2008 Cadillac CTS Fuel Pump Relay Location: A Complete Guide for DIY Owners

The fuel pump relay on a 2008 Cadillac CTS is located inside the Rear Compartment Fuse Block, which sits on top of the battery on the right side of the trunk. You will need to remove the battery access door to reach it. The relay itself is labeled "FUEL/PUMP RLY," and it works alongside a dedicated 20A "FUEL/PUMP" mini fuse in the same block. This guide walks you through finding it, testing it, and replacing it, plus common failure symptoms and what to check before you buy any parts.

If your CTS cranks but won't start, the fuel pump relay is one of the first things to check. The 2008 model year belongs to the second generation of the CTS, also known by the internal code "GMX322," and it uses two separate fuse boxes: one under the hood and one in the rear compartment. The fuel pump relay is not under the hood. It is in the trunk-mounted block. Knowing exactly where to look saves you from wasting time rummaging through the engine bay and pulling the wrong covers.

Where Exactly Is the Rear Compartment Fuse Block?

The rear compartment fuse block is mounted on top of the battery, which is located on the right side of the trunk. The battery access door must be removed to access the fuse block. This is a common source of confusion for owners, because many other GM vehicles from that era keep their main fuse boxes under the hood or under the rear seat. The 2008 CTS is different. The rear fuse block controls several critical circuits, including power windows, door locks, the audio system, and the fuel pump.

To get to it:

  1. Open the trunk and locate the battery on the right side. It sits behind a trim panel.
  2. Remove the battery access door. On most 2008 CTS models, this involves pulling back the carpet or releasing a few plastic retainers. You do not need to disconnect the battery to inspect the relay, but you should disconnect the negative terminal before doing any electrical testing or replacing parts.
  3. Once the access door is off, you will see the fuse block sitting directly on top of the battery case. It has a cover with a diagram printed on it. Pull the cover off to expose the fuses and relays.

Rear compartment fuse block location on 2008 Cadillac CTS
The rear compartment fuse block sits on top of the battery, right side of the trunk.

Finding the Fuel Pump Relay and Fuse

Inside the rear fuse block, you are looking for two specific items:

Fuel Pump Relay:

  • Label: FUEL/PUMP RLY
  • Type: Relay (physical, rectangular component)

Fuel Pump Fuse:

  • Label: FUEL/PUMP
  • Type: Mini fuse, rated at 20A

Both are located in the rear compartment fuse block. The relay and the fuse work together. Power flows from the battery, through the fuse, to the relay, and then to the fuel pump itself. If either one fails, the fuel pump will not receive power, and the engine will not start.

The relay is a small black or gray cube with four or five metal prongs on the bottom. It plugs directly into a socket in the fuse block. The fuse is a standard blade-type mini fuse that pulls straight out with a fuse puller or a pair of needle-nose pliers.

How the Fuel Pump Relay Works in the 2008 CTS

The engine control module (ECM) controls the fuel pump relay. When you turn the ignition key to the "ON" position, the ECM provides ignition voltage to the coil side of the fuel pump relay. This is a momentary signal. The relay closes, sending power to the fuel pump. The pump primes the fuel system for about two to three seconds, then the relay opens and the pump shuts off.

Once the engine is cranking and the ECM detects ignition reference pulses from the crankshaft position sensor, it keeps the relay closed. The pump runs continuously while the engine is running. If the ECM stops receiving reference pulses, it opens the relay within two seconds. The system is designed this way for safety. If the engine stalls or the car is in an accident, the pump shuts off to prevent fuel from being pumped into a wrecked engine bay.

The fuel system on the 2008 CTS is a returnless, on-demand design. The fuel pressure regulator is built into the primary fuel tank module, and the pump itself sits inside the fuel tank. Normal fuel pressure should be between 380 and 410 kPa (55 to 60 psi). If the pump is weak, you might see pressure below that range. If the pressure is too high, the regulator or the pump may be failing.

Common Symptoms of a Bad Fuel Pump Relay

A failing relay can produce the same symptoms as a failing fuel pump itself. Here are the most common signs:

  1. Engine cranks but does not start. This is the classic symptom. You turn the key, the starter spins the engine, but it never fires up. If the relay fails to close, the pump never runs.

  2. No fuel pump prime sound. When you turn the ignition to "ON" (not START), you should hear a brief whirring noise coming from the rear of the car, near the fuel tank. If you hear nothing, the pump is not receiving power. This could be a bad relay, a blown fuse, or a faulty pump.

