Locating the Fuse and Relay
First things first, you need to find these components. In most vehicles, the primary fuse box is under the hood. It’s a black plastic box with a lid that often has a diagram showing which fuse or relay is which. There’s usually a secondary fuse panel inside the cabin, located under the dashboard on the driver’s side, in the glove compartment, or even on the side of the dashboard when you open the driver’s door. Your owner’s manual is your best friend here—it will have the exact location and a detailed diagram specific to your car’s model and year. If you don’t have the manual, a quick online search for “[Your Car Year, Make, Model] fuse box diagram” will usually yield results. The relay is typically a small, cube-shaped device plugged in alongside the fuses. It might be labeled “Fuel Pump,” “FP,” “Pump,” or “EFP.”
How to Test the Fuel Pump Fuse
Testing a fuse is straightforward and doesn’t require many tools. The most common type in modern cars is a blade fuse. Here’s a step-by-step guide with detailed data points.
Visual Inspection: Pull the fuse straight out using a fuse puller (often found in the main fuse box lid) or a pair of plastic tweezers. Never use metal tools, as this can cause a short circuit. Hold the fuse up to the light. The key component is a thin metal wire or strip visible through the plastic body. If this wire is broken, melted, or has a cloudy/blown appearance inside the window, the fuse is blown. A good fuse will have a continuous, intact wire. Studies of component failure rates show that visual inspection can correctly identify over 95% of blown fuses.
Multimeter Test for Certainty: For a definitive answer, use a multimeter set to the continuity setting or ohms (Ω). This is the most reliable method. Touch one probe to each of the two metal prongs sticking out of the fuse. A reading near zero ohms (or a beep, indicating continuity) means the fuse is good. A reading of “O.L.” or infinity (no beep) confirms it’s blown. This method is 100% accurate. Fuses are rated by their amperage, which is clearly printed on top (e.g., 10A, 15A, 20A). It is critical to replace a blown fuse with one of the exact same amperage rating. Using a higher-rated fuse can cause severe damage to the vehicle’s wiring by allowing excessive current to flow.
| Fuse Rating | Typical Color (Blade Fuse) | Purpose / Circuit |
|---|---|---|
| 10 Amp | Red | Often for low-power circuits like instruments |
| 15 Amp | Blue | Common for fuel pumps, power windows |
| 20 Amp | Yellow | Higher draw components like blower motors |
| 25/30 Amp | Green / Light Green | Heavy-duty circuits like sunroofs |
How to Test the Fuel Pump Relay
Relay testing is a bit more involved but still very doable. A relay is an electrically operated switch. It uses a small current from the ignition switch to activate an electromagnet, which then closes a separate set of contacts to carry the large current required by the Fuel Pump. A typical automotive relay has four or five pins.
The Swap Test (Easiest Method): Find another relay in the fuse box that has the same part number printed on it (e.g., a relay for the horn, A/C compressor, or radiator fan). Swap the suspected fuel pump relay with this known-good relay. If the car starts after the swap, you’ve found the problem. This is a quick, practical field test with a high success rate for diagnosis.
Multimeter Testing (Pin-Out Test): For a precise diagnosis, you’ll need the relay’s pin diagram, which is often molded into its plastic case. The standard 4-pin relay pins are:
85 & 86: Coil terminals (the electromagnet).
30: Common terminal (power input from the battery).
87: Normally Open terminal (power output to the fuel pump when activated).
Set your multimeter to measure resistance (Ohms, Ω). Test across pins 85 and 86. You should get a resistance reading typically between 50 and 120 ohms. This confirms the relay’s internal coil is intact. If you get an “O.L.” reading, the coil is broken and the relay is faulty.
Bench Test with a Power Source: This test confirms the relay’s switching action. You’ll need a 9-volt battery or a set of jumper cables connected to your car’s battery.
1. Connect the multimeter to pins 30 and 87, set to continuity. It should show no continuity (O.L.).
2. Apply the 9V battery’s positive lead to pin 86 and the negative lead to pin 85. You should hear a distinct and satisfying *click* as the relay energizes.
3. With power applied, the multimeter should now show continuity (a beep or 0 ohms) between pins 30 and 87. This means the internal contacts have closed correctly. If you hear a click but get no continuity, the internal contacts are burned out—a very common failure point due to the high current the fuel pump draws, which can be 5 to 10 amps or more.
| Relay Pin | Function | Multimeter Test (No Power) |
|---|---|---|
| 85 | Coil Ground | Resistance (50-120Ω) to pin 86 |
| 86 | Coil Power (from ignition switch) | Resistance (50-120Ω) to pin 85 |
| 30 | Power In (from battery via main fuse) | No connection to 87 |
| 87 | Power Out (to fuel pump) | No connection to 30; Continuity to 30 when 12V is applied to 85/86 |
Safety Considerations and Next Steps
Always disconnect the negative terminal of your car battery before removing or installing fuses and relays to prevent any accidental short circuits. When testing with a multimeter, be careful not to touch the probes together on active circuits. If you find a blown fuse, replacing it might get the car running, but it’s vital to ask *why* it blew. A fuse is a protective device; it blows due to an overload. A one-time event might be a fluke, but if the new fuse blows immediately or soon after, it indicates a deeper problem like a short circuit in the wiring to the pump or a failing pump motor that’s drawing too much current (amperage). Similarly, a faulty relay is often a symptom of age and wear, but repeated failures can point to underlying electrical issues. If your diagnostics point to a failing pump motor itself, that’s a more complex repair involving depressurizing the fuel system and accessing the pump, which is often located in or on the fuel tank.
Understanding the electrical demands is also helpful. A typical in-tank electric fuel pump in a modern passenger vehicle can draw between 4 and 8 amps during normal operation. However, if the pump is failing or there’s a restriction in the fuel line, the amperage draw can spike significantly, which is what leads to blown fuses or burned relay contacts. Using a clamp meter to measure the actual current draw of the pump while it’s running is a professional-level diagnostic step that can confirm the health of the pump itself.