Engine start bypass...

[marty1mc]

I just finished a rewire to allow me to crank my motor without it starting. Since the Z only gets driven once or twice a week and sometimes not even once a week, I wanted a way to be able to crank the motor and not have it start. I looked at a few threads on various sites. Most just put an a switch on the fuel pump relay and call it a day. However, residual fuel pressure can allow the car to start for a few seconds. That defeats cranking the motor to allow oil to move through the crank and rod bearings. So, I tried a few different things...

Things I tried that didn't work...

1). Interrupt pin 58 (W/B from ECU) with a switch. Since cutting this wire and moving it to pin 45 was written up as the way to change the voltage from unswitched to switched at the injectors, I reasoned the ECU was supplying the voltage. However, cutting this wire made no difference at all! The engine started and ran without pin 58 in the circuit (switch off). I suspected this may be the case since the beginning of the circuit is connected to the fusible link box. This also makes me suspicious of the rewire post for switching injectors from battery voltage to switched voltage. Without removing the fusible link from the circuit, I don't see how that procedure does anything but crossfeed circuits.

I also looked at another writeup on isolating the injector wires (W/B) by digging into the harness 6-8 inches from the blue connector, cutting the three injector wires out of a crimp bundle there and moving those to a switch. This is the same pin 58 white/Black (W/B) wire, but further up the harness. However, my harness is new. The new harnesses have moved this point from 6 - 8 inches from the blue connector to the other side of the firewall, very near the injectors. It was an EC Nissan put in to save wire and all new harnesses (90-94) have it. Therefore, I would have to open the harness on the engine side and run wire back into the cabin to the switch. I didn't like that idea at all.

2). ECCS Relay - I pulled the relay to see if the ECU would be dead, allow me to crank the car. I would then rewire the pick side of the relay and be done. Again, the car started and ran without the relay plugged in. I honestly don't see the purpose of this relay.

What worked:

I ended up modifying two circuits - The fuel pump relay and the ignition coil relay. The ignition coil relay by itself will do the trick. When I pulled it, the motor doesn't start. However, the injectors are still pumping fuel and that ain't good either. I could smell fuel odor when I stopped. The car would start, but I don't want to fuel wash my cylinders each time I crank for oil pressure. I wanted to stop both spark and fuel. While the residual pressure can still find it's way into the cylinder, since the fuel pump will be dead, it isn't that much. By taking the ignition coil out of the circuit, the residual fuel won't fire the motor anyway. It isn't the exact solution I was looking for, but it works well enough.

So, the modification turned out to be fairly simple. I needed a Double pole single throw switch (Radio Shack), some wire, shrink tubing, solder and a couple of hours to do this. The circuits to be modified are as follows:

Spark -
1) Ignition coil relay - Very simple mod - cut the orange wire at pin 16 on the ecu, solder wire from both ends to the DPST switch. The switch will now interrupt the ignition relay from picking and there will be no spark. I cut it about 6 inches up so the solder joints would be outside the plastic cover for the blue connector. MAKE SURE YOU HAVE PIN 16, not any other pin. There is more than one orange wire on that connector. Pin 16 is easily identifiable. It is in the middle connector bundle. It has a blank slot in front of it (pin 15) and two blank slots behind it (Pins 17 and 18).

Fuel -

2). Fuel pump relay - the relay is located in the driver's side kick panel. When you look at the socket for the relay, you will see 2 White/Black wires and one black/pink wire. The smallest diameter wire (B/P) is the pick for the relay. The other two fat ones are the power wire for the pump. Simply cut the skinny B/P (Black/Pink wire, solder in wire to both ends and route it to the other pole of the DPST switch.

3). Find a suitable spot for the switch. I prefer to hide mine as it can also act as an anti-theft device.

That's it. Now, when I throw the switch, I can crank the motor until I see the oil pressure begin to rise. I don't smell a ton of gas and the car never fires due to residual pressure in the fuel rail.

If there is enough interest, I can modify this writeup with pics and make it a complete how-to. I hope this helps some members. It drove me nuts trying to find a way to keep the car from kicking over so I could build oil pressure.

ETA: after looking at the wires in the daylight and in the diagrams, the pick wire for the fuel pump relay is actually Black/Pink, not black/white. The above was changed to reflect this...

[minlow88]

I didnt read all of that cause there was alot. But what first came to my mind would be to just kill the injectors. But also dont you think it would be hard on the starter and then some to just continue cranking it? and if you do kill the gas you would need to kill spark to right?

I would just start it and let it idle for about 20 min. call it good and park it til next week.

[marty1mc]

Sorry for the long post, but killing the injectors is exactly what I set out to do. Unfortunately, I have a new harness. Nissan EC'd the thing and moved the injector wires (+12v side) connections from inside next to the ECU to outside on the back of the motor. I would have had to run wire from the middle of the engine to the switch inside the cabin and back. Plus, I would have had to open a good portion of the harness behind the engine to find the connection. The connection I am referring to mates the 3 injector wires (+12v side) to the voltage source and has to be broken away from the voltage source to work.

[carsonsowers]

This is of no use since you already figured a way out, but you could have run a wire from hot wire on your starter to a switch and back. then hit the switch without turning on the key. The guy that had mine did it because I think the starter relay was bad. It cranks with no key but needs the key to start.

[marty1mc]

I also looked at wiring the starter relay on a switch. But, I prefer a setup that can't damage the motor. If that switch accidentally gets hit or bumped while the motor is running, it can damage the flywheel and starter. The flywheel is expensive, so for a little more effort, I could do it properly.