  3. Intermittent starting problems. A relay that is wearing out may work sometimes and fail others. You might go days with no issues, then the car refuses to start. This is often temperature-related. As the relay heats up, internal contacts expand and lose connection.

  4. Engine stalls while driving. If the relay opens while the engine is running, the pump loses power and the engine dies immediately. This is dangerous, especially at highway speed.

  5. Whining noise from the fuel tank area. This is more often a sign of a failing pump, not a relay. But if the relay has high resistance, the pump may run at reduced voltage, causing it to run louder than normal.

Testing the Fuel Pump Relay

Before you buy any parts, you should test the relay. A visual inspection is not enough. A relay can look perfectly fine on the outside and still be dead inside.

Step 1: Listen for the Pump

Turn the ignition key to "ON" but do not start the engine. Listen near the fuel filler cap or under the rear seat. You should hear the fuel pump run for two to three seconds and then shut off. If you hear this, the relay is likely working, and the problem is elsewhere.

If you do not hear the pump, proceed to the next steps.

Step 2: Check the Fuse

The fuel pump fuse is a 20A mini fuse. Pull it out and look at the metal strip inside. If the strip is broken or blackened, the fuse is blown. Replace it with a new 20A mini fuse.

Important: If the fuse blows again immediately, you have a short circuit, not just a bad fuse. The most common causes include damaged wiring shorting to ground, a fuel pump with an intermittent internal failure, or a faulty fuse itself. Do not keep replacing the fuse without finding the root cause.

Step 3: Test the Relay Coil

You can test the relay with a digital multimeter (DMM) without removing it from the car. But it is easier to remove the relay and test it on a bench.

With the relay in your hand, measure resistance between terminals 85 and 86. This is the coil side. A good fuel pump relay should measure between 135 and 185 ohms. If the reading is outside this range, the relay coil is damaged, and you should replace the relay.

Step 4: Test the Relay Contacts

With the relay still removed, measure resistance between the following terminal pairs. The multimeter should display "O.L." (open loop, or infinite resistance) for each of these pairs:

  • Terminals 30 and 86
  • Terminals 30 and 87
  • Terminals 30 and 85
  • Terminals 85 and 87

If you get a low resistance reading on any of these pairs, the relay has an internal short, and it should be replaced.

Step 5: Bench Test the Relay

To confirm the relay actually works, you can power it up directly.

  1. Connect a 15-amp fused jumper wire from the positive battery terminal to relay terminal 85.
  2. Connect a jumper wire from the negative battery terminal to relay terminal 86.
  3. With the relay energized, measure resistance between terminals 30 and 87. You should see less than 2 ohms. If the reading is higher, the contacts inside the relay are worn out, and the relay cannot carry enough current.

Testing for Power at the Relay Socket

If the relay tests good, the problem might be in the wiring or the ECM. Here is how to check for power at the relay socket:

  1. Turn the ignition OFF. Remove the fuel pump relay from the socket.
  2. Turn the ignition ON. Do not start the engine.
  3. Use a multimeter to check for battery voltage between the B+ voltage circuit terminal of the relay socket and ground. If you read less than battery voltage, there is an open or high resistance in the B+ circuit. You will need to trace the wiring back to the fuse block.

If you have power at the socket, you can bypass the relay to test the pump itself. Connect a 15-amp fused jumper wire between the B+ circuit terminal and the supply voltage circuit terminal of the fuel pump in the socket. The fuel pump should operate. If it does not, check for open or high resistance in the supply voltage circuit and the ground circuit. If those test normal, the fuel pump itself is faulty and needs replacement.

Why Does the Fuel Pump Fuse Blow?

A blown fuse is a symptom, not the disease. If you replace the fuse and it blows again, you need to find the cause. The most common reasons are:

  1. Short circuit in the wiring. Damaged, frayed, or exposed wires can touch the vehicle body and short to ground. This is more common after rear-end collisions or if the car was improperly modified.

  2. Failing fuel pump. As the pump wears out, it draws more current. Eventually it can overload the circuit and blow the fuse. This is often accompanied by a whining noise before the failure.

  3. Clogged fuel filter. A severely restricted filter makes the pump work harder. The increased load can push current draw above the fuse rating. This is rare on a returnless system, but it can happen.

  4. Wrong fuse installed. If someone previously replaced the fuse with a lower amperage rating, it will blow under normal conditions. Always use the specified 20A mini fuse.

What to Check If the Relay Is Not the Problem

If the relay and the fuse are both good, you need to look further. The fuel pump itself may be failing, or there may be an issue with the ECM or its sensors.

