1999 Cadillac DeVille Fuel Pump Relay Location: The Ultimate Troubleshooting Guide

Finding the fuel pump relay location in your 1999 Cadillac DeVille is crucial when diagnosing a no-start condition or fuel delivery problem. The primary fuel pump relay is located in the Rear Compartment Fuse Panel, situated in the trunk behind the back seat on the driver's side. A secondary power relay related to the fuel pump circuit is found in the Underhood Electrical Center near the battery.

This guide provides the definitive, step-by-step instructions to locate, test, and replace the fuel pump relay in your 1999 Cadillac DeVille. Understanding where this critical relay hides will save you significant time and frustration during troubleshooting, potentially avoiding unnecessary repairs.

Understanding the Fuel Pump Relay's Vital Role
The fuel pump relay acts as an electronically controlled switch. Your DeVille's Powertrain Control Module (PCM) sends a signal to activate this relay when you turn the ignition key to the "ON" position or crank the engine. Upon activation, the relay closes its internal contacts. This action delivers high-amperage battery power directly to the electric fuel pump located inside the gas tank. Without this relay functioning correctly, the pump will not receive power, leading directly to a "cranks but doesn't start" situation. Recognizing this failure pattern is the first step in diagnosis.

Precise Location of the Primary Fuel Pump Relay: Rear Compartment Fuse Panel
For the 1999 DeVille, the main fuel pump relay resides inside the vehicle's trunk area. Locate this fuse panel as follows:

  1. Access the Trunk: Open the trunk lid completely.
  2. Locate the Fuse Block: Position yourself facing the back of the rear seat.
  3. Identify the Panel: Find the rectangular black plastic fuse box cover mounted vertically on the driver's side of the trunk, near the rear corner and behind the carpet trim. It is secured by plastic push-pins or small bolts.
  4. Remove the Cover: Carefully detach the cover by prying gently at its edges or removing securing fasteners. Set the cover aside safely.
  5. Identify the Relay: Look inside the fuse box itself. You will see an array of fuses (smaller, often colored plastic) and several larger, square or rectangular cubes – these are the relays. Consult the diagram printed on the underside of the fuse box cover you just removed. This diagram is essential. Locate the relay position labeled "FUEL PUMP," "FP," "F/P," "Pump," or similar. It is Relay position R4 or R8 (common locations for this generation DeVille, though always verify via the cover diagram). The relay itself is typically a black or gray cube with four or five metal prongs on the bottom.

Location of the Secondary Power Relay: Underhood Electrical Center
While the main control relay is in the trunk, another relay in the engine compartment supplies primary power to the fuel pump relay circuit. This is often referred to as the "Main Fuel Pump Power Relay" or simply the "Power Relay". Its failure can mimic a bad trunk relay or pump.

  1. Open the Hood: Secure the hood prop rod.
  2. Locate the Underhood Center: Identify the large black plastic fuse and relay box situated near the driver's side front of the engine compartment, typically close to the battery and the strut tower. It has a large lid covering it.
  3. Remove the Center Cover: The lid usually has clips or nuts securing it. Release these and lift the cover off.
  4. Find the Relay: Inside this box is a complex array of fuses and larger relays. Consult the diagram printed on the underside of the Underhood Electrical Center lid. Look specifically for the relay labeled "PCM BATT," "ECM B," "ECM IGN," "PCM IGN," or sometimes "Fuel" (though 'Fuel' might point back to the trunk relay). Common locations are relay positions 6, 10, or 15, but ABSOLUTELY rely on the lid diagram for your specific vehicle. It will be another cube-shaped component similar in appearance to the trunk relay.

Step-by-Step Guide to Safely Remove a Relay
Before handling relays, prioritize safety and identification:

  1. Disconnect Battery Power: Always start by disconnecting the negative (black) battery terminal. Use a wrench to loosen the clamp bolt and move the cable away from the battery post. This prevents accidental shorts.
  2. Identify Correct Relay: Double-check you have located the correct relay per the fuse box cover diagrams before removal.
  3. Relay Removal: Relays pull straight out from their sockets. Grasp the relay firmly. Rock it gently side-to-side while pulling upwards with steady pressure. Avoid excessive force or prying that could damage the socket. The relay will release from the socket.
  4. Inspect: Examine the relay's prongs for signs of corrosion, burning, or melting. Visually check the relay socket contacts for similar damage or pushed-in pins. Note the relay's identification number if replacing it.

