2008 Cadillac DTS Fuel Pump Relay Location: Solved (Underhood #15)

Finding the elusive fuel pump relay in your 2008 Cadillac DTS is surprisingly straightforward: look for the black plastic relay clearly labeled with the number "15" inside the underhood electrical center (fuse box), located near the battery on the driver's side. Don't waste time checking under dashboards or seats – that's a common misconception. If your DTS cranks but refuses to start, or you hear no fuel pump prime when turning the key to "ON," checking and replacing this relay is your crucial first diagnostic step before suspecting the pump itself. Locating it requires just basic tools and a few minutes.

Preparing to Locate the Fuel Pump Relay

Park the vehicle on a level surface. Apply the parking brake firmly. Turn the ignition completely off and remove the key from the ignition switch. Open the driver's door. Locate the hood release lever beneath the dashboard, usually on the lower left side near the door frame. Pull this lever firmly. Walk to the front of the vehicle. Locate the secondary hood release latch underneath the center front edge of the hood. Slide this latch to the side and lift the hood completely. Secure the hood with the prop rod if necessary. Wait a few minutes for any hot components near the underhood electrical center to cool down. Wear safety glasses during this process to protect eyes from falling debris. Collect a small flat-head screwdriver and a flashlight. Have replacement fuses and relays available before proceeding.

Identifying the Underhood Electrical Center

The underhood electrical center, sometimes called the main fuse box or power distribution center, is the primary electrical hub. The 2008 DTS positions this large, rectangular black plastic box prominently in the engine compartment. Stand facing the front of the vehicle. Locate the vehicle's battery. The battery resides on the driver's side front corner. Look directly next to the battery, towards the front of the vehicle. A large plastic housing is secured in place with fasteners. This housing typically measures around 12 inches long and 8 inches wide. It features a removable lid sealed against moisture. Wiring harnesses emerge from underneath the housing into various parts of the engine bay. The lid itself often has clear labels or embossed symbols indicating its purpose as the main fuse/relay center.

Accessing the Relay and Fuse Compartment

Position yourself securely over the front driver's side fender. Identify the securing mechanisms for the electrical center cover. Most 2008 DTS models utilize simple plastic latches integrated into the cover sides. Locate one or two latches on the longer sides of the rectangular cover. Inspect the latch design; most require pressing the latch inward slightly while lifting upward. Place your fingers firmly on one latch. Apply steady inward pressure toward the center of the lid. While holding this pressure, lift that corner of the lid upwards. Repeat this press-and-lift motion for any additional latches on the lid. Avoid excessive force to prevent breaking the brittle plastic latches. If a latch feels stuck, double-check its mechanism; never pry at corners without releasing latches first. Once all latches are released, lift the entire cover straight up and away. Set the cover aside in a safe location away from moving parts or dirt.

Reading the Relay and Fuse Map

Locate the reference information once the underhood center cover is removed. The cover's underside usually contains a detailed diagram. Remove any protective plastic film obscuring labels on the cover's underside. Locate the legend or map printed on this inner surface. This legend serves as the key identifying every component inside the electrical center. Study the legend carefully. Note its layout: it typically mirrors the arrangement of slots within the center itself. Common elements include circles representing mini fuses and larger squares representing relays. Search specifically for the fuel pump relay designation. Standard labels include "FUEL PUMP," "FP," "F/P," or "F/PMP". Scan the diagram systematically until identifying this label. Simultaneously note the relay's position number within the grid layout. Confirm the assigned number for the fuel pump relay. Record this number mentally or on paper. Observe any relevant adjacent fuses listed for the fuel pump circuit.

