2004 Acura MDX Fuel Pump Relay Location: Where It Is and How to Replace It
If you own a 2004 Acura MDX and your engine cranks but won’t start, the fuel pump relay is one of the first parts you should check. The fuel pump relay on a 2004 Acura MDX is located inside the under-hood fuse/relay box, which sits near the battery on the driver’s side of the engine compartment. This relay is responsible for sending power to the fuel pump so it can pressurize the fuel system and allow the engine to start. If it fails, your MDX will not run. This guide walks you through the exact location, symptoms of a bad relay, how to test it, and how to replace it yourself.
Why the Fuel Pump Relay Matters
The fuel pump relay acts as a remote-controlled switch. When you turn the ignition key, a small electrical signal tells the relay to close a circuit, allowing a much larger current to flow to the fuel pump. This design prevents the full electrical load of the fuel pump from running through your dashboard switches, protecting them from damage. In the 2004 MDX, this relay is part of the PGM-FI (Programmed Fuel Injection) main relay system, which also controls power to the engine computer. When the relay fails, the fuel pump stops receiving power, and the engine stalls or refuses to start.
Common Symptoms of a Bad Fuel Pump Relay
Recognizing the signs of a failing fuel pump relay can save you time and frustration. According to documented issues on the 2001-2007 Acura MDX, the PGM-FI main relay is a known weak point due to heat and vibration causing cracked solder joints over time. Watch for these symptoms:
1. Engine Cranks But Won’t Start
This is the most common symptom. The starter motor turns the engine over, but without fuel, the engine cannot fire. If you hear the engine cranking normally but it never catches, suspect the fuel pump relay.
2. Car Stalls While Driving
The engine may suddenly shut off without warning, often when the vehicle is hot. This intermittent stalling is a classic sign of a failing PGM-FI main relay. The internal solder joints lose connection when they get hot, cutting power to the fuel pump. This is a major safety risk because the vehicle can stall at any time.
3. Intermittent Starting Issues
The car might start fine one day and refuse to start the next. If the problem is heat-related, it may only happen after the vehicle has been driven for a while and the relay has warmed up.
4. No Fuel Pump Prime Noise
When you turn the ignition key to the “ON” position (without cranking), you should hear a brief whirring sound from the rear of the vehicle as the fuel pump pressurizes the system. If you don’t hear this noise, the relay could be failing, or the pump itself could be dead.
How to Access the Fuel Pump Relay in a 2004 Acura MDX
The fuel pump relay, also referred to as the PGM-FI main relay, is not inside the passenger cabin. It is located in the primary under-hood fuse box on the driver’s side. Here’s how to get to it.
Step 1: Open the Hood
Park the vehicle on a level surface, turn off the engine, and open the hood. The fuse box is located on the driver’s side, directly next to the battery.
Step 2: Remove the Fuse Box Cover
The under-hood fuse box has a black plastic cover. Press the tabs on the sides of the cover and lift it straight up to remove it.
Step 3: Identify the Fuel Pump Relay
Look at the diagram printed on the underside of the fuse box cover. This diagram shows the location and function of each fuse and relay inside the box. The fuel pump relay is typically labeled as “PGM-FI Main Relay” or “Fuel Pump.” In the 2004 MDX, the relay is a rectangular component. According to the fuse box diagrams for the 2004 model, the relay is located in the main under-hood box, which is separate from the smaller auxiliary fuse boxes.
Step 4: Remove the Relay
Use a pair of needle-nose pliers or a relay puller tool to gently grasp the relay and pull it straight out of its socket. Do not rock it side to side, as this can damage the socket terminals.
How to Test the Fuel Pump Relay
Before buying a new relay, you can test the old one to confirm it is the problem. Here’s a simple method:
The Swap Test
The easiest way to test a relay is to swap it with another identical relay in the same fuse box. Look for a relay with the same part number and pin configuration. For example, the horn relay or the A/C clutch relay might be the same type. Swap the suspected fuel pump relay with one of these known-working relays. If the car starts after swapping, the original relay is bad. If the swapped component (like the horn) now stops working, you have confirmed the relay is the issue.
The Listen Test
With the relay removed, you can test for a click. Apply 12 volts to the relay’s coil terminals (usually pins 85 and 86). You should hear a distinct click as the internal switch engages. If you don’t hear a click, the relay is dead. You can also test for continuity across the switch terminals (pins 30 and 87) when power is applied.