Testing the 1999 DeVille Fuel Pump Relay (Simple Methods)
Replacing relays gets expensive fast. Testing before replacement is highly recommended:

  1. The "Click" Test (Power Applied):
    • Ensure vehicle is in Park, parking brake applied.
    • Have an assistant turn the ignition key to the "ON" (Run) position.
    • Place your fingers firmly on the relay body.
    • Your assistant should hear and you should feel a distinct "click" inside the relay within 1-2 seconds of turning the key to "ON". This "click" signifies the internal electromagnet is activating.
    • Warning: No click does not definitively condemn the relay. It could be a faulty PCM signal, wiring to the relay, or the relay itself. A click confirms activation but not necessarily that the relay is passing power.
  2. The Swap Test (Best Practice):
    • Locate another relay in the same fuse panel with identical appearance and the same terminal numbers printed on it. Horn, A/C compressor, and headlight relays are often suitable candidates (verify ratings!).
    • Safely remove this identical, known-good relay.
    • Remove the suspected fuel pump relay.
    • Install the known-good relay into the fuel pump relay socket. Ensure it's fully seated.
    • Reconnect battery negative cable temporarily.
    • Turn ignition to "ON". Listen near the gas tank filler for a brief (2-second) fuel pump priming hum and check if the engine will now start. If the problem is resolved, your original relay is faulty.
    • Disconnect battery negative cable again before swapping relays back.

Advanced Relay Socket Terminal Testing
Using a multimeter provides definitive confirmation by checking for power and ground signals at the socket:

  1. Tools: Digital Multimeter (Set to DC Volts & Ohms).
  2. Ensure Safety: Battery Negative Terminal MUST be disconnected.
  3. Identify Socket Terminals: With the relay removed, study its bottom. Prongs are numbered (look closely or check relay body imprint):
    • 85 and 86: Control Circuit (Low current - from PCM & Ground).
    • 30: Battery Feed (Constant High-Current Power - Should always be hot).
    • 87: Switched Output (High-Current Power to Fuel Pump - Only hot when relay active).
      (Note: Some relays use 87a - ignore if present in the DeVille, as the fuel pump relay uses standard 85, 86, 30, 87).
  4. Testing Power at Terminal 30:
    • Reconnect battery negative cable.
    • Set multimeter to DC Volts (20V scale).
    • Place the black meter lead on a clean, unpainted metal engine bolt or strut tower bolt (a good ground).
    • Touch the red meter lead to the socket terminal identified as 30.
    • The meter should read battery voltage (approx. 12.6V), with the ignition OFF.
    • If no power, check the high-amperage fuse feeding the Underhood Center fuse block.
  5. Testing Control Signal at Terminal 86:
    • Keep black lead on ground.
    • Touch red lead to socket terminal 86.
    • Turn ignition key to the "ON" position (Engine not running).
    • The multimeter should read battery voltage (approx. 12.6V) only while the key is ON. It should read 0V when key is OFF.
    • If missing signal at key ON, suspect PCM fault, blown fuse protecting PCM (like IGN fuse in Underhood Center), or wiring issue between PCM and relay.
  6. Testing Control Ground at Terminal 85:
    • Disconnect battery negative cable.
    • Set multimeter to Ohms (Ω) or Continuity.
    • Place one meter lead on socket terminal 85.
    • Place the other meter lead on a clean, unpainted metal engine bolt or strut tower bolt.
    • The meter should read very low resistance (less than 1 Ohm) or beep for continuity. This confirms the ground path controlled by the PCM is intact through the wiring harness.
    • If high resistance/no continuity, check ground points (G107 near battery, G117 on engine, G201 on body) related to the PCM or fuel pump circuit. Refer to a wiring diagram.
  7. Testing Output Terminal 87:
    • Reconnect battery negative cable.
    • Set multimeter to DC Volts (20V scale).
    • Black lead on good ground.
    • Red lead on socket terminal 87.
    • Have an assistant turn the ignition key to "ON".
    • If the relay and its controls are good, the meter should briefly display battery voltage for 1-2 seconds at key ON.

Selecting the Correct Replacement Relay
If testing confirms a bad relay, choose wisely:

  1. OEM Specification: Find the GM part number stamped on your old relay (e.g., 12135049, 12183687, etc.). Purchase an identical GM relay from an auto parts store or dealership parts counter. This is the most reliable option.
  2. Industry Standard: Cadillac used common ISO Mini relays in this era. Ensure the replacement matches:
    • Physical shape and pin layout (4 or 5 pins - DeVille FP is usually standard 4-pin).
    • Terminal numbers on case must match (85, 86, 30, 87).
    • Amperage Rating: Typically 20, 30, or 40 Amp - Match the rating printed on the old relay.
  3. Brand Reliability: Avoid the absolute cheapest options. Brands like Bosch, Delphi, Standard Motor Products (SMP), or genuine ACDelco offer better quality control and longevity.

