I have had a number of email requests for this and keep running into people (usually JWT types) that are having problems "keeping it up" in terms of fuel pressure, due to maintaining the stock fuel pump power supply. I know this is old hat to many of you, but perhaps someone will get some mileage out of it. Feel free to linkinate or post anywhere you want.
The very important thing to note about stock Z31 fuel pump circuit is that it does NOT supply a constant 12 v to the pump if you leave it stock. The ECU controls how much ground it gets, and varies the voltage. With my fuel pressure guage in the car, its obvious just how much it varies your pressure. You need to have a constant source of nice clean 12v to the pump in order to maintain the increased base pressure and anything beyond that. Here's a simple circuit modification that corrects the fluctuating pump voltage problem. I have mine mounted back in the spare tire well.
What you need:
Standard relay that is a normally closed switched side. (I am using layman's terms here)
10 or 12 guage wire
Four female spade connectors
Solder iron, solder, electrical tape or….crimp connectors and shrink tubes
1/2 of a brain (optional)
A sheetmetal screw (optional)
Take a blue Nissan relay (everyone should have a box of these things lying around - they are very useful, and "free" at any junkyard), or any standard relay. What you want to do is to create a ground source that is controlled by this relay. I have tapped into my power antenna power supply to activate the switch, since I don't use the power antenna, you can also tap into the power supply for the fuel tank sending unit, or even the power supply to the fuel pump itself (this is the most efficient, but I didn't think of that until I had already sourced it from the antenna). Either of those are hot signal +12 when the ignition is on, and the ecu ground will be controlling when the relay opens anyway. So you activate your relay with that power source. Cut the ground wire that runs from the fuel pump through the harness to the ECU (umm-yeah it is black it think, use an ohmmeter to make sure). (leave plenty of room to solder connectors on both sides). (Use fairly heavy guage wire 10 or 12) Solder/crimp a ground wire to the ECU side of this ground wire and run it to the power supply (the actuator) side of the relay (this will be the ground that completes your circuit from the positive hot ignition source earlier). On the fuel pump side of the ground wire, solder in a wire and connect to one side of the switched section of the relay. On the other side of the switched section of the relay, run another heavy ground wire to any good body ground. I just use one of the screws that goes to the cover on the sending unit.
So now what you have is that when you turn the key ON, the relay gets power from the positive source, and the ECU activates the ground side. This throws the relay switch that allows the body ground to reach the fuel pump and complete the circuit.
We have maintained the fuel ground to the ECU so it thinks everything is normal and just does its thing - it's loaded with the power supply from the relay. Now you will have 12volts full power to your fuel pump whenever the car is on (for five seconds) or running. The reason you have to use a relay is since the fuel pump positive is always on, even when the car is off. The stock fuel pump circuit only gets completed by the ecu when the ecu grounds that wire.
There are other ways you can design this circuit but this one has built in safety: If you wire your pump to run full time whenever the ignition switch is on, you could find yourself in a situation where you are upside down unconscious in a ditch and your fuel pump is still running. NOT GOOD.
This circuit allows the ECU to control the relay. The ecu is pretty smart, you notice how on turning on the key, the pump runs for five seconds and shuts off. Similarly, if the engine stalls out, the ecu shuts off the pump. So by wiring it this way you don't have to listen to the pump running if you are just checking something with the ignition on, and you reduce the chances of dieing in a fiery blaze if there is an accident, but you still get your full power to the fuel pump.
Here is a cheesy diagram:
http://devster.net/roni/86z/Z31%20Fu...plications.doc
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The very important thing to note about stock Z31 fuel pump circuit is that it does NOT supply a constant 12 v to the pump if you leave it stock. The ECU controls how much ground it gets, and varies the voltage. With my fuel pressure guage in the car, its obvious just how much it varies your pressure. You need to have a constant source of nice clean 12v to the pump in order to maintain the increased base pressure and anything beyond that. Here's a simple circuit modification that corrects the fluctuating pump voltage problem. I have mine mounted back in the spare tire well.
What you need:
Standard relay that is a normally closed switched side. (I am using layman's terms here)
10 or 12 guage wire
Four female spade connectors
Solder iron, solder, electrical tape or….crimp connectors and shrink tubes
1/2 of a brain (optional)
A sheetmetal screw (optional)
Take a blue Nissan relay (everyone should have a box of these things lying around - they are very useful, and "free" at any junkyard), or any standard relay. What you want to do is to create a ground source that is controlled by this relay. I have tapped into my power antenna power supply to activate the switch, since I don't use the power antenna, you can also tap into the power supply for the fuel tank sending unit, or even the power supply to the fuel pump itself (this is the most efficient, but I didn't think of that until I had already sourced it from the antenna). Either of those are hot signal +12 when the ignition is on, and the ecu ground will be controlling when the relay opens anyway. So you activate your relay with that power source. Cut the ground wire that runs from the fuel pump through the harness to the ECU (umm-yeah it is black it think, use an ohmmeter to make sure). (leave plenty of room to solder connectors on both sides). (Use fairly heavy guage wire 10 or 12) Solder/crimp a ground wire to the ECU side of this ground wire and run it to the power supply (the actuator) side of the relay (this will be the ground that completes your circuit from the positive hot ignition source earlier). On the fuel pump side of the ground wire, solder in a wire and connect to one side of the switched section of the relay. On the other side of the switched section of the relay, run another heavy ground wire to any good body ground. I just use one of the screws that goes to the cover on the sending unit.
So now what you have is that when you turn the key ON, the relay gets power from the positive source, and the ECU activates the ground side. This throws the relay switch that allows the body ground to reach the fuel pump and complete the circuit.
We have maintained the fuel ground to the ECU so it thinks everything is normal and just does its thing - it's loaded with the power supply from the relay. Now you will have 12volts full power to your fuel pump whenever the car is on (for five seconds) or running. The reason you have to use a relay is since the fuel pump positive is always on, even when the car is off. The stock fuel pump circuit only gets completed by the ecu when the ecu grounds that wire.
There are other ways you can design this circuit but this one has built in safety: If you wire your pump to run full time whenever the ignition switch is on, you could find yourself in a situation where you are upside down unconscious in a ditch and your fuel pump is still running. NOT GOOD.
This circuit allows the ECU to control the relay. The ecu is pretty smart, you notice how on turning on the key, the pump runs for five seconds and shuts off. Similarly, if the engine stalls out, the ecu shuts off the pump. So by wiring it this way you don't have to listen to the pump running if you are just checking something with the ignition on, and you reduce the chances of dieing in a fiery blaze if there is an accident, but you still get your full power to the fuel pump.
Here is a cheesy diagram:
http://devster.net/roni/86z/Z31%20Fu...plications.doc

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