The 1998 Chevy 1500 Fuel Pump Relay Location (& What To Do Next)

Answer: The fuel pump relay for a 1998 Chevrolet Silverado/GMC Sierra 1500 (GMT400 platform) is located in the Underhood Electrical Center. Specifically, it occupies position number #46 in the relay/fuse box mounted on the driver's side fender wall, near the brake master cylinder. This relay is typically a standard 4-pin mini ISO relay.

Knowing where your fuel pump relay is located is crucial when diagnosing a "no-start" condition where the engine cranks but refuses to fire. This guide details the exact location, how to identify it, test it, and what to do if it's faulty.

Finding the Underhood Electrical Center (Fuse/Relay Box)

  1. Open the Hood: Securely prop open the hood of your 1998 Chevy 1500.
  2. Locate the Driver's Side Fender Wall: Stand facing the engine bay. Look towards the back of the engine compartment on the driver's side.
  3. Identify the Black Plastic Box: You'll see a relatively large, rectangular black plastic box mounted vertically on the inner fender wall. This is the primary underhood electrical center, housing numerous fuses and relays responsible for critical engine and vehicle functions.
  4. Access the Cover: Lift the cover off the electrical center. It usually has clips on both ends or along the sides.

Pinpointing Relay #46

  1. Check the Diagram: The inside of the electrical center cover should have a detailed diagram showing the positions and functions of every fuse and relay. Always check this diagram first for the most accurate map of your specific truck.
  2. Look for Position #46: If the diagram is faded or missing, locate the printed numbering inside the box itself. Fuses and relays are arranged in rows and columns. Position #46 is usually located near the center or slightly offset towards the front of the box. It's positioned horizontally with other relays.
  3. Identify the Relay: The fuel pump relay in position #46 is a small, square, black plastic component approximately 1 inch by 1 inch. It will have four electrical prongs extending down into the socket base. It may have markings like "30", "87", "85", "86" on top near each prong. It may be labeled "FP" or "FUEL PUMP" next to the socket.
  4. Confirm the Type: It is a standard 4-pin mini ISO relay common in GM vehicles of this era. Other relays in the box (like the AC Compressor relay or Horn relay) are often identical in appearance.

Visually Distinguishing Relay #46

  • Location: Near other critical relays like the AC Compressor Relay or Powertrain Control Module (PCM) Relay.
  • Appearance: Square/rectangular black plastic housing with four prongs. Size: approx. 1" x 1". Top usually has a faint number grid like "30", "87", "85", "86".
  • Comparison: While other relays look the same physically, the position (#46) or adjacent label is the definitive identifier. Don't rely solely on looks; use the position.

Why Your Fuel Pump Relay Matters

The fuel pump relay is a critical electrical switch controlled by the Powertrain Control Module (PCM). Its job is simple but vital:

  1. Ignition On: When you turn the key to "ON" (before cranking), the PCM briefly energizes the fuel pump relay for about 2 seconds. This powers the fuel pump to pressurize the fuel system. You might hear a slight hum from the rear of the truck (the fuel pump location).
  2. Engine Cranking/Running: Once the engine begins cranking or running, the PCM keeps the relay energized continuously. This provides constant power to the fuel pump to maintain fuel pressure while driving.
  3. Engine Off: The PCM de-energizes the relay once the engine is switched off, stopping the fuel pump.

Symptoms of a Faulty Fuel Pump Relay (Position #46)

A failing or failed relay in socket #46 causes the fuel pump to lose power. Symptoms mirror a completely failed fuel pump:

  1. No Start (Cranks But Doesn't Run): The most obvious sign. The engine cranks normally but absolutely will not start, as no fuel reaches the engine.
  2. No Initial Prime Sound: When you turn the ignition key to "ON" (not start), you should hear a faint humming or buzzing for 1-2 seconds from the rear fuel tank area. No sound at all is a strong indicator the pump isn't getting power, potentially due to the relay.
  3. Engine Stalling: An intermittent relay might cause the engine to stall suddenly while driving and refuse to restart immediately.
  4. Intermittent Starting Problems: The truck may start fine sometimes and fail to start at others, indicating the relay contacts are failing.

Testing & Diagnosing Relay #46

Before condemning the relay, a simple test or swap is prudent:

  1. The Swap Test: Locate another identical relay in the underhood box. Common choices known to be the same type as the fuel pump relay (#46) include:
    • AC Compressor Relay (Position #42)
    • Horn Relay (Position #41 or similar - see diagram)
    • Cooling Fan Relay (Position #43 - note: may be same type but potentially higher amp rating, confirm diagram)
    • Check your underhood diagram! Find a relay marked the same type as #46 (e.g., "Mini ISO").
    • Important: Remove the relay you intend to swap. Cycle the key to ON and confirm the horn doesn't honk (if swapping the horn relay) or the AC doesn't engage (if swapping AC relay). This ensures the component is inactive.
    • Swap the suspected fuel pump relay (#46) with this known good, identical relay.
    • Try starting the truck. If it starts and runs reliably, the original relay in position #46 is faulty. Replace it.
  2. Listen Test (Key On): Have a helper turn the ignition key to "ON" while you listen near the fuel tank fill neck (driver's side rear). You should hear the pump hum for 2 seconds. No sound strongly suggests a power delivery failure (relay, fuse, wiring, pump).
  3. Voltage Test (Requires Multimeter):
    • At the Relay Socket: Carefully remove relay #46. Set multimeter to DC Volts (20V scale). Check for voltage between ground and specific terminals in the socket with the key ON:
      • Terminal 30: Should have constant battery voltage (12V+) at all times.
      • Terminal 86: Should have 12V+ when the key is turned ON (briefly) and during cranking/running. This is the PCM control signal.
      • Terminal 85: Is the ground side for the control circuit. Grounds through the PCM.
      • Terminal 87: This is the "output" terminal that sends power to the fuel pump when the relay is activated. It should have no voltage when the relay is out and the key is ON.
    • At the Fuel Pump Harness (Advanced): The connector near the top of the fuel tank or along the frame rail (access may require removing a kick panel in the cab near the fuel tank) should have voltage on the power wire (usually thick pink or gray wire) during key ON and cranking. No voltage points back to relay/fuse issues.

