Brand New 260z Owner Looking for Fuel Pump Relay Help :c

[zee.rowe]

Hello, everyone. I just acquired a 1974 260z in December 2021 (I believe later 74 because of the bumpers???). I finally got a dream Z!! It's rough around the edges, but I've been chipping away at the thing to get it to a daily ish car.

Problem is, I hit a hard wall to overcome. I have pretty much no electrical experience with cars. I've done the small "wire the horn into the aftermarket race wheel" job, and a cruise control wiring chase with an e46. Pretty light work. Well, I jumped head first into what I consider intense wiring: Adapting newer relays to the fuel pump system. I only have one of the two original relays, and the previous owner says the one I have is also fried. Might as well replace them with more modernized ones, right? It seems to be a popular adaptation, but I've found nothing on this modification.

When I say I'm completely new to electrical, I'm very new. This took me days to come up with a circuit because I didn't know about relays and figuring out what each wire means, and reverse engineering the one old relay I have to figure out which wire was going to be the power, trigger source, coil contacts, and even what NC and NO means... So I guess let's jump into my drawings here and uh, please help me out (SteveJ, I saw you help someone else with a similar problem with the same relays, I hope you're still around LOL).

The first image is a compacted redraw from the original diagram. It's the basic wiring of what each wire was leading to from the relays (excluding some branches that weren't necessary to the fuel pump relays, I think). The second image is about the one original relay that I do have that I peeled open. I'm pretty sure it's Relay #2, because it's wired to the NC contact, not the NO. With these 2 images, I was able to more or less connect the dots and draw up the third image: the adaptation of the new relays and where the wires on the Z go to them. Again, I'm new to Z's and wiring, so please be patient with me, but is this right? I really don't want to fry anything by wiring the wrong wire to the wrong post, so I'm trying to eliminate the guess work.

The ignition switch goes to Common Contact (or wiper, or 30 post, or the switch?) connected to empty 87. Yellow from alternator is coil positive and ground is coil negative. NO is green/white heading to Relay #2 Common Contact with NC contact leading to fuel pump. NO is empty. Yellow is from an emergency switch and is coil positive, ground still coil negative.

I'm pretty sure this is right, but I could be dangerously wrong, which, again, is why I'm asking for help at last. I tried to tackle this alone, I tried scraping every forum for any information, I really got nothing. A good chunk of what I got so far was also from SteveJ helping another person with a similar problem. I can't do this alone anymore, and I could do with some electrical wisdom. Thank you.

[Patcon]

@SteveJ

[SteveJ]

Just now, Patcon said:

@SteveJ

What? Do you think I can just spit out a response?  😉 I've been working on it for about 45 minutes now. 

[SteveJ]

Let's go over the theory of operation in an effort to help explain what is going on.

When Nissan changed carburetors between the 72 and 73 model years, more complaints were received about vapor lock. Nissan implemented a couple of "fixes".

1. Insulate the heck out of the fuel rail including the individual hoses from the rails to the carburetors. (Mmm, asbestos!)
2. Add a helper electric fuel pump to the mechanical fuel pump to push cool fuel through the lines blocked by the higher pressure vapor.

When the car is starting, it uses the fuel in the float bowls, and you don't need fuel pouring into the float bowls. As a matter of fact, it is easier to push the vapor if the carburetors have used some fuel during starting, lowering the float to open the float valve. 

So now let's look at the fuel pump relay circuit and talk about what happens. The drawing is from page EF-7 in the 74 FSM. I enhanced the descriptions so you can see it better.

 

Let's first talk about Fuel Pump Relay #2 (FPR2). The coil of FPR2 is energized when the key is in the START position. When the coil is energized, the fuel pump circuit opens, preventing the operation of the fuel pump. When you release the key to allow it to go from START to ON, the coil for FPR2 de-energizes, the contact closes, and that part of the circuit is completed again.

Now let's go to Fuel Pump Relay #1 (FPR1). The coil of FPR1 is energized by the neutral of the alternator. So what is the voltage of the neutral? Let's go down the rabbit hole a little bit.

If you measured the voltage coming off the windings of the alternator you would see AC voltage, specifically 3 phase AC voltage. In power generation terms, we call the configuration of the alternator WYE because each of the three phases connects to a common "neutral" point. As you can see, when I rotated the first image, it looks like the letter "Y". If you ground the neutral, the voltage from neutral to ground becomes zero, but the neutral isn't grounded in our cars. Now, the output from the alternator goes through a rectifying bridge. This uses diodes to stop the current from flowing in the reverse direction, and with the 3 phase waveforms, the output is close enough to a DC square wave that it is usable to the DC electrical system.

