How Do I Test a Fuel Pump Relay? Simple Step-by-Step Methods

Testing a fuel pump relay is a crucial diagnostic step when your car cranks but won't start. You can accurately test it yourself using simple tools like a multimeter or by safely bypassing it. This guide details multiple reliable methods to determine if your relay is faulty.

A malfunctioning fuel pump relay is a frequent culprit behind a "no-start" condition where the engine cranks over normally but fails to fire. The relay acts as a high-power switch, turning on the fuel pump for a few seconds when you turn the key to "ON," and continuously once the engine cranks. If it fails, the pump gets no power, leaving your engine starved for fuel. Before condemning the pump itself, testing the relay is a critical, relatively simple check. Here's how to do it properly.

Understanding the Basics

Fuel pump relays are typically located in the engine bay fuse box, the interior fuse panel (often under the dashboard or kick panels), or sometimes near the battery. Consult your vehicle's owner's manual or a service manual (easily found online) for its exact location and appearance. The relay itself is a small, usually cube-shaped component, often black or grey, plugged into a socket with multiple electrical terminals. It generally has four or five terminals, labeled with numbers or standard identifiers:

  • 85 and 86: Coil Terminals. These receive a small control signal (usually 12 volts) from either the ignition switch or the Powertrain Control Module (PCM) to activate the relay.
  • 30: Power Input. This is the terminal that receives constant battery power (typically 12 volts) directly from the fuse box, usually via a large fuse.
  • 87: Power Output. This terminal is connected to 30 only when the relay is activated. The fuel pump gets its power through this terminal.
  • (Sometimes 87a): Normally Closed Output. Only present on 5-pin relays. This terminal is connected to 30 when the relay is OFF, and disconnected when ON. Not used for a standard fuel pump relay, which is typically a 4-pin "normally open" type.

Understanding these terminals is key to effectively testing the relay.

Essential Tools & Safety Precautions

  • Tools: You'll need:
    • A basic Digital Multimeter (DMM) - Crucial for voltage and continuity checks.
    • A set of jumper wires with alligator clips.
    • A test light (can supplement the multimeter).
    • Your vehicle's wiring diagram or fuse/relay assignment chart.
    • Mechanics gloves and safety glasses.
  • Safety:
    • ALWAYS disconnect the negative (-) battery cable before working near fuse boxes or relays to prevent shorts and sparks. This is non-negotiable.
    • Work in a well-ventilated area away from open flames or sparks – fuel vapors are explosive.
    • Ensure the vehicle is in Park (automatic) or Neutral (manual) with the parking brake firmly engaged.
    • Avoid pinching wires or damaging terminals when removing or installing the relay.

Method 1: The Swap Test (Quickest Initial Check)

If you locate the fuel pump relay in its fuse box, you might find other identical relays nearby (like for the horn, A/C compressor, headlights, or ECM). This is often the fastest first check.

  1. Identify & Access: Locate the fuel pump relay. Locate another relay in the same box that has the same part number or physically looks identical. Common candidates are horn or cooling fan relays (confirm in your manual!). Note their positions carefully.
  2. Swap: Carefully remove both relays (firm, straight pull). Plug the known good relay (e.g., the horn relay) into the fuel pump relay socket. Plug the suspected fuel pump relay into the horn relay socket.
  3. Test: Reconnect the battery negative cable briefly. Turn the key to the "ON" position (do not crank). Listen carefully near the rear of the car for the distinct 2-3 second humming sound of the fuel pump priming. If it now primes, your original fuel pump relay is likely faulty. Test the horn – if it no longer works with the suspected relay installed, that further confirms the suspected relay is bad. Turn key off and disconnect battery again before swapping back.
  4. Interpretation: If the pump still doesn't prime, the relay might still be good, or there could be other problems (like power supply issues, blown fuse, wiring fault, or even a bad pump). Proceed to more definitive tests. This test is circumstantial but fast.

Method 2: Listen & Feel (Preliminary On-Vehicle Check)

A simple, non-invasive check involves listening and feeling:

  1. Position: Have someone sit in the driver's seat. Position yourself near the fuse box containing the fuel pump relay.
  2. Activate: Instruct your helper to turn the ignition key firmly to the "ON" (Run) position, but DO NOT crank the engine. You should only move the key from "OFF" to "ON."
  3. Observe: As the key is turned "ON," you should hear and feel a distinct, audible click from the relay within the fuse box. This "click" means the relay coil is energizing and physically switching the internal contacts. Wait a few seconds - you might hear another click as the relay de-energizes (since the PCM usually stops sending the "prime" signal after a few seconds).
  4. Interpretation: If you hear a solid click, the relay's coil circuit (terminals 85/86) is likely working. This doesn't guarantee the power switching contacts (30/87) are good, only that the coil received its signal. If you hear no click at all, there's a problem with the relay coil circuit (dead relay, blown fuse supplying the control side, wiring fault, or bad signal from PCM/ignition). Proceed to voltage tests.

