How Do You Test a Fuel Pump Relay? A Step-by-Step Guide for DIY Mechanics

Testing a fuel pump relay is a straightforward diagnostic task you can often perform yourself with basic tools. A faulty relay is a common culprit behind a car that cranks but won't start, as it controls power to the fuel pump. Identifying and testing it can save you time and money compared to immediately replacing the fuel pump itself. Here’s exactly how to do it safely and effectively.

Understanding the Fuel Pump Relay's Role

The fuel pump relay acts as a switch for the fuel pump circuit. Your car's engine control unit (ECU) sends a small signal to the relay. This signal activates an electromagnet inside the relay, which then closes a set of larger internal contacts. These closed contacts allow high-current battery power to flow directly to the fuel pump, enabling it to pressurize the fuel system. The relay protects the ECU from handling the high current the pump requires and allows remote control of the pump (like priming for a few seconds when you turn the key to "ON" before starting). When the relay fails, power cannot reach the fuel pump, preventing the engine from starting due to lack of fuel pressure.

Essential Safety Precautions Before Starting

Always prioritize safety:

  1. Work in a Well-Ventilated Area: Avoid potential fumes.
  2. Disconnect the Battery: Locate your car's negative battery terminal (usually marked with a - sign and black cable). Loosen the clamp nut and carefully remove the cable from the terminal. Secure it away from the battery post. This prevents accidental electrical shorts or sparks while working near relays and wiring.
  3. Let the Engine Cool: If the engine was recently running, allow it to cool before touching components.
  4. Have Your Vehicle Information Ready: Know your car's year, make, model, and engine size. This is crucial for locating the correct relay and understanding its specific function in your fuse box layout.
  5. Gather Necessary Tools: You'll typically need:
    • Basic screwdrivers (if needed to access fuse boxes)
    • A multimeter (digital preferred for ease of use)
    • A test light (optional but helpful for quick checks)
    • A wiring diagram for your specific vehicle (highly recommended, often found in repair manuals or online resources)
    • A jumper wire or fused jumper wire kit (for bypass testing - use with extreme caution)

Step 1: Locating the Fuel Pump Relay

Finding the relay is the first practical step:

  1. Consult Your Owner's Manual: This is the best source. Look for the fuse box diagram section, which usually includes relay locations and identities.
  2. Identify Fuse Box Locations: Common locations include:
    • Under the Hood: In the main engine bay fuse/relay box (often near the battery or firewall).
    • Inside the Cabin: Often under the dashboard (driver or passenger side), near the kick panels, or sometimes within the glove compartment.
  3. Examine the Fuse Box Lid/Cover: Most fuse box covers have a diagram listing each fuse and relay position and its function. Look for labels like "Fuel Pump," "FP," "Pump," "EFP," "Main Relay" (sometimes combined), or simply "Relay" with a number corresponding to the diagram.
  4. Physical Identification: Relays are typically small, cube-shaped or rectangular black plastic boxes, larger than fuses. They plug into sockets within the fuse box. They usually have 4 or 5 prongs (terminals) on the bottom. There might be multiple identical-looking relays. Never rely solely on looks; always confirm using the diagram.
  5. Listen for the Click: Have an assistant turn the ignition key to the "ON" position (not start). You should hear a distinct audible click from the fuel pump relay as it energizes. Place your finger lightly on suspected relays to feel the click. The relay that clicks when the key is turned on is a prime candidate. (Note: Some systems may only energize the pump for 2-3 seconds in "ON" and then de-energize the relay until cranking starts).

Step 2: Performing a Basic Visual and Auditory Check

Before removing anything, do these quick checks:

  1. Inspect the Relay: Look for obvious signs of damage: melting plastic, cracks, burn marks, or corrosion on the terminals. If visible, the relay is likely faulty and needs replacement.
  2. The "Click" Test (Revisited): With the ignition in "ON," listen and feel for the relay clicking on. No click could indicate a faulty relay, but it could also mean the relay isn't receiving the control signal from the ECU or power from the battery. Don't assume the relay is bad just yet.
  3. Swap with a Known Good Relay (Identical Type): This is one of the most reliable quick tests if you can find an identical relay in the fuse box serving a non-critical function (like the horn relay, A/C relay, or sometimes a rear window defogger relay - check your diagram!).
    • Carefully pull out the suspected fuel pump relay.
    • Find another relay of the exact same type (same part number, same terminal layout).
    • Swap their positions.
    • Try starting the car. If the car now starts, your original fuel pump relay is faulty. If the problem persists, the relay might be okay, and the issue lies elsewhere (like the pump, wiring, or ECU signal).