The output of the alternator is about 14.5 volts. Knowing that, we can calculate the voltage to ground. (You may have seen @Captain Obviousand I discussing that on another thread. He keeps me from going down the wrong rabbit holes.) To get the neutral to ground voltage, we can divide the alternator voltage by the square root of 3 (1.732), giving us about 8.3V. That is enough to energize the coil of FPR1.

With the key in ON and the alternator turning, FPR1 completes the full circuit to the fuel pump. (With the key in ON, FPR2 is not energized, so the contact is closed, allowing current to flow through it.)

If the engine dies, the alternator stops turning. The voltage from neutral to ground goes to zero, causing FPR1 to open the fuel pump circuit even if the key is on. There is no separate emergency switch.

So here is the maybe useful trivia. You may notice posts saying that instructions for going to an internally regulated alternator won't work on a 260Z. This is because of how FPR1 is energized. If you change the wiring for an internally regulated alternator, you lose the neutral output from the alternator. This means you would need to find another source for the FPR1 coil and probably incorporate an inertia switch into the circuit for safety.

Edited January 22, 2022 by SteveJ

[zee.rowe]

Ok, so, the alternator has 3 phases in AC voltage, a rectifying bridge to prevent reverse flow, and by going through the rectifier, you get a DC wave too?

From what you're saying, FPR1 is energized by the neutral of the alternator (8.3V) which completes fuel pump circuit, and FPR2 is energized when the key is in the start position, because the carb doesn't need more fuel on startup...?

And WHEN the car is ON with the alternator turning, then FPR1 is going, and so is FPR2 because at that point, FPR2 is not energized, and FPR1 is, both completing the circuit? Does that mean the wiring that I designed is correct or...?

And I don't know what the emergency switch is, that's just what it says in the diagram I received from the previous owner hahaha...

[SteveJ]

42 minutes ago, zee.rowe said:

Ok, so, the alternator has 3 phases in AC voltage, a rectifying bridge to prevent reverse flow, and by going through the rectifier, you get a DC wave too?

If you're looking at the output with an oscilloscope, you would see the ripples, but with the 3 phase full wave rectifier setup in the alternator, we can think of it as just a DC output. 

42 minutes ago, zee.rowe said:

From what you're saying, FPR1 is energized by the neutral of the alternator (8.3V) which completes fuel pump circuit, and FPR2 is energized when the key is in the start position, because the carb doesn't need more fuel on startup...?

And WHEN the car is ON with the alternator turning, then FPR1 is going, and so is FPR2 because at that point, FPR2 is not energized, and FPR1 is, both completing the circuit? Does that mean the wiring that I designed is correct or...?

And I don't know what the emergency switch is, that's just what it says in the diagram I received from the previous owner hahaha...

When the key is in ON with the engine running, both FPRs complete the circuit for power to the fuel pump.

I do not know why the previous owner made the drawing you included. What I described is the stock wiring and is shown in the image from the factory service manual. There is an "Emergency Switch" in the 260Z, but it is bypass the (*gag* *choke* *puke* 🤮) Seat Belt Interlock Relay on a one-time basis. Most 260Z owners (and any other owners of 1974 model year cars) have bypassed the interlock relay, negating the need for the emergency switch.

[zee.rowe]

Seatbelt Interlock Relay???

3 minutes ago, SteveJ said:

When the key is in ON with the engine running, both FPRs complete the circuit for power to the fuel pump.

So that means my wiring is somewhat right, besides the weird yellow wire leading to what is saying is an emergency switch for FPR2???

[SteveJ]

12 minutes ago, zee.rowe said:

Seatbelt Interlock Relay???

So that means my wiring is somewhat right, besides the weird yellow wire leading to what is saying is an emergency switch for FPR2???

The last part of the diagram that shows the common relays is correct except I would call the yellow wire "starter solenoid" instead of "emergency switch"

You can also buy connectors from Vintage Connections to match up to the connectors in the wiring harness.

 

[zee.rowe]

Thank you for your help, SteveJ! I put it in my notebook that the "Emergency Switch" is the Starter Solenoid, and thank you for the link to the vintage connections. I have a feeling that will be useful when I clean up the rest of the harness on this Z. Feels pretty ratty under the passenger dash. I also really appreciate the additional electrical knowledge.

[SteveJ]

You are welcome. I just ask that you pay it forward to others when you have the chance.