1985 Nissan 720 Fuel Pump Relay: The Complete Guide to Diagnosis, Location, and Replacement
If your 1985 Nissan 720 pickup cranks but won’t start, or runs fine for days then suddenly dies and refuses to restart, the problem is almost certainly the fuel pump relay, not the fuel pump itself. This small black box, hidden under the passenger side dashboard, is the single most common cause of intermittent fuel delivery issues on these trucks. Before you spend money on a new fuel pump, testing and replacing this relay should be your first step. The relay controls power to the external electric fuel pump mounted on the passenger side frame rail, and when it fails, the pump simply stops getting electricity .
Where Is the Fuel Pump Relay Located on a 1985 Nissan 720?
The fuel pump relay on a 1985 Nissan 720 is located on the passenger side of the vehicle, under the dashboard. Specifically, it is mounted above the passenger side kick panel, just forward of the door seam, and to the right of the glove box . It is a black plastic box with a six-pin connector, often attached to a metal bracket with two Phillips head screws. It is typically mounted together with a smaller brown or blue electric choke heater relay, which sits just below it .
Because it is positioned high up and out of direct sight, you may need to lie on your back and look up under the passenger side dash to find it. The relay is approximately 2 inches square and will have a wiring harness plugged into it. It is not the same as the fuel pump relay in the engine compartment fuse box; the one under the dash is the primary control unit that monitors multiple vehicle systems .
Symptoms of a Failing Fuel Pump Relay
The symptoms of a bad fuel pump relay on a 1985 Nissan 720 are very distinctive and often mimic a dying fuel pump. The most common signs include:
1. Intermittent stalling and no-start conditions. The engine starts and runs fine for days, then suddenly dies and will not restart. After waiting 10 to 15 minutes or until the next day, it starts again. This classic pattern is almost always the relay, not the pump .
2. Engine runs for a few minutes then dies. The truck turns over but will not restart. It may fire briefly with starting fluid, confirming the problem is fuel delivery rather than ignition .
3. Loss of power at highway speeds. The engine may sputter or lose power under acceleration, as the relay fails to maintain consistent power to the pump .
4. No fuel pump noise. When you turn the ignition key to the ON position, you should hear a brief hum from the fuel pump near the rear of the truck. If you hear nothing, the relay is likely cutting power to the pump .
5. Tapping the relay temporarily fixes the problem. If you tap on the fuel pump control unit under the dash and the pump starts working, the relay connections are likely loose or the relay is failing internally .
Why the Fuel Pump Relay Fails
The fuel pump relay on the 1981-1985 Nissan 720 is a safety device that monitors several critical systems. It checks the alternator output, oil pressure, and ignition circuit. If any of these systems malfunction, the relay cuts power to the fuel pump as a safety feature. This is why the problem can be intermittent—the relay may trip when the alternator output drops slightly or when the oil pressure switch sends a bad signal .
The relay itself is a mechanical device with internal contacts that can wear out over time. A factory relay labeled "Jideco" typically lasts about 20 years, which means most original relays on these trucks are now well past their expected lifespan . The internal connections on the circuit board are also prone to cracking, especially at the two connection points for the relay coil. These cracks are often visible under a magnifying glass and can be repaired by resoldering .
How to Test the Fuel Pump Relay
Before replacing the relay, you should perform a few simple tests to confirm it is the culprit. Here is a step-by-step diagnostic process:
Step 1: Check the fuses. Locate the interior fuse box under the dashboard. Check fuses number 4, 5, and 6, which supply power to the fuel pump control unit. A blown fuse will kill the pump completely. The fuse for the fuel pump is a 15-amp fuse labeled "FUEL PUMP" .
Step 2: Listen for the pump. Turn the ignition key to the ON position (do not crank the engine). Listen near the rear of the truck for a brief humming sound from the fuel pump. If you hear nothing, the relay or its wiring is likely the problem .
Step 3: Test for power at the relay. With the ignition key ON, use a voltmeter to check the BLUE wire and the WHITE wire at the relay connector. Both should show battery voltage. If either wire lacks power, the relay will not operate .