Method 3: Voltage Testing (Definitive On-Vehicle Check)

This method uses a multimeter to verify critical voltages at the relay socket with the relay removed. This tests the car's wiring to the relay socket. Disconnect battery negative before removing relay!

  1. Set Multimeter: Set your digital multimeter to measure DC Voltage (usually a "V" with straight and dashed lines). Select the 20V DC range if not auto-ranging.
  2. Locate Ground: Find a reliable grounding point, like an unpainted bolt on the engine block, chassis, or battery negative terminal. Clip the black (negative) multimeter probe firmly here.
  3. Identify Socket Terminals: Consult your vehicle's wiring diagram to identify the function of each terminal in the empty relay socket. Most sockets have small numbers molded into the plastic next to each socket terminal. If not, diagrams are essential.
  4. Test Terminal 30 (Power Input):
    • Reconnect the battery negative cable for this test. CAUTION: Live Circuit.
    • Touch the red (positive) multimeter probe to the socket terminal identified as 30.
    • With the key turned OFF, you should measure battery voltage (approx. 12.6V if engine off). If no voltage, check the main power fuse supplying this circuit (often a large fuse in the main under-hood fuse box) and the wiring from the battery/fuse to this terminal.
  5. Test Terminal 86 (Coil Ground):
    • Keep the key OFF.
    • Move the red probe to the socket terminal identified as 86. This terminal usually connects to ground through the PCM or directly.
    • You should measure very low voltage (less than 0.5V). Sometimes a "buzz" or continuity test setting might be easier: Set meter to continuity or low Ohms (Ω), touch probes to 86 and your ground point. If the terminal is grounded, it should beep/show low resistance (< 5 Ohms). High resistance or no continuity indicates a broken ground path.
  6. Test Terminal 85 (Coil Power/Signal Input):
    • Keep the black probe grounded.
    • Turn the ignition key to the ON (Run) position.
    • Touch the red probe to the socket terminal identified as 85. This terminal should receive a 12V signal when the key is turned on.
    • The meter should read battery voltage (~12V). If it reads zero, the issue could be a blown ignition or PCM fuse, a faulty ignition switch, a wiring fault, or a problem within the PCM not sending the signal.
  7. Interpretation:
    • Good Readings: If you have solid battery voltage at 30 (Key Off), a good ground at 86 (Key Off), and battery voltage at 85 (Key On), the wiring to the relay socket is good. Proceed to testing the relay itself (Method 4 or 5). If the relay was clicking but the pump doesn't run, the fault is likely within the relay (bad contacts) or downstream (wiring, fuse, pump).
    • Bad Readings: Identify the missing voltage or ground source found in steps 4-6. Fix that wiring/fuse/switch/PCM ground issue before suspecting the relay.

Method 4: Relay Bench Test (Definitive Relay Check)

This is the most conclusive method to test the relay itself off the vehicle. You'll need your multimeter and two jumper wires.

  1. Set Multimeter: Set the multimeter to Continuity Test or Ohms (Ω). This measures if there's a connection between two points. The "beep" setting is often easiest.
  2. Identify Relay Terminals: Look at the bottom of the relay. The pins correspond to the terminals. You need to identify pins 85, 86, 30, and 87. Many relays have a schematic diagram printed on the side. If not, diagrams or terminal numbering conventions (typically 85 & 86 are smaller/thinner pins than 30 & 87) can help. Use Method 3 identification as a guide for your specific relay socket if needed.
  3. Check Normally Open Contacts (30 & 87):
    • Touch the multimeter probes to the relay pins 30 and 87.
    • Relay De-energized (OFF): With the relay not activated, there should be no continuity (no beep, high resistance OL (Over Limit) on display). The circuit is open.
    • Pass: If you get no continuity, this part is good initially.
  4. Activate & Re-Check Contacts (30 & 87):
    • Take two jumper wires. Clip one wire from the positive (+) terminal of your 12V car battery to the relay pin 85.
    • Clip the other wire from the negative (-) terminal of your 12V car battery to the relay pin 86. CAUTION: Ensure connections are solid. Be prepared for a small spark at the battery terminal when making the last connection.
    • As you make the final connection to energize the coil, you should hear and feel the relay click.
    • Relay Energized (ON): With the relay activated (coil powered), touch the multimeter probes to pins 30 and 87 again. You should now have continuity (a beep and low resistance reading, near 0 Ohms). The internal switch has closed, connecting these terminals.
    • Deactivate: Remove one of the jumper wires from 85 or 86. The relay should click off. Re-check pins 30 and 87 with your meter - continuity should be gone.
  5. Interpretation:
    • Passes Both: The relay clicks when energized AND the 30-87 connection changes state correctly (open when off, closed when on). This relay is functioning.
    • Fails:
      • No Click: The relay coil is likely open or damaged internally. Replace relay.
      • Clicks but 30-87 always open: The internal switching contacts are burnt, pitted, or stuck open. Replace relay.
      • Clicks but 30-87 always closed: The internal switching contacts are welded shut. Dangerously unsafe, replace relay immediately.
      • 30-87 partially conducts when off (low resistance but not zero): Contacts may be contaminated or arcing. Faulty, replace relay.