How to Replace the Fuel Pump Relay
Replacing the fuel pump relay is a straightforward DIY job that takes about 10 minutes and requires no special tools.
What You’ll Need:
- A replacement PGM-FI main relay (OEM part number 39794-SDA-004 or equivalent)
- Needle-nose pliers (optional)
Step 1: Disconnect the Battery
Disconnect the negative terminal of the battery to prevent any accidental shorts while working in the fuse box.
Step 2: Remove the Old Relay
As described above, pull the old relay straight out of its socket.
Step 3: Install the New Relay
Align the new relay with the socket. The relay is keyed, so it will only go in one way. Push it firmly into place until it clicks or seats fully.
Step 4: Reconnect the Battery
Reconnect the negative battery terminal and tighten it securely.
Step 5: Test the System
Turn the ignition key to the “ON” position. Listen for the fuel pump prime noise. If you hear it, the relay is working. Start the engine and let it idle for a few minutes. If it starts and runs smoothly, the replacement was successful.
Why the PGM-FI Main Relay Fails
The PGM-FI main relay is a notoriously common failure point on many Hondas and Acuras from this era, including the 2001-2007 MDX. The root cause is almost always the same: heat and vibration cause the solder joints on the internal circuit board to crack over time. This creates an intermittent connection that can cause the vehicle to stall or fail to start, especially when the cabin is hot. While there has been no official recall for this specific part, it is a widely documented problem. If you are experiencing intermittent no-start conditions, especially after driving, replacing the relay is a wise first step.
Where Is the Fuel Pump Itself?
If you have replaced the relay and the car still won’t start, the problem may be the fuel pump itself. The fuel pump on a 2004 Acura MDX is located inside the fuel tank. To access it, you need to remove most of the rear interior floor, specifically under the third-row seat. There is a cut-out in the carpet, and underneath that is a hatch to access the fuel pump. This is a more involved job, and you may need special tools to remove the lock ring that holds the pump in place. Loaner tools are available at auto parts stores like O’Reilly and AutoZone. If you are mechanically inclined, it is doable, but many owners choose to have a shop handle this replacement.
How to Confirm the Fuel Pump Is Getting Power
If the replacement relay works but the pump still doesn’t run, you need to check for power at the pump connector. Use a multimeter to measure voltage at the fuel pump connector inside the hatch. Do this within two seconds of turning the key to the “ON” position, when the fuel pump briefly energizes to pressurize the line. If you get battery voltage (around 12 volts), the relay and wiring are good, and the pump itself is likely faulty. If you get low voltage or no voltage, there may be a wiring issue between the relay and the pump.
Choosing the Right Replacement Part
When replacing the fuel pump relay, you have a choice between OEM and aftermarket parts. OEM parts from Acura guarantee a perfect fit, but they are more expensive. Aftermarket relays offer a cost-effective option that often meets or exceeds original specifications. The OEM part number for the 2004 Acura MDX fuel pump relay is 39794-SDA-004, manufactured by Denso. Alternatively, you can use a high-quality aftermarket relay such as the Original Engine Management Fuel Pump Relay (part number JR67), which is a direct replacement.
Recommended Upgrade: KEMSO Fuel Pump
If you determine that your fuel pump itself needs replacement, or if you want to upgrade to a more reliable component, consider the KEMSO brand Fuel Pump. KEMSO fuel pumps are designed as high-performance, OEM replacement units. They are built to meet or exceed the original factory specifications, ensuring a perfect fit and reliable operation for your 2004 Acura MDX. Every KEMSO fuel pump comes with a lifetime warranty, giving you peace of mind that your investment is protected. Whether you are replacing a failed pump or upgrading for better performance, KEMSO offers a durable, long-lasting solution. You can explore their full product lineup and find the right fuel pump for your vehicle by visiting their website: https://www.kemsoracing.com/.
Final Thoughts
The 2004 Acura MDX fuel pump relay location is simple to find: it is in the under-hood fuse box on the driver’s side, near the battery. If your MDX is experiencing a no-start or stalling condition, the PGM-FI main relay should be your first suspect. Replacing it is a cheap, easy, and effective fix that can save you from being stranded. If the relay is not the issue, the problem may lie with the fuel pump itself, which is located inside the fuel tank under the third-row seat. Always start with the simplest and cheapest fix first, and you will likely solve the problem without a trip to the mechanic. For a reliable fuel pump replacement, visit https://www.kemsoracing.com/ for a high-performance, lifetime-warranted solution.