1998 Chevy S10 Fuel Pump Relay Location: Find, Test & Fix Your Relay

The fuel pump relay on a 1998 Chevrolet S10 (and Sonoma) is located inside the primary Underhood Electrical Center, specifically in position #3 (or sometimes labeled #31 in some diagrams) of the relay/fuse box near the battery.

If your 1998 Chevy S10 cranks but won't start, experiences sputtering or power loss, or you hear no fuel pump prime when you turn the key to "ON," a faulty fuel pump relay is a common and relatively inexpensive culprit. Knowing exactly where to find it is the critical first step in diagnosing and solving the problem yourself. Here’s the definitive guide to locating, identifying, testing, and replacing the fuel pump relay on your 1998 S10.

The Specific Location: Inside the Underhood Fuse Box

1. Open the Hood: Secure the hood safely in the open position.
2. Locate the Main Underhood Electrical Center: On the driver's side of the engine compartment, next to the battery, you will find a large, rectangular, black plastic box. This is the primary fuse and relay box for most of the vehicle's critical systems.
3. Remove the Cover: The box has a lid secured by plastic tabs or clips near each corner. Press these tabs inward or lift the clips gently and remove the cover. Set it aside safely.
4. Identify the Relay Position: Once the cover is off, you'll see an array of fuses and relays inside the box. Look closely at the diagram printed on the underside of the cover you just removed. This diagram is essential. Find the label for the "Fuel Pump" relay.
   • Standard Labeling: On most 1998 S10s, the fuel pump relay occupies position #3 within the relay block section of the underhood box. You will usually see this "3" molded into the plastic base of the box near the relay socket.
   • Alternate Labeling: Some diagrams or box bases might label this specific relay position as #31. Always refer to the diagram on your specific vehicle's cover first. Look for "Fuel Pump," "FP," "ECM B" (as the relay also powers the Powertrain Control Module when the ignition is on), or sometimes "PCM." Ignore generic position numbers like "R1"; the cover diagram is the key.
5. Identify the Physical Relay: Once you've identified the correct socket position from the diagram (Position #3/31), locate the physical relay plugged into that socket. It will be a small (roughly 1" x 1" x 1.3"), typically cube-shaped or rectangular black plastic component. It will have four or five metal blade terminals (pins) protruding from the bottom, plugged into the socket.

What the Relay Looks Like: The fuel pump relay in your S10 is a standard automotive ISO mini relay. It has:

• A black plastic housing.
• A standard blade terminal pattern.
• A circuit diagram and ratings (e.g., 12V, 20A, 30A) printed on its side.
• Do not confuse it with fuses (which are clear or colored and have exposed metal links) or larger relays (like the starter relay) that might also be in the same box.

Verifying the Relay: Swapping (If Applicable)

The underhood fuse box often contains several identical-looking relays. If your vehicle is equipped, there might be identical relays powering other systems. Common candidates for swapping include the A/C Compressor relay or the Horn relay – but only if they are listed as the same part number or physically identical on your cover diagram.

1. Carefully note the position of your fuel pump relay (#3/31).
2. Carefully note the position of an identical spare relay (check the diagram under the cover).
3. With the ignition OFF, gently pry the suspect fuel pump relay straight up and out of its socket using your fingers or careful leverage with a small flathead screwdriver (be gentle to avoid breaking the socket).
4. Remove the known-good spare relay from its socket.
5. Plug the suspected bad fuel pump relay into the known-good spare's socket. Turn the ignition to "ON". If the system controlled by the spare relay (e.g., A/C clutch engages when A/C is on, horn sounds) now does not work, the suspected relay is likely faulty.
6. Plug the known-good spare relay into the fuel pump relay socket (#3/31). Turn the ignition to "ON". If you now hear the fuel pump prime (a distinct humming/buzzing sound from the rear of the truck lasting 2-3 seconds) and/or the engine starts, your original fuel pump relay was definitely faulty.
7. Important: Return the relays to their correct original positions once testing is complete, even if swapping temporarily got the truck running.