Method 5: The Bypass Test (Direct Circuit Test - Caution Advised)

This test bypasses the relay entirely, supplying power directly to the fuel pump circuit to confirm it functions. It definitively tests the circuit from the relay output (87) through the fuse, wiring, pump connector, and the pump itself. Use extreme caution.

  1. Disconnect Battery: ALWAYS disconnect the negative battery terminal first.
  2. Locate Terminals: Identify the 30 (Constant Power) and 87 (Fuel Pump Output) sockets in the fuel pump relay socket (use wiring diagram/terminal ID from before).
  3. Prepare Jumper: Create a heavy-gauge jumper wire with alligator clips on both ends (a fused jumper wire is safest but often not practical for quick tests). Ensure the wire can handle the pump's current (10-20A typically).
  4. Make Connections (KEY OFF):
    • Clip one end of the jumper to the 30 socket terminal (constant battery power).
    • Clip the other end of the jumper to the 87 socket terminal (pump feed).
  5. Safety Check: Double-check connections. Be mindful of sparks. Ensure no tools or metal objects are bridging terminals.
  6. Apply Power: Reconnect the battery negative terminal.
  7. Listen: With the key still OFF, you should immediately hear the fuel pump run continuously. WARNING: The pump will run non-stop! Only leave the jumper connected for a few seconds. It's solely to confirm the pump runs. Extended operation without engine running is unnecessary and risks washing down cylinder walls with fuel or building unsafe pressure in a faulty system. Turn key off, disconnect battery before removing jumper!
  8. Interpretation:
    • Pump Runs: The wiring, fuse, and fuel pump are functional. The problem lies upstream of 87: specifically, the fuel pump relay itself, the control signal to the relay (85), or the ground path (86). Go back and test the relay (Method 4) or relay socket voltages (Method 3).
    • Pump Does Not Run: There is a problem downstream of the relay socket 87 terminal. Causes include:
      • Blown fuse specific to the pump circuit (though often 30 fuse supplies both the socket AND the pump fuse...check all related fuses!).
      • Bad wiring connection or broken wire in the circuit feeding the pump.
      • Faulty fuel pump connector or wiring at the tank.
      • Seized or burned-out fuel pump.
      • Poor ground connection for the pump itself.

Troubleshooting Based on Results

  1. No Relay Click (On-Vehicle - Method 2):
    • Verify voltage at relay socket 85 when key is ON (Method 3).
    • Verify ground at relay socket 86 (Method 3).
    • Check fuses for the ignition switch or PCM.
    • Suspect bad relay coil (Test with Method 4).
  2. Relay Clicks, But Pump Doesn't Run:
    • Test relay contacts on the bench (Method 4).
    • Verify good constant power at socket 30 (Method 3).
    • Perform the Bypass Test (Method 5, cautiously).
      • If pump runs: Faulty relay (confirmed bad contacts).
      • If pump doesn't run: Problem downstream (fuse, wiring, pump ground, pump itself).
  3. Swap Test Indicates Bad Relay: Test the swapped relay on the bench (Method 4) to confirm.
  4. All Tests Point to Good Relay, But Problem Persists:
    • Double-check all related fuses (main power to relay, pump circuit fuse).
    • Re-examine wiring diagrams for PCM control strategy - sometimes PCM grounds the coil (86), other times it supplies switched power (85). Ensure correct diagnosis of socket terminals.
    • Investigate wiring harness damage near the relay box or fuel tank.
    • Confirm the vehicle security system isn't immobilizing the fuel pump.
    • Consider PCM problems (less common than relay failure but possible).

Why Test the Relay?

Testing the fuel pump relay matters because:

  1. Common Failure Point: Relays wear out over time. Internal contacts get dirty, pitted, or burn. Coils can fail.
  2. Inexpensive Part: Relays are significantly cheaper than fuel pumps.
  3. Simplest Fix: Replacing a relay is usually much easier and faster than dropping a fuel tank to access a pump.
  4. Quick Diagnosis: Several tests (like the swap, click, or basic voltage checks) can rapidly point you towards or away from the relay as the problem.
  5. Avoids Unnecessary Pump Replacement: Diagnosing a relay failure prevents the costly mistake of replacing a perfectly good fuel pump.

Conclusion

Testing a fuel pump relay is an essential diagnostic skill for any DIY mechanic facing a "cranks but no start" situation. By systematically using the methods outlined above – starting with the simple Swap and Listen tests, then moving to voltage checks and definitive bench testing – you can confidently determine if the relay is the culprit or if the problem lies elsewhere in the fuel delivery system. Remember to prioritize safety: disconnect the battery, avoid sparks near fuel vapors, and understand the circuits you're testing. Proper diagnosis saves time, money, and hassle, putting you back on the road quickly. If testing reveals a faulty relay, replacement is usually straightforward once you have the correct part number. When the relay checks out good, your focus correctly shifts downstream to fuses, wiring, or the pump itself.

Back to blog