Locating Relay Position 15

Place the electrical center cover aside securely. Look down into the opened fuse/relay panel cavity. Observe the grid pattern formed by the internal structure. Numerous relay slots are populated within this grid. Most slots contain either a blue or black plastic relay. Relays are modular components plugged vertically into their sockets. Each position within the grid has a unique identification number. Locate the numbering system: these numbers are clearly embossed or printed directly onto the plastic housing next to each relay position. They are usually white or black numerals on the housing's surface. Methodically scan across the rows and columns of relay positions. Systematically search for the number "15" ("FIFTEEN"). Position 15 is typically found within the second or third row from the front edge. It is located approximately in the center or slightly offset towards the driver's side of the box. Verify visually that you have found the correct position marked with the number "15".

Identifying the Fuel Pump Relay Itself

Concentrate now on the specific relay plugged into socket position number 15. The fuel pump relay in the 2008 DTS adheres to a common automotive standard. It features a standard "ISO mini" relay form factor. The relay body is typically colored black. Dimensions are roughly 1 inch long by 1 inch wide and 1.25 inches high. Four metal blade terminals protrude from its base, inserted into the receptacle below. The top surface of the relay displays its electrical specifications. Expect to see ratings like "12V" or "30/40A". A part number is usually molded onto the top or side plastic casing; common examples include GM part numbers like 88988995 or aftermarket equivalents like AC Delco D1743A. Notice any manufacturer branding imprinted on the relay housing. Visually inspect the relay to ensure it matches the typical description.

Understanding Relay Function and Related Components

The fuel pump relay serves as the high-current switch within the fuel pump electrical circuit. The Powertrain Control Module (PCM) activates this relay. Upon turning the ignition key to the "ON" position, the PCM applies control voltage to the relay coil terminals (usually positions 85 and 86 within the socket). This creates a magnetic field inside the relay. The magnetic field pulls an internal switch closed. Closing this switch connects the main power feed (terminal 30) to the output terminal (terminal 87). Power flows from the large battery cable attached to the electrical center through terminal 30. Closing the relay routes this power through terminal 87 and directly to the fuel pump itself. The fuel pump relay requires protection from excessive current. Locate the associated fuel pump fuse near position 15. Identify this fuse using the legend under the cover. Common fuse positions include FUSE 45 or FUSE 48 (confirm using the legend). Check this fuse concurrently with the relay if a fuel delivery problem exists. The fuse protects wiring from overloads caused by shorts.

Verifying the Fuel Pump Relay Location

Double-check your identification of relay position number 15. Confirm the number embossed on the fuse box housing directly adjacent to the relay. Verify this position corresponds precisely with the "FUEL PUMP" label on the cover diagram. Physically remove the relay if location confirmation requires further validation. Grasp the relay body firmly between thumb and forefinger. Pull straight upwards steadily. Avoid rocking the relay sideways to prevent socket damage. Listen for a soft click as terminals disengage. Examine the now-empty relay socket. Four cavities accept the relay blades. Notice the position indicators are fixed to the plastic housing and cannot change accidentally. Reference the factory service manual schematics if uncertainty persists; they confirm power distribution center relay position R15 controls the fuel delivery system.

Performing Initial Relay Testing (Swapping Method)

Swapping relays offers the fastest initial test when diagnosing fuel delivery problems. Begin this method only after locating position 15 correctly. Identify other relays within the underhood center sharing the identical physical shape and part number as the suspected fuel pump relay. Common candidates include the horn relay or cooling fan relays in positions like 26, 27, or 41. Select one known-good relay matching the fuel pump relay specifications. Ensure the ignition key is turned completely to the "OFF" position. Extract the suspected fuel pump relay from position 15 using the removal method described. Extract the known-good donor relay from its socket. Insert the known-good donor relay carefully into position 15. Align its terminals correctly and press down firmly until fully seated. Listen for a distinct click upon proper insertion. Turn the ignition key to the "ON" position without cranking the engine. Listen intently near the fuel tank (rear underside) for a distinct two-second buzzing or humming noise. This confirms the fuel pump is receiving power during priming. Restoring prime noise after the swap strongly indicates the original relay was faulty. Perform this test multiple times if the initial result is unclear.