Simple On-Vehicle Electrical Test (Using a Multimeter)

You can perform a basic voltage test at the relay socket with the relay removed:

1. Set Multimeter: Set your multimeter to DC Volts (20V range).
2. Identify Socket Pins: Looking at the socket where the relay plugs in (Position #3/31):
   • Terminal 30: Should be HOT (Battery Voltage ~12V) at all times, even with ignition off. This is constant power from the battery feed via a fuse (likely the "ECM/BATT" fuse).
   • Terminal 85: Ground circuit controlled by the Powertrain Control Module (PCM).
   • Terminal 86: Should become HOT (Battery Voltage) only when the ignition is turned to "ON" or "START". This is switched power controlled by the ignition switch via the "IGN 0" fuse.
   • Terminal 87: Output terminal that sends power to the fuel pump when the relay is energized. Should be dead with ignition off.
   • Terminal 87a: Not used on ISO mini relays; only terminals 30, 85, 86, and 87 are used.
3. Test Ground (Pin 85): Place multimeter black probe on battery negative terminal or known good engine ground. Touch red probe to pin 85 in the socket. With ignition OFF, you should have very low resistance (continuity to ground). The PCM provides this ground path when commanded.
4. Test Constant Power (Pin 30): Touch red probe to pin 30, black probe to ground. Should read ~12V at all times. If not, check the "ECM/BATT" fuse.
5. Test Switched Power (Pin 86): Touch red probe to pin 86, black probe to ground. Should read 0V with ignition off. Turn ignition to "ON" – should read ~12V. If not, check the "IGN 0" fuse.
6. Test Output (Pin 87): Should read 0V with ignition off and when initially turning ignition to ON (relay not energized yet). When the relay clicks (you should hear it) about 2 seconds after turning ignition ON, pin 87 should show ~12V for 2-3 seconds as the pump primes. If pins 30, 85, and 86 have correct power/ground but pin 87 never gets voltage when it should (while listening for a click), and a swapped relay works, the original relay is bad.

Why the Fuel Pump Relay Matters

The fuel pump draws significant electrical current. The relay acts as a remote-controlled heavy-duty switch. It uses a small electrical signal from the Powertrain Control Module (PCM) when you turn the key on to activate an electromagnet inside the relay. This magnet pulls internal switch contacts closed, allowing high current to flow directly from the battery (through the relay) to the fuel pump located in the gas tank at the rear of the truck. Without a functioning relay, the signal from the PCM cannot turn on the high current needed by the fuel pump, leading to a no-start condition or drivability problems. Testing and replacing it yourself saves significant time and money compared to towing or a mechanic.

Troubleshooting Beyond the Relay

If replacing or testing confirms the relay works, the fuel pump still won't prime, and you don't hear the relay click:

1. Check Fuses:
   • "ECM/BATT" Fuse: In the underhood box. Provides constant power to the relay (Pin 30). If blown, the relay has no power input.
   • "IGN 0" Fuse: Also in the underhood box. Provides switched ignition power to the relay coil (Pin 86). If blown, the relay cannot be energized.
   • "FUEL PUMP" Fuse: Located inside the dash fuse panel (driver's side, lower left edge of dash near the door). This fuse protects the wire running from the relay output (Pin 87) back to the fuel pump. If blown, power leaves the relay but never reaches the pump.
   • "PCM/IGN" Fuse: Check this in the underhood box as well. It powers the PCM itself. A blown PCM fuse will prevent the PCM from turning the relay on.
2. Inertia Switch: The S10 has a fuel pump inertia safety switch, usually located on the passenger side firewall in the engine compartment or under the dash on the passenger side near the kick panel. This switch cuts power to the pump in the event of a collision. If tripped (a button on top pops up), firmly press the button back down to reset it. Verify its wiring connections are secure.
3. Bad Fuel Pump: The pump itself can fail. Before condemning it, ensure all fuses, the relay, and the inertia switch are good and power is getting to the pump connector near the tank. You can try tapping the bottom of the fuel tank lightly (carefully!) with a rubber mallet while someone else turns the key to "ON" – sometimes a failing pump will temporarily start working. Otherwise, listen carefully at the gas tank filler neck with the key turned to "ON" – a healthy pump will make a distinct 2-3 second hum/buzz.
4. Wiring Issues: Corrosion, damaged wires, or poor connections anywhere between the battery, relay, fuse, inertia switch, and fuel pump can interrupt power.
5. Ignition Switch Issues: A failing ignition switch might not send the "ON" signal correctly to energize the relay coil (Pin 86).
6. PCM Problem: If fuses are good, Pin 85 has a solid ground path during cranking/prime cycle (test with multimeter), Pin 86 gets power when ignition is ON, and Pin 30 has constant power, but the relay doesn't click and Pin 87 has no output, and a known-good relay doesn't help, a PCM malfunction preventing the ground command could be the issue (less common).