Step 3: Testing the Relay Itself (Bench Testing)

If the swap test isn't conclusive or possible, remove the relay for direct testing:

  1. Remove the Relay: Gently pull it straight out of its socket. Wiggle slightly if needed, but avoid excessive force.
  2. Identify the Terminal Layout: Look at the relay itself. The terminals are usually labeled on the bottom or side. Common labels include:
    • 85 & 86: These are the coil terminals (the electromagnet). They receive the low-current control signal from the ECU or ignition switch.
    • 30: This is the common terminal connected to the battery power source (usually constant 12V).
    • 87: This is the normally open (NO) terminal. When the relay coil is energized, it connects internally to terminal 30, sending power out to the fuel pump.
    • 87a: (Only on 5-pin relays) This is the normally closed (NC) terminal. It's connected to terminal 30 when the relay coil is de-energized and disconnects when energized. Fuel pump relays are usually 4-pin (85, 86, 30, 87) or 5-pin (85, 86, 30, 87, 87a). The 87a terminal is typically not used in a standard fuel pump circuit. Refer to your specific relay's diagram or vehicle wiring diagram.
  3. Testing Coil Resistance (Terminals 85 & 86):
    • Set your multimeter to measure resistance (Ohms, Ω).
    • Touch one probe to terminal 85 and the other to terminal 86.
    • You should read a resistance value. This value varies significantly between relays (anywhere from 50Ω to 200Ω is common). The key is that it should not be infinite (OL or Open Loop on the meter) or zero (0Ω). Infinite resistance means the coil is broken inside (open circuit). Zero resistance indicates a shorted coil. Either means the relay is faulty.
  4. Testing the Switch Contacts (Terminals 30 & 87):
    • Set your multimeter to measure continuity (often indicated by a diode symbol or sound wave symbol; it beeps when there's a connection).
    • De-energized State: Touch one probe to terminal 30 and the other to terminal 87. The meter should indicate no continuity (no beep, OL reading). This is the "Normally Open" state.
    • Energized State: You need to simulate the control signal to activate the coil. Apply 12 volts to terminal 85 and ground to terminal 86. You can use:
      • A small 12V power supply.
      • Jumper wires connected to your car battery (after reconnecting the negative terminal temporarily, then disconnect again after this test).
      • A specialized relay tester.
    • While applying 12V to 85 and Ground to 86 (you should hear/feel a click), touch the multimeter probes to terminals 30 and 87. The meter should now indicate continuity (beep, low resistance reading near 0Ω). This confirms the internal switch contacts close properly when the coil is energized.
    • (For 5-pin relays): Also check terminals 30 and 87a. In the de-energized state, there should be continuity. In the energized state, there should be no continuity. Fuel pump circuits typically don't use 87a.

Step 4: Testing the Relay in the Vehicle (Power and Signal Testing)

This tests if the relay is receiving the correct signals and power in its actual socket:

  1. Reconnect the Battery Negative Terminal. You need power for this test.
  2. Access the Relay Socket: Carefully plug the relay back into its socket. You need to access the metal terminals on the top or side of the socket where the relay plugs in. Sometimes you can back-probe the wire connector behind the socket, or carefully probe the small slots next to the relay terminals. Be extremely careful not to short terminals together with the probe.
  3. Identify Socket Terminals: Use your wiring diagram to determine which socket terminal corresponds to relay terminals 85, 86, 30, and 87.
  4. Test for Power at Terminal 30:
    • Set your multimeter to DC Volts (20V scale).
    • Connect the black (negative) probe to a known good ground point on the chassis or engine (e.g., bare metal bolt).
    • Connect the red (positive) probe to socket terminal 30 (battery power input).
    • With the ignition OFF, you should read battery voltage (around 12.6V). If not, there's a problem in the power feed (fuse, wiring).
  5. Test for Ground Signal at Terminal 86 (Common Setup):
    • Keep the black probe on ground.
    • Touch the red probe to socket terminal 86 (one side of the coil).
    • Turn the ignition key to the "ON" position.
    • You should see battery voltage (12V+) appear at terminal 86 for a few seconds (or continuously, depending on the vehicle). This indicates the ECU or ignition switch is providing a power signal to one side of the coil.
    • (Alternative Setup - Ground Controlled): Some vehicles apply constant power to terminal 85 and the ECU grounds terminal 86 to activate the relay. Check your wiring diagram. If this is the case, you would test for constant 12V at terminal 85 (ignition OFF or ON), and then test terminal 86 for continuity to ground when the ignition is turned ON (using the continuity setting on the meter, ignition ON, probe between 86 and ground - should show continuity briefly).
  6. Test for Control Signal Completion (Coil Activation):
    • This test checks if the coil circuit is complete when activated. Set your multimeter to DC Volts.
    • Connect the black probe to socket terminal 85 (or 86, depending on the control type - see diagram).
    • Connect the red probe to socket terminal 86 (or 85, the opposite coil terminal).
    • Turn the ignition key to "ON."
    • You should see battery voltage (12V+) appear across the two coil terminals. This voltage should drop slightly and stabilize but remain significant. If you see 0V or very low voltage (like 1-2V), the control circuit is incomplete (faulty ECU signal, bad ground, wiring issue).
  7. Test for Output Power at Terminal 87:
    • This is the critical test to see if power is reaching the fuel pump circuit.
    • Connect the black probe to ground.
    • Connect the red probe to socket terminal 87 (output to fuel pump).
    • Turn the ignition key to "ON."
    • You should see battery voltage (12V+) appear at terminal 87 for the same few seconds that the relay clicks. If you have voltage here when the relay is commanded on, but the fuel pump doesn't run, the problem is downstream (wiring to pump, pump ground, or the pump itself). If you have no voltage at terminal 87 when the relay should be on, but you have voltage at terminal 30 and the coil is being activated (confirmed by click or voltage across 85/86), then the relay itself is faulty internally (even if it bench tested okay – internal contacts can fail under load).