Installation Procedure for New Relay
Installation is straightforward once you have the correct replacement:

  1. Disconnect Battery Power: Ensure the Negative battery cable is disconnected.
  2. Align Correctly: Orient the new relay so its prongs align with the socket holes. Look for orientation marks or keying slots. It should only fit one way.
  3. Firm Seating: Push the relay down firmly and evenly into the socket until it clicks or is fully seated. Ensure all prongs disappear properly into the socket contacts. Loose relays cause intermittent failures.
  4. Reconnect Battery: Reconnect the Negative battery cable.
  5. Test Operation: Turn the ignition key to "ON". Listen for the fuel pump prime. Attempt to start the engine.
  6. Replace Fuse Panel Covers: Ensure both trunk and underhood fuse block covers are securely replaced to protect components from debris and moisture.

Symptoms Pointing Directly to Fuel Pump Relay Failure
Suspect this relay if you experience:

  • Sudden No-Start: Engine cranks normally but refuses to fire, especially after the vehicle sat briefly or suddenly shut down while driving. No fuel pump prime noise heard at key-on.
  • Intermittent Starting: Vehicle sometimes starts perfectly, other times cranks without starting, particularly after being warm or sitting in hot weather. Worn relay contacts often fail when hot.
  • Silent Fuel Pump: Listening near the gas tank filler neck or rear seat fuel pump access panel reveals no audible 2-second "whirring" or "humming" sound when the ignition is turned to "ON".
  • No Check Engine Light? The PCM doesn't always set a diagnostic trouble code (DTC) for a failed fuel pump relay, especially if the control circuit signals are still present.

Important Considerations Beyond the Relay
A faulty relay is common, but not the only possibility. Keep these in mind:

  1. Check Related Fuses: Always check fuses first! The primary fuel pump fuse is usually located in the Underhood Electrical Center. Consult the lid diagram – often labeled "FUEL PUMP," "FP," "F/P," "Pump," or sometimes "ECM IGN." Another important fuse is the "PCM BATT" or "ECM B" fuse (usually 10A or 15A) in the same panel. Blown fuses signal a downstream problem that caused the overload (like a wiring short or failing pump motor).
  2. The Fuel Pump Itself: A completely silent pump after confirming the relay and main fuse are good (power and ground at pump connector) strongly indicates a failed pump.
  3. Wiring Problems: Corrosion, damaged wires, or poor connections anywhere in the circuit – especially between the relay in the trunk and the pump module at the tank – will cause failure.
  4. Inertia Safety Switch: Cadillacs of this era usually have an inertia fuel cutoff switch. Its function is to kill the pump in a collision. It can sometimes trip accidentally from minor impacts or jarring, or suffer internal failure. It's often located near the base of the trunk trim on the driver's side or near the rear wheel well. Push its reset button if tripped (consult owner's manual). Test for power input and output or bypass it temporarily for diagnosis.
  5. Security System: The Passlock™ security system can sometimes disable the fuel injectors (not necessarily the pump) if triggered, mimicking a fuel delivery issue. Look for a flashing security light.
  6. PCM Issues: A faulty Powertrain Control Module could fail to send the activation signal to the relay (No power at socket terminal 86 during key ON).

Tools Required for the Job
Gather these tools beforehand:

  • Standard Hand Tool Set (Sockets, Wrenches, Screwdrivers)
  • Digital Multimeter (Essential for proper diagnostics)
  • Needle Nose Pliers (Helpful for pulling fuses/relays)
  • Work Light or Good Flashlight
  • Owners Manual (For fuse/relay diagram confirmation)
  • Service Manual Wiring Diagrams (Highly Recommended for complex troubleshooting)

Conclusion: Master the Relay Location for Quick Diagnosis
Now equipped with this detailed guide, locating and resolving a faulty fuel pump relay in your 1999 Cadillac DeVille becomes a manageable task. Remember the dual locations: the primary control relay resides in the Rear Compartment Fuse Panel (trunk, driver's side), while its critical power feed relay sits in the Underhood Electrical Center. Always start diagnosis by listening for the pump prime, then move on to checking fuses before testing the relays using the swap method or multimeter procedures. Prioritizing safety by disconnecting battery power prevents electrical mishaps. Addressing a failed relay promptly restores reliable performance, avoiding the inconvenience and potential hazard of a stranded vehicle. Bookmark this comprehensive resource to empower your next troubleshooting venture.

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