge or thicker is generally suitable).
* **Battery (12V):** You’ll need a 12-volt power source to activate the relay. A car battery is ideal, but a bench power supply set to 12V can also be used.
* **Safety Glasses:** Always wear safety glasses when working with electrical components.
* **Work Gloves (optional):** Gloves can provide a better grip and protect your hands.
* **Relay Puller (optional):** If the relay is difficult to remove, a relay puller can help prevent damage.

Understanding a Fuel Pump Relay

Before testing, it’s crucial to understand the basic components and function of a fuel pump relay. A typical automotive relay consists of:

* **Coil:** This is a wire winding that creates a magnetic field when electricity flows through it. This magnetic field is what activates the switch.
* **Contacts (Switch):** These are the physical contacts that open and close the circuit to the fuel pump. There are typically two types of contacts: normally open (NO) and normally closed (NC). In a fuel pump relay, the contacts are usually normally open, meaning the circuit is broken until the relay is activated.
* **Terminals:** These are the connection points on the relay that allow you to connect it to the vehicle’s wiring harness and to provide power to the coil.

Relays typically have four or five terminals. Here’s a breakdown of their common functions:

* **Terminal 85 or A1:** One side of the coil.
* **Terminal 86 or A2:** The other side of the coil.
* **Terminal 30:** Common terminal, connected to the power source.
* **Terminal 87:** Normally open (NO) terminal, connected to the fuel pump.
* **Terminal 87a:** Normally closed (NC) terminal (may not be present on all relays).

**Important Note:** Terminal numbers can vary depending on the relay manufacturer. Always refer to the relay’s diagram (usually printed on the relay itself) or your vehicle’s wiring diagram to identify the correct terminals.

Testing Procedure: Step-by-Step Guide

Now, let’s get to the testing process. Follow these steps carefully:

**Step 1: Locate the Fuel Pump Relay**

The fuel pump relay is typically located in the under-hood fuse box or in a relay panel under the dashboard. Consult your vehicle’s owner’s manual or a repair manual to find its exact location. The manual will usually have a diagram showing the location of each fuse and relay.

**Step 2: Remove the Relay**

Once you’ve located the relay, carefully remove it from its socket. Use a relay puller if necessary to avoid damaging the relay or the socket. Before removing the relay, disconnect the negative terminal of the car battery for safety. Although low voltage, disconnecting the battery minimizes the risk of electrical shorts during testing.

**Step 3: Visual Inspection**

Before testing with the multimeter, visually inspect the relay for any signs of damage, such as:

* **Cracked or broken casing:** This could indicate physical damage that might affect the relay’s functionality.
* **Burned or corroded terminals:** Corrosion or burning can prevent proper electrical contact.
* **Melted plastic:** This suggests overheating, which can damage the relay’s internal components.

If you see any of these signs, the relay is likely faulty and should be replaced.

**Step 4: Testing the Coil Resistance**

This test checks the integrity of the relay’s coil. A broken or shorted coil will prevent the relay from functioning properly.

1. **Set the Multimeter to Ohms (Ω):** Select the ohms setting on your multimeter. You might need to choose a specific range, such as 200 ohms or 2k ohms, depending on your multimeter.
2. **Connect the Multimeter Probes:** Connect the multimeter probes to terminals 85 and 86 (or A1 and A2, depending on the relay’s markings). It doesn’t matter which probe goes to which terminal for this test.
3. **Read the Resistance:** Observe the resistance reading on the multimeter. A typical coil resistance for a 12V automotive relay is between 50 and 120 ohms. Refer to the relay’s specifications or your vehicle’s repair manual for the exact expected value. A reading of zero ohms (or a short circuit) indicates a shorted coil. An infinite resistance (or open circuit) indicates a broken coil. Both of these conditions indicate a faulty relay.

**Step 5: Testing Continuity of the Switch (Without Power)**

This test verifies that the switch contacts are normally open (NO) as they should be when the relay is not energized.

1. **Set the Multimeter to Continuity Mode:** Select the continuity setting on your multimeter. This setting is often represented by a diode symbol or a sound wave symbol. The multimeter will beep when there is a closed circuit (continuity).
2. **Connect the Multimeter Probes:** Connect the multimeter probes to terminals 30 and 87. These are the terminals connected to the switch contacts.
3. **Check for Continuity:** The multimeter should NOT beep. This indicates that the switch contacts are normally open, as expected. If the multimeter beeps, it means the switch contacts are stuck in the closed position, and the relay is faulty.

**Step 6: Testing Relay Activation (Click Test)**

This test verifies that the relay is able to switch when power is applied to the coil.

1. **Connect Jumper Wires to the Relay Coil:** Connect jumper wires with alligator clips to terminals 85 and 86 (or A1 and A2). These wires will be used to supply power to the relay’s coil.
2. **Connect to 12V Power Source:** Connect the other ends of the jumper wires to a 12V power source (car battery or bench power supply). Ensure correct polarity: typically, terminal 86 is connected to the positive (+) terminal and terminal 85 to the negative (-) terminal, but double-check your wiring diagram. **CAUTION:** Incorrect polarity can damage the relay.
3. **Listen for a Click:** When you connect the power source, you should hear a distinct “click” sound from the relay. This indicates that the coil is energizing and the switch contacts are moving. If you don’t hear a click, the relay is likely faulty. Try reversing the polarity of the connections to terminals 85 and 86. Some relays require a specific polarity to operate. If it still doesn’t click, the relay is most likely bad.
4. **Disconnect the Power Source:** Disconnect the jumper wires from the 12V power source *before* proceeding to the next step.