Performing Advanced Relay Testing (Multimeter Method)

Use a digital multimeter for conclusive electrical testing. Ensure the multimeter has voltmeter functionality. Select the DC voltage scale up to 20 volts. Turn the ignition key to the "LOCK" position. Safely remove the fuel pump relay from socket position 15. Identify the four socket terminals exposed within position 15. These terminals correspond directly to the relay blade numbers: 30 (Permanent Battery Positive), 85 (Control Coil Ground), 86 (Control Coil Positive from PCM), and 87 (Output to Fuel Pump). Consult a socket diagram if terminal markings are unclear. Set the multimeter to test voltage (V DC). Attach the multimeter's black probe securely to a known good ground point (unpainted metal bolt on engine or chassis). Switch the ignition key to the "ON" position. Touch the red probe to the socket terminal for position 86. A normal reading should show approximately 11-12 volts with ignition "ON" – this confirms the PCM is providing the signal. Touch the red probe to terminal 30. A normal reading should show permanent battery voltage of 12.6V – confirms hot feed active. Touch the red probe to terminal 87. This terminal should measure zero volts when the relay is removed. Check terminal 85 for continuity to ground: Turn multimeter to resistance/ohms setting. Touch one probe to terminal 85, the other probe to a good ground point. Resistance should measure near zero ohms, confirming ground path. These readings establish socket integrity. Terminal 30 and 86 should possess voltage with the key "ON". Terminal 85 should demonstrate continuity to ground. Terminal 87 should lack voltage until relay activated. Test the relay itself if the socket checks pass without issues. Use resistance mode to test between coil terminals 85 and 86; expect roughly 50-100 ohms. Test between switch terminals 30 and 87; should measure infinite resistance open normally, and near zero ohms when 12V applied to 85/86. Replace relay failing any test.

Replacing a Faulty Fuel Pump Relay

Purchase an exact specification replacement relay. GM part number 88988995 or AC Delco D1743A are standard. Generic equivalents labeled ISO 40A Mini Relay function identically. Confirm the terminal layout numbering matches the old unit. Turn the vehicle ignition completely off. Open the hood securely. Position yourself safely near the electrical center. Identify the faulty relay in socket position 15. Grasp the relay body firmly, avoiding terminal contact. Pull the relay vertically upwards out of its socket without tilting it. Place the removed relay aside. Retrieve the new replacement relay. Compare the terminal pattern to the socket cavity. Ensure correct alignment before proceeding. Position the new relay directly above socket position 15. Guide the relay terminals precisely over the corresponding socket contacts. Push the relay firmly straight down into the socket until it seats fully. Listen and feel for a definitive click indicating secure engagement. Visually confirm the relay sits flush like adjacent units. Test immediately: Turn the ignition key to the "ON" position. Listen for the characteristic 2-second fuel pump prime noise near the rear fuel tank. Attempt to start the engine normally after hearing prime confirmation. Close the underhood electrical center cover securely after replacement and testing. Press downward evenly on the cover until all retaining clips fully engage. Avoid leaving the electrical center exposed unnecessarily.

Differentiating Relay Failure from Pump Failure

Correct relay diagnosis prevents unnecessary fuel pump replacement. Relay failures manifest characteristic symptoms: An intermittent no-start condition occurs. Operation resumes only after waiting minutes or hours. Failure often correlates with elevated ambient temperatures underhood. Lack of fuel pump prime sound remains the most consistent indicator; ignition "ON" yields silent operation without audible prime from the rear. The vehicle cranks normally but never fires. Occasionally, gentle tapping the relay itself might restore temporary function. In contrast, fuel pump failures exhibit different signs: Prime sound weakens progressively over time. Pump operation produces unusual screeching or whining noises, especially during colder operation. Engine experiences power loss during acceleration or struggles maintaining higher speeds. Vehicle stalls suddenly during driving then restarts later inconsistently. Relay swapping produces zero audible prime noise change. Fuel pressure testing reveals inadequate pressure readings at the test port. Advanced diagnostics include verifying voltage actually reaches the pump connector during prime command. If 12 volts exists at the pump harness but pump remains silent, the pump motor is confirmed faulty.