What To Do If Relay #46 Is Faulty

  1. Replace It: Purchase a standard GM/ISO 4-pin mini relay from any auto parts store (ACDelco, BWD, Standard Motor Products). They are inexpensive (typically 20).
  2. Purchase the Correct Part: Ensure it matches the physical size and terminal configuration of the relay you removed from socket #46. Denso 056700-5050 or 25540747 (GM OE Number) are examples, but confirm visually. Auto parts stores can cross-reference "1998 Chevy 1500 fuel pump relay".
  3. Installation: Orient the new relay correctly using the prong pattern and the diagram printed on the relay. Push it firmly down into socket #46 until it clicks/seats fully.
  4. Test: Try starting the truck immediately. You should hear the fuel pump prime briefly when you turn the key to "ON".

Crucial Next Steps: When Replacing the Relay Doesn't Fix It

If you've replaced the fuel pump relay in position #46 and the truck still won't start, or you hear the pump run but it doesn't start, further diagnosis is needed:

  1. Check the Fuel Pump Fuse: The fuel pump power circuit also has a main fuse.
    • Location: In the same underhood electrical center. Refer to the diagram. Common locations include:
      • Fuse #21 (20 Amp)
      • Fuse #42 (10 Amp - feeds relay control circuit)
      • Fuse #16 (20 Amp)
    • Check the diagram inside the lid or box. Visually inspect the fuse blades for breakage or melted filament. Test with a multimeter for continuity or replace it if any doubt.
  2. Inspect Wiring: Look for obvious damage or corrosion at the relay socket terminals. Check wiring near the underhood box, along the frame towards the tank, and at the fuel pump access point.
  3. Verify Fuel Pump Operation: If the fuse and relay are good and you have power confirmed at the pump connector (with key ON/Crank), but the pump makes no sound, the fuel pump itself is likely the culprit. If the pump runs but the engine doesn't start, consider fuel pressure testing, clogged filter, or injector issues.
  4. Check Inertia Switch: Some GMT400 trucks (especially older models in this generation) may have an inertia fuel shutoff switch located on the passenger side firewall inside the cab (behind the glovebox area, under the dash, mounted on the transmission hump). This switch cuts fuel pump power during a collision. Check if it's been accidentally tripped (the button will be popped up). Press the button down firmly to reset it if tripped.
  5. Consider Ignition Switch: A failing ignition switch can sometimes prevent the PCM from sending the signal to energize the fuel pump relay. Listen for the relay clicking when turning the key to ON (with relay installed). It should click on for about 2 seconds then click off.
  6. Security System (PASS-Key): On trucks with the factory VATS/PASS-Key system, a problem reading the resistor pellet in the key can disable the fuel injectors and the fuel pump circuit. Does the security light on the dash stay on? Does it start briefly and then die? Try a second key. Wiggle the key gently while in RUN position before cranking. The fuel pump won't run if the PCM disables the system.
  7. Grounds: Check for clean and tight ground connections, especially the main engine ground strap and grounds around the PCM/underhood fuse box.

Preventative Maintenance & Reliability Tips

  • Carry a Spare Relay: Given their critical role and common failure point, keeping a spare fuel pump relay (and main fuse) in your glove box is cheap insurance against being stranded.
  • Relay Quality: While cheap relays work, spending a little extra on a name-brand relay (ACDelco, Bosch, Standard) might offer better longevity.
  • Terminal Care: When installing, ensure the relay socket terminals are clean and not spread out (losing tension). Apply a tiny amount of dielectric grease to the relay prongs to prevent corrosion.
  • Address Other Issues: If replacing the relay fixes an intermittent start problem, consider it a warning that the truck's wiring or connectors might be aging. Inspect related connections.
  • Listen Regularly: Make a habit of listening for the brief fuel pump prime sound each time you turn the key to "ON". Its absence is an early warning sign.

Conclusion: Knowing #46 Saves Time and Money

Locating the fuel pump relay (position #46 in the underhood electrical center) is the first critical step in diagnosing a no-start condition on your 1998 Chevy Silverado or GMC Sierra 1500. Understanding its role, how to test it simply (the swap test), and what to do next if it fails empowers you to fix this common problem quickly and affordably. Always start with the relay and fuse checks before jumping to the conclusion of needing a costly fuel pump replacement. Keep a spare relay on hand for peace of mind on the road.

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