Electric fuel pump conversion question

[Jasonxp]

Hello, great to be a part of this forum. Been coming here for answers on getting my project '74 260z running. The old engine is shot and I am replacing it with a L28, but I am using the carbs from the old engine. As the L28 doesn't have accommodation for a mechanical fuel pump, I was installing a new electric one when I realized the FSM says the electric fuel pump only activates when the engine is above 400 rpm. Is there something I need to do to change this to make the engine start correctly? Modify a relay maybe? I appreciate your help. 

[sweatybetty]

12 volts to the fuel pump when the ignition switch is in the "run" position. that is the most basic, simple answer there is

Edited January 17, 2017 by sweatybetty

[Zed Head]

He's just asking how to be sure the pump will supply his carbs so that his engine will start and run.

Once the carb's float bowls get filled after the first start it will probably be fine.  It will start on float bowl fuel.  BUT, study the FSM description closely again.  Usually Nissan bypassed the RPM or oil pressure or AFM (for EFI engines) control when the key is at Start.  So there might not actually be a problem.

SteveJ is a 260Z guy.  He might know.  @SteveJ.  

[SteveJ]

Thanks for the heads up on this thread, @Zed Head. 

The OP has read the manual, and I think he understands the operation as designed.

The electric fuel pump circuit in the 260Z was designed to run the electric fuel pump only when the car is running. The Nissan engineers intended for the mechanical fuel pump to handle the starting procedure. If you look at page EF-7, you will see that relay identified by number 4 is a normally closed contact. (In the Nissan convention, the bar on the switch is pulled toward the coil. If the contacts are between the bar and coil, the relay is normally open. If the bar is between the contacts and the coil, the relay is normally closed.) When the key is in start, the fuel pump will not run.

So, if you want to run the fuel pump while starting the car, here is what I would do. (YMMV)

1. Add a relay that in parallel to the relay labeled number 3 in Fig EF-15. The 85 pin on the relay should be wired to the black/yellow wire coming off of the ignition switch. I would tap into it around connector C-5. That is near the fuel pump relay. Ground the 86 pin. Jumper the wire on the 85 pin over to the 87 pin. At Electric Fuel Pump Relay #1, (This relay has a 4 wire connector) tap into the green/yellow wire and take it to the 30 pin on the new relay. Consider installing a 10A inline fuse before the relay to cut power if something is shorted out.

2. On the existing fuel pump relay #2, jumper together the black/white and green/yellow wires. Better, yet, install an inertia switch between those two wires. That will kill the fuel pump power in an accident. It will be easy to access in the passenger compartment, too.

Note: If you convert your car to an internally regulated alternator, you'll take away the signal for the coil on the Electric Fuel Pump Relay #1. You'll need to bring in a switched source for power, AND the inertia switch becomes mandatory. Adding in an oil pressure switch isn't a bad idea, either.

By the way, thank you for bringing this up, @Jasonxp. I have to do the same thing to my car for essentially the same reason. If you're lucky, I'll remember to document this with photos.

 

 

[Captain Obvious]

Clearly academic, but just in case anyone wants to know... That "400 RPM" cutout is a bit of a misnomer.

They use the "N" output from the alternator to pull in the fuel cut relay #1, and that "N" output from the alternator is actually the "Neutral" connection of a three phase "Y" connected generator. The voltage on this neutral output is the normal alternator output voltage divided by the square root of three. So for example, when the alternator output is twelve Volts, the neutral wire should have about seven Volts on it:

Datsun assumes that by the time the engine is spinning at 400 RPM or higher, the alternator is up and operating, and the voltage on the neutral output is high enough to pull in the fuel cut relay.

So the "400 RPM" threshold is a little arbitrary, but the intent is "when the engine is running, the electric fuel pump will run". Geek mode off... 

[Zed Head]

Whoa....  I broke a sweat just reading those two posts.  And we're in the middle of an ice-storm.

 

He could also just carry a can of starting fluid.

Edited January 17, 2017 by Zed Head

[SteveJ]

I was looking at the oil pressure sensors for the 78. Since they are designed to work with a fuel pump relay, I figured I would try one out. 

[SteveJ]

2 hours ago, Captain Obvious said:

Clearly academic, but just in case anyone wants to know... That "400 RPM" cutout is a bit of a misnomer.

They use the "N" output from the alternator to pull in the fuel cut relay #1, and that "N" output from the alternator is actually the "Neutral" connection of a three phase "Y" connected generator. The voltage on this neutral output is the normal alternator output voltage divided by the square root of three. So for example, when the alternator output is twelve Volts, the neutral wire should have about seven Volts on it:

Datsun assumes that by the time the engine is spinning at 400 RPM or higher, the alternator is up and operating, and the voltage on the neutral output is high enough to pull in the fuel cut relay.

So the "400 RPM" threshold is a little arbitrary, but the intent is "when the engine is running, the electric fuel pump will run". Geek mode off... 

Interesting. I wasn't thinking about the neutral was floated. I'm so used to the neutral being bonded to the ground in low voltage applications I work with. Of course, shipboard power is usually done in the fashion you described. 

[Jasonxp]

Thanks for the replies, especially @SteveJ and @Captain Obvious, I really appreciate all the technical info... been searching the web for awhile to no avail. 

[gnosez]

Might I say, I have nothing to add or for that matter had any useful information to provide to start with, but wanted to commend SteveJ, the Captain and our new member Jason for showing why this is a great forum.

A request for help written in a manner understandable to others and then really great replies that even I understood that provided a workable answer.

(Insert the sound of two hands clapping)

It's been a horrible day and this cheered me up.

 

Edited January 18, 2017 by gnosez

[Zed Head]

Nada for moi?  

[SteveJ]

19 minutes ago, Zed Head said:

Nada for moi?