Emergency Starting Procedures for Relay Failure

Jumpering the relay socket provides temporary operation during relay failure. Understand this bypass permanently powers the pump when engaged; use only briefly for emergency movement. Locate socket position 15 carefully. Obtain a fused jumper wire rated for high current or a suitable piece of 12-gauge copper wire. Turn the vehicle ignition off completely. Remove the fuel pump relay exposing its socket. Identify the two large terminals: terminal 30 and terminal 87. Terminal 30 is continuously powered from the battery. Terminal 87 connects directly to the fuel pump motor. Insert one end of the jumper wire into socket terminal 30. Insert the other end of the jumper wire into socket terminal 87. This connects power directly to the pump circuit, bypassing normal controls. Listen carefully near the fuel tank; continuous pump running noise should be audible immediately. Start the engine normally with the jumper installed. Operate the vehicle immediately only as necessary to reach a repair location. Prolonged jumpering poses overheating risks. Disconnect the jumper immediately upon arrival. Have replacement relay ready for installation after shutdown. Jumpering is strictly a temporary roadside assistance technique. Relay replacement remains mandatory for correct operation.

Important Safety Precautions and Handling Guidelines

Follow safety protocols diligently when handling relays and electrical systems. Power down systems: Turn ignition fully off and remove key before relay handling. Isolate terminals: Never allow metal tools to bridge terminals within fuse box during inspection. Prevent corrosion: Avoid touching bare relay terminals with fingers; skin oils accelerate terminal oxidization. Match specifications precisely: Install only relays matching original voltage, amperage, and physical form factor. Discard damaged components: Replace any relay exhibiting cracks, melted plastic, or physical distortions. Secure connections: Ensure relay snaps fully into socket to prevent intermittent connections and heat buildup. Beware shock hazard: Avoid contact with exposed battery terminals during procedures underhood. Contain hot pieces: Dropped metal screws near battery terminals create extreme short circuit dangers. Maintain part organization: Keep old and new relays distinctly separated during replacement to prevent errors. Utilize correct tools: Employ plastic trim tools if necessary for fuse box covers instead of metal screwdrivers. Manage static: Discharge static electricity by touching grounded metal before handling sensitive PCM circuitry parts. Install fuse protection: Never replace a fuel pump fuse with a higher amperage rating under any circumstances. Respect vehicle battery: Disconnect negative terminal for extended work sessions involving wiring modification.

Confirming Application Years and Model Variations

The relay location information applies primarily to the 2008 Cadillac DTS. Verify your specific year matches. The fifth-generation DeVille/DTS platform covers model years 2000 through 2011. Underhood electrical center layout changed minimally throughout this extended production period. Relay position 15 consistently controls the fuel pump across these model years. This includes the 2005 and earlier DeVille models before the DTS badge introduction. Subsequent DTS models through the final 2011 year retain the same position. Northstar engine versions share the identical fuse box layout and relay assignment. Verify the physical position using the lid map if vehicle model year differs slightly. Some model years designate the relay numerically as position R15 for clarity. Dual overhead cam (DOHC) Northstar V8 engine architecture uses this control strategy universally. Aftermarket vehicle databases like Alldata or Mitchell ProDemand corroborate position "PDC R15" across the generation. Earlier fourth-generation DeVille models (1994-1999) feature significantly different engine layouts and relay positions. Later Cadillac XTS or CT6 models employ disparate fuse box configurations entirely. Always rely on the specific diagram attached to your vehicle's underhood center lid for absolute certainty. This lid map constitutes the factory-approved reference documentation location.