Replaced Oil Sending Unit Now Fuel Pump Runs Continuously
1981–1985 Nissan and Datsun Pickup Fuel Pump Problems
Dan has been an auto mechanic, truck driver, engineer, and many other things.
Look under here!
Nissan Hardbody Fuel Pump Problems
This article is for owners of the 1981–1985 Datsun or Nissan 720 pickups with the electric fuel pump. On this page, you will learn how to go beyond the typical service manual fuel delivery problem diagnosis.
If, like me, you have trusted in your Haynes manual to troubleshoot a fuel delivery problem, you may have found that nothing seems to work and the problem persists. You've checked the fuse (#2) and it's good! You've checked the output of your electric fuel pump (just in front of the passenger side rear tire) and find that it works most of the time—or maybe it only works if you power the pump with a jumper wire! Yet, your pump just won't put fuel to the carburetor when it's supposed to . . . or it will for a while, but then the truck dies and leaves you stranded for 10–15 minutes, or until tomorrow.
It is likely that the trouble lies in your fuel pump relay. Now I know that the book doesn't say anything about a fuel pump relay, and many mechanics don't even know about it (they're probably using the same book). I'm here to tell you that the relay exists and should be called a fuel shutoff relay. You will also find that the auto-choke relay is mounted on the fuel pump relay. Trust me, it exists (don't believe Chapter 13, figure 5.2b). I have a 1981 Datsun and a 1984 Nissan and they both have this troublesome black box. Your repair guide not only ignores it, but it is conspicuously absent from the wiring diagrams, making diagnosing and repairing electric fuel pump problems virtually impossible. I tore apart my wiring harness to discover it and later found other shade-tree mechanics discussing it in a few online forums.
To find it, look under the dash on the passenger's side. It is just visible above the kick panel in front of the door and will have the auto choke relay attached, and in automatic trucks, the fuel cut solenoid.
Look closer
The fuel pump relay itself has a 6-pin connector going into it. Those are six wires that you will not find on your wiring diagrams. Do not try to remove the harness from the relay unless you have forearm muscles like Popeye. Remove the attached relay(s) and then undo the two screws holding the fuel pump relay to the panel. I found it easiest to remove the knee bar and heater blower motor first, but if you have very short screwdrivers and small hands, you can probably manage less disassembly than I did.
B=black, W=white, L=Blue, G=green, and Y=yellow.
Wiring Diagram of Relay
These six mystery wires monitor several important systems in your truck so that if any one of them fails, the electric fuel pump will not run. Before you unbolt the relay and spend $50 or more to replace it, first make sure that these systems are functioning correctly!
They are:
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- The ignition switch accessory position (power to blue wire when on).
- The ignition switch start position (power to white wire at start using fuse #5).
- The alternator (power to white/blue wire when running).
- The oil pressure switch or oil sending unit (4WD) (yellow/green).
This is a wiring diagram for the fuel pump relay terminal that I created using the Haynes color codes found in my harness. Hopefully, yours is the same. Use this to check continuity in your wiring. It's better to find a broken or corroded wire now than to replace the relay and still have the same problem.
Repair Manual Discrepancies
I've noted some discrepancies in my Haynes manuals which may or may not apply to yours as well.
- The wire from this relay to the body harness connector (ultimately to the fuel pump) is white/black. Why Nissan used a white/black wire from the relay to the body harness connector (found under the carpet beneath the passenger's seat) is a mystery to me. All other fuel-related wires throughout the truck are brown as is the connecting terminal in the body harness connector which goes directly to the fuel pump.
- The white wire connects to the fuse block at fuse #5. In my two Haynes books (chapter 13, figure 3.1b), fuse #5 is shown to be empty. I'm currently running a 10A fuse in this position and haven't had any trouble. Also, pulling this fuse doesn't seem to shut off any start features other than the fuel pump (via the relay).
- Not related to the fuel pump relay so much (but since I've breached the subject) the wire out of fuse #1 (ignition coil no. 2) should have been listed as WB and not WY. All other diagrams seem to show it correctly.
Test Your Fuel Pump Again
If you have not already tested your fuel pump by running a jumper wire to it, you can do it now. Build a short jumper wire and connect the blue terminal to the white/black terminal. When you turn the ignition over to the accessory position, you should now hear the fuel pump running constantly (not recommended as a permanent fix)!
If you decide to buy a replacement, go with quality parts.
The red arrows indicate the contacts that probably need repaired.
The Fuel Pump "Shutoff" Relay
Assuming you've narrowed down your problem to this relay, you can order a new one from an online parts warehouse or your local dealer. However, you may be able to save a few dollars. What have you got to lose trying? The relay is already not working and you're a do-it-yourselfer after all!
- Here's your little trouble-maker once you've removed it from the truck. A factory relay is labeled Jideco, and if you choose to replace it, they seem to last about 20 years.
- On the back of the assembly are four Phillips screws. Once removed, the cover and insulator come off and you're looking at the circuit board of the relay itself. On the opposite side are the resistors, amps, and the big coil that should trip the relay when all is well—but has probably failed. If you look at the solder points under magnification, you may notice cracks. If you're careful, you can resolder these contacts and have your relay working again in a few minutes.
- Now put it all back together, pat yourself on the back, and go for a drive. You just potentially saved yourself hundreds of dollars at the dealer!
This article is accurate and true to the best of the author's knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
© 2009 Dan
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