Need clarification on Fuel Pump wiring for 2000 Miata
 

I want to double check some information concerning doing the fuel pump specific wiring on my 2000 Miata build.
Hopefully somebody here is wiring savvy enough to double check me before I go to hacking the wrong wires.
My question is this.
Should I cut and ground the wire mentioned in the blog?

This blog Nathan's LS1 Miata
lists the wiring connections necessary to connect the LS1 wiring harness to the Miata chassis harness wires.
No. 9 is the specific instruction dealing with the Fuel Pump wiring.

His car is earlier than mine and has slightly different colors on the wires. For the power wire going from the relay to the pump, his wire is blue/red while mine is red/blue and his two power wires are listed as being white/red while mine are white/blue. I did look at the 2000 Miata factory wiring diagram to verify I am looking at the proper connection and the manual agrees with my colors.

The part that has me scratching my head is where he says
"To modify the Miata relay, cut the white/red that is located directly next to the blue/red wire and ground the white/red wire."
I looked at his picture and located the same wire on my connector. It is the one closest to the green trigger wire that I will connect to the LS1 harness wire.
Do I really actually ground the wire he mentions or has something changed enough on the 2000 model to change the instructions?
If it needs to be done differently, what specifically do I need to do?

Thanks for helping.

Isn't Nathans Miata an NA? The fuel pump relay is different, and wire colors will be too, as you mentioned.

The Miata ECU uses a ground to trigger the fuel pump relay, I believe the GM ECU uses 12V to trigger the fuel pump relay. The light green wire is the ground side of the relay coil that usually goes to the Miata ECU, this now needs to go to constant ground. The wiring diagram shows the white/blue input power wire splicing and going to both the power side of the coil, and the high power input of the relay. The smaller gauge white/blue wire will be cut and connected to the GM ECU output wire.

Both of my white/blue wires are the same gauge.
The light green wire is much lighter gauge on my car.

The Flyin Miata instructions say to hook the ECU trigger wire to the light green wire, which matches my connector layout.

So now I am really confused.

It seems the thing to really figure out is what kind of signal your ECU is putting out. I'm not sure if they