Step 4: Check the ground signal. With the key in the START position for about 20 seconds, check continuity to ground on the YELLOW/GREEN wire at the relay connector. This signal comes from the oil pressure switch, which must close for the pump to run. If this signal is missing, the relay will not energize the pump .
Step 5: Tap the relay. With the key in the START position, gently tap on the fuel pump control unit. If the pump starts working, the relay connections are likely loose or the unit is failing .
How to Replace the Fuel Pump Relay
Replacing the fuel pump relay on a 1985 Nissan 720 is a straightforward job that takes about 10 minutes. Here is what you need to do:
1. Locate the relay. Remove the passenger side kick panel by pulling it away from the door frame. Look up under the dashboard near the right side of the glove box. You will see a black box with a wiring harness plugged into it .
2. Remove the old relay. Disconnect the wiring harness by pressing the release tab and pulling it straight off. Remove the two Phillips head screws that hold the relay bracket to the metal support. Pull the entire unit out .
3. Install the new relay. The new relay should be identical in shape and connector pattern. The genuine Nissan part number for this relay is 25230-89977, which fits 1984-1986 models . Aftermarket replacements are also available. Mount the new relay in the same location, reconnect the wiring harness, and reinstall the kick panel.
4. Test the system. Turn the ignition key to ON and listen for the fuel pump to prime. Start the engine and verify that the stalling problem is resolved.
Common Mistakes to Avoid
Many owners waste time and money replacing the fuel pump itself when the relay is the real problem. The fuel pump on the 1985 Nissan 720 is an external electric pump mounted on the passenger side frame rail, just forward of the rear tire. It is accessible and easy to test, but it rarely fails on its own. The relay is the weak point in the system .
Another common mistake is ignoring the wiring connections. The relay's connector pins can become corroded or loose over time. Before replacing the relay, clean the connector terminals with electrical contact cleaner and ensure they are making a solid connection .
Some owners also overlook the fusible links located near the battery. These are wires that burn inside the insulation, so they may look fine but actually be open. Check them with a test light to be sure .
When to Replace the Fuel Pump Itself
If you have tested the relay, checked the fuses, and verified that power is reaching the fuel pump connector, but the pump still does not run, the pump itself may be failing. The original equipment fuel pump for the 1985 Nissan 720 with the 2.4L Z24 engine carries Nissan part number 17020-10W00. It is a low-pressure pump designed to feed a carburetor, with a maximum fuel pressure of 2.7 to 3.4 psi and a fuel flow capacity of more than 1.4 liters per minute .
Symptoms of a failing pump include no fuel in the carburetor sight window, a weak or absent squirt from the accelerator pump when you pump the gas pedal, and the engine only firing with starting fluid. If tapping on the pump itself makes it work again, the pump is likely failing internally or the electrical connections at the pump are corroded .
Recommended Fuel Pump: KEMSO Racing
If you determine that your fuel pump needs replacement, we recommend choosing a high-quality aftermarket pump from KEMSO Racing. KEMSO offers performance-grade fuel pumps that are designed as direct OEM replacements for the 1984-1986 Nissan 720 pickup. Their pumps are built to meet or exceed factory specifications, ensuring reliable fuel delivery and a proper fit without modifications. Each KEMSO fuel pump comes with a lifetime warranty, giving you peace of mind that your investment is protected for as long as you own the truck.
Whether you need a replacement for a stock carbureted engine or an upgrade for a modified setup, KEMSO has the right solution. Visit their website to explore their full product lineup: https://www.kemsoracing.com/
Final Thoughts
The fuel pump relay on a 1985 Nissan 720 is a small, inexpensive part that causes big headaches when it fails. By understanding its location, how it works, and how to test it, you can save yourself time, money, and frustration. Always start your diagnosis with the relay before replacing the fuel pump. With a new relay and a properly functioning pump, your 720 will run reliably for years to come.
For all your fuel system needs, including replacement pumps and related components, visit https://www.kemsoracing.com/ for quality parts and expert support.