Replacing the Faulty Relay

1. Obtain Replacement: Purchase a standard automotive ISO mini relay. They are inexpensive and widely available at auto parts stores, dealerships, and online. Common GM part numbers include AC Delco D1747A or equivalent generics (e.g., BOSCH 0332017305 or similar ISO 280 type). Get the correct one – bring the old relay to match if possible.
2. Turn Ignition OFF: Ensure the key is removed.
3. Locate Position: Re-confirm Position #3 (or #31) in the underhood box using the cover diagram.
4. Remove Old Relay: Gently pry it straight up and out of its socket. If it's stubborn, wiggle it slightly side-to-side while pulling up.
5. Insert New Relay: Orient the new relay correctly based on the pin layout of the socket and the terminals on the relay base. It will only fit one way. Firmly press it straight down into the socket until fully seated.
6. Test: Turn the ignition to "ON" and listen for the 2-3 second fuel pump prime sound and/or try starting the engine. If successful, the problem is fixed.
7. Replace Fuse Box Cover: Securely snap the fuse box cover back on.

Frequently Asked Questions (FAQ)

• Q: What are the symptoms of a bad fuel pump relay?
  A: Engine cranks but won't start, no fuel pump prime sound when turning key to "ON", engine sputtering or stalling unexpectedly, loss of power while driving, sudden engine cut-out.
• Q: Can I drive with a failing fuel pump relay?
  A: It is highly inadvisable and dangerous. The engine could stall completely at any moment, including while driving at speed, potentially causing an accident. Have it diagnosed and replaced immediately.
• Q: Are all the relays in the underhood box the same? Can I swap any?
  A: No. While some relays might be identical part numbers (like A/C compressor, horn), many are not interchangeable. They may look similar but have different internal wiring configurations or amperage ratings. Always refer to the diagram under the fuse box cover to identify a suitable spare relay for swapping tests (confirm it's listed as the same part number or function). Only swap during testing and return them to original positions immediately after.
• Q: I replaced the relay, but the pump still doesn't run. Now what?
  A: Proceed through the troubleshooting steps outlined earlier: Check all relevant fuses ("ECM/BATT", "IGN 0", "FUEL PUMP" in dash, "PCM/IGN"). Verify the inertia switch is reset. Test for power at the fuel pump wiring connector near the tank (refer to repair manual for connector details). Check for wiring damage. Consider professional diagnosis if these don't resolve it.
• Q: Where is the fuel pump fuse?
  A: The "FUEL PUMP" fuse is located inside the interior fuse panel on the driver's side lower dash, near the door. The underhood fuses that feed the relay ("ECM/BATT", "IGN 0", "PCM/IGN") are also critical.
• Q: How much does a replacement relay cost?
  A: Typically $10-$25 USD, sometimes less, depending on brand and source. Very affordable compared to other potential causes like the fuel pump itself.
• Q: Can a bad relay drain my battery?
  A: It is very unlikely with the ISO mini style relay used. A stuck-closed relay would continuously power the fuel pump until the battery died, which is easily heard and noticed quickly. A more common parasitic draw would come from another component.

Conclusion

Finding and checking the fuel pump relay on your 1998 Chevy S10 (Position #3 / #31 in the underhood electrical center) is a straightforward task that can save you significant hassle and expense. By following this guide—using the diagram on the fuse box cover to locate the correct relay, performing simple swap tests or voltage checks, understanding the related fuses and inertia switch, and replacing with the correct inexpensive part—you can often diagnose and fix a common cause of no-start and drivability problems yourself. Remember that a failing fuel pump relay can lead to sudden stalling; prioritize its diagnosis and repair for both convenience and safety. If troubleshooting beyond the relay and its immediate fuses doesn't resolve the problem, further electrical diagnosis focusing on the pump circuit or potential PCM issues may be needed.