Step 5: The Jumper Wire Test (Bypassing the Relay - Use Extreme Caution)

This test directly applies power to the fuel pump circuit, bypassing the relay. It definitively checks the pump and its wiring but carries risk:

  1. WARNING: This sends full battery power directly to the fuel pump circuit. Ensure you know the correct terminals! Applying power to the wrong terminals can damage sensitive electronics (like the ECU). Using a fused jumper wire (e.g., 15A or 20A fuse) is HIGHLY recommended to mitigate risk.
  2. Identify Terminals: Locate socket terminal 30 (Battery Power In) and terminal 87 (Power Out to Fuel Pump).
  3. Perform the Bypass:
    • With the ignition OFF.
    • Carefully insert one end of the fused jumper wire into the socket slot for terminal 30.
    • Insert the other end into the socket slot for terminal 87. Ensure the jumper only touches these two terminals.
  4. Listen for the Pump: You should immediately hear the fuel pump whirring/humming sound from the rear of the car (usually near the fuel tank). This sound typically lasts as long as the jumper is connected.
  5. Interpretation:
    • Pump Runs: This confirms the fuel pump, its wiring, and its ground are functional. The problem lies with the relay itself or the control circuit (ECU signal, wiring to relay coil).
    • Pump Does NOT Run: This indicates a problem after the relay socket: a blown fuse (check main fuel pump fuse!), faulty wiring between the relay socket and the pump, a bad fuel pump ground connection, or a failed fuel pump itself. Further diagnosis downstream is needed.

Troubleshooting Common Scenarios

  • Relay Clicks, No Voltage at 87: Relay is faulty internally (contacts burned or corroded). Replace relay.
  • Relay Clicks, Voltage at 87, Pump Doesn't Run: Problem is downstream: pump fuse (if separate), wiring harness damage, bad pump ground, or failed fuel pump. Check fuse, inspect wiring, test pump power and ground directly at the pump connector.
  • No Relay Click, No Voltage Across Coil Terminals (85/86): Problem is in the control circuit. Check fuses related to ECU or ignition. Check for power/ground at the relay socket coil terminals as described in Step 4.5 & 4.6. Could be faulty ignition switch, wiring break, bad ground connection for the ECU or relay control, or faulty ECU.
  • No Relay Click, Voltage Present Across Coil Terminals (85/86): The relay coil is faulty (open circuit), even if voltage is present to activate it. Replace relay.
  • Relay Gets Very Hot: Indicates internal resistance or a short. Replace relay immediately.

What to Do If the Relay Tests Bad

If your tests confirm the fuel pump relay is faulty:

  1. Purchase the Correct Replacement: Relays are relatively inexpensive. Get the exact type specified for your vehicle (check part number on the old relay or consult an auto parts store). Using an incorrect relay can cause malfunction or damage.
  2. Installation: Simply plug the new relay firmly into the correct socket in the fuse box. Ensure the terminal orientation matches.
  3. Reconnect Battery: Reattach the negative battery cable.
  4. Test Operation: Turn the ignition key to "ON" and listen for the relay click and the fuel pump priming for a few seconds. Attempt to start the engine.

When Testing Points Elsewhere

If your relay tests good but the fuel pump still doesn't run when bypassed (jumper test), focus on:

  1. Check the Main Fuel Pump Fuse: Locate it using your diagram (often underhood or in-cabin). Inspect visually or test with a multimeter for continuity. Replace if blown. Investigate why it blew (short circuit?).
  2. Inspect Wiring: Look for damaged, frayed, or corroded wires in the harness running from the relay box to the fuel pump, especially where it passes through the firewall or near sharp edges or heat sources. Check connectors for corrosion or bent pins.
  3. Test Fuel Pump Power and Ground: Access the electrical connector at the fuel pump (often requires accessing the fuel tank sender unit/pump assembly). With ignition ON (or during jumper test):
    • Check for 12V+ at the pump's power wire (refer to diagram).
    • Check the pump's ground wire for good continuity to chassis ground.
  4. Test the Fuel Pump Itself: If power and ground are good at the pump connector, the pump itself is likely faulty and needs replacement.

Conclusion

Knowing how to test a fuel pump relay empowers you to diagnose a common no-start condition efficiently. By methodically following the steps – locating the relay, performing visual/swap checks, bench testing the relay, testing power and signals in the socket, and cautiously using a jumper wire – you can isolate whether the relay is the problem or if the issue lies elsewhere in the fuel pump circuit. Always prioritize safety, disconnect the battery before handling relays, use a wiring diagram, and proceed carefully, especially with jumper wires. Successfully testing and replacing a faulty fuel pump relay is a highly satisfying and cost-effective DIY repair.

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