  • Defective fuel pump relay: This is the most common relay-related failure. We covered this above.
  • Impact sensor (inertia switch): Some vehicles have an impact sensor that cuts power to the fuel pump in a crash. If it was accidentally triggered, the pump will not run. This is less common on the 2008 CTS, but it is worth checking if you recently hit a pothole or had a minor fender bender.
  • Engine Control Unit (ECU) problems: The ECM controls the relay ground circuit. If the ECM is faulty, or if it is not receiving ignition reference pulses from the crankshaft sensor, it will not enable the relay.
  • Fuel pressure regulator: The regulator is part of the fuel tank module. If it fails, the pump may run but you will not get proper fuel pressure. A diagnostic scan tool can read fuel rail pressure. Normal pressure is 380–410 kPa (55–60 psi).
  • Clogged fuel lines or filter: A restriction in the fuel feed pipe can cause the pump to work harder, drawing more current and possibly blowing the fuse.

If you have confirmed power reaching the pump during startup and the pump does not run, the pump itself is faulty. If there is no power reaching the pump, the issue is electrical, either in the relay circuit, the ECM, or the wiring.

How to Replace the Fuel Pump Relay

Replacing the relay is a five-minute job. You do not need any special tools, just a fuse puller or pliers and a new relay.

  1. Disconnect the negative battery cable. This is a safety precaution. The fuel pump circuit is live whenever the battery is connected.
  2. Remove the battery access door and the fuse block cover.
  3. Locate the FUEL/PUMP RLY relay. It is a small cube plugged into the fuse block.
  4. Grip the relay firmly and pull it straight out. Do not twist or rock it. The socket terminals are fragile.
  5. Compare the new relay to the old one. The terminal layout should match. Use only a relay with the same part number or an approved equivalent.
  6. Push the new relay straight into the socket until it clicks into place.
  7. Reinstall the fuse block cover and the battery access door.
  8. Reconnect the negative battery cable.
  9. Turn the ignition to "ON" and listen for the prime sound.

Fuel pump relay and fuse location in the rear fuse block
The fuel pump relay (labeled FUEL/PUMP RLY) and its 20A fuse (labeled FUEL/PUMP) are side by side in the rear fuse block.

Where to Buy a Reliable Fuel Pump for Your 2008 CTS

Once you have confirmed the relay and fuse are good, and you have verified that power is reaching the pump, the next step is likely a fuel pump replacement. This is a bigger job, and the quality of the pump you choose matters. A cheap pump may fail within a year, leaving you stranded again.

We recommend the KEMSO brand for your fuel pump replacement. KEMSO fuel pumps are built to high-performance standards and serve as direct OEM replacements. They match or exceed the factory specifications for your 2008 Cadillac CTS, so you can install them without modifying anything. Each KEMSO fuel pump comes with a lifetime warranty, which gives you peace of mind that the replacement will last.

You can browse the full KEMSO lineup and check application fitment at:

https://www.kemsoracing.com/

Preventing Future Fuel Pump and Relay Problems

A few simple habits can extend the life of both the relay and the pump:

  1. Keep the fuel tank at least a quarter full. The fuel pump uses the gasoline around it to stay cool. Running the tank dry or near-empty repeatedly can overheat the pump and shorten its life.
  2. Replace the fuel filter on schedule. A clogged filter loads the pump and increases current draw. Refer to your owner's manual for the recommended interval.
  3. Use quality fuel. Poor-quality fuel can leave deposits in the pump and the regulator.
  4. Address starting issues early. If the car cranks but takes longer than usual to start, do not ignore it. A weak pump or a failing relay will eventually leave you stranded.
  5. Inspect the wiring. If you notice burnt smells, corroded connectors, or exposed wires near the battery tray, fix them right away. Shorts in this area can blow the fuse and damage the ECM.

Final Thoughts

Finding the fuel pump relay on the 2008 Cadillac CTS is straightforward once you know it is in the trunk, not the engine bay. The rear compartment fuse block sits on top of the battery on the right side of the trunk, and the fuel pump relay (FUEL/PUMP RLY) and the 20A fuel pump fuse (FUEL/PUMP) are both inside that block.

Before you replace anything, test the fuse and the relay. A simple multimeter and a 15-amp fused jumper wire are all you need. If the relay and fuse are good, check for power at the relay socket and at the pump. If the pump is getting power and still does not run, the pump itself is the problem.

When it is time to replace the fuel pump, choose a KEMSO unit. It is a high-performance, OEM-replacement part with a lifetime warranty. Visit https://www.kemsoracing.com/ to find the correct pump for your 2008 Cadillac CTS and get back on the road with confidence.


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