**Step 7: Testing Continuity of the Switch (With Power)**

This test confirms that the switch contacts close when the relay is energized.

1. **Keep Jumper Wires Connected (But Disconnected from Power):** Ensure the jumper wires are still connected to terminals 85 and 86 of the relay but *disconnected* from the 12V power source.
2. **Set the Multimeter to Continuity Mode:** Select the continuity setting on your multimeter (as in Step 5).
3. **Connect the Multimeter Probes:** Connect the multimeter probes to terminals 30 and 87 (as in Step 5).
4. **Apply Power to the Coil:** Briefly connect the jumper wires to the 12V power source. You should hear the relay click.
5. **Check for Continuity:** While the relay is energized (clicking), the multimeter should now beep, indicating continuity between terminals 30 and 87. If the multimeter does not beep, the switch contacts are not closing properly, and the relay is faulty.
6. **Disconnect the Power Source:** Disconnect the jumper wires from the 12V power source. The multimeter should stop beeping when the relay de-energizes.

**Step 8: Testing the Normally Closed (NC) Terminal (If Applicable)**

If your relay has a terminal 87a (normally closed), you can test it as follows:

1. **Set the Multimeter to Continuity Mode:** Select the continuity setting on your multimeter.
2. **Connect the Multimeter Probes:** Connect the multimeter probes to terminals 30 and 87a.
3. **Check for Continuity (Without Power):** The multimeter should beep, indicating continuity between terminals 30 and 87a when the relay is not energized.
4. **Apply Power to the Coil:** Briefly connect the jumper wires to the 12V power source. You should hear the relay click.
5. **Check for Continuity (With Power):** While the relay is energized, the multimeter should *not* beep, indicating that the circuit between terminals 30 and 87a is now open.
6. **Disconnect the Power Source:** Disconnect the jumper wires from the 12V power source. The multimeter should beep again when the relay de-energizes.

If the relay does not behave as described above, it is likely faulty.

Interpreting the Results

Here’s a summary of how to interpret the test results:

* **Coil Resistance:**
* **Normal Resistance (50-120 ohms, or as specified):** Coil is likely good.
* **Zero Ohms (Short Circuit):** Coil is shorted, relay is bad.
* **Infinite Resistance (Open Circuit):** Coil is broken, relay is bad.
* **Continuity Test (Without Power):**
* **No Continuity between 30 and 87:** Switch is normally open, as expected.
* **Continuity between 30 and 87:** Switch is stuck closed, relay is bad.
* **Activation Test (Click Test):**
* **Click Heard:** Relay is likely activating.
* **No Click Heard:** Relay is not activating, relay is bad.
* **Continuity Test (With Power):**
* **Continuity between 30 and 87 (While Energized):** Switch is closing properly.
* **No Continuity between 30 and 87 (While Energized):** Switch is not closing, relay is bad.
* **Normally Closed Terminal (If Applicable):**
* **Continuity between 30 and 87a (Without Power):** Normally closed circuit is intact.
* **No Continuity between 30 and 87a (With Power):** Normally closed circuit is opening when energized.

If the relay fails any of these tests, it should be replaced.

Replacing the Fuel Pump Relay

If your testing indicates that the fuel pump relay is faulty, replacing it is a simple process:

1. **Purchase a Replacement Relay:** Obtain a new fuel pump relay that is compatible with your vehicle. Check your vehicle’s owner’s manual or a parts catalog to find the correct part number.
2. **Install the New Relay:** Simply plug the new relay into the socket where the old relay was located. Ensure it is fully seated.
3. **Reconnect the Battery:** Reconnect the negative terminal of the car battery.
4. **Test the System:** Start the engine to verify that the fuel pump is now working properly. Listen for the fuel pump priming sound when you turn the ignition key to the “on” position.

Troubleshooting Tips

* **Double-Check Connections:** Ensure all connections are clean and secure before testing.
* **Refer to Wiring Diagrams:** Use a wiring diagram specific to your vehicle to identify the correct terminals on the relay.
* **Test the Socket:** After removing the relay, you can test the socket itself to ensure that it’s receiving power and ground. Use a multimeter to check for voltage between the power and ground terminals when the ignition is in the “on” position.
* **Consider Other Causes:** If replacing the relay doesn’t solve the problem, consider other potential causes, such as a bad fuel pump, clogged fuel filter, wiring problems, or a faulty fuel pump fuse.
* **Consult a Professional:** If you’re not comfortable performing these tests or if you’re unable to diagnose the problem, consult a qualified mechanic.

Safety Precautions

* **Disconnect the Battery:** Always disconnect the negative terminal of the car battery before working on electrical components.
* **Wear Safety Glasses:** Protect your eyes from debris and potential sparks.
* **Use Caution with Electricity:** Be careful when working with electricity. Avoid touching exposed wires and ensure your hands are dry.
* **Work in a Well-Ventilated Area:** If you’re working with fuel, work in a well-ventilated area to avoid inhaling fumes.
* **Consult a Professional:** If you’re not comfortable performing these tasks, consult a qualified mechanic.

By following these steps and tips, you can effectively test your fuel pump relay and diagnose potential fuel delivery problems. Remember to always prioritize safety and consult a professional if you’re unsure about any aspect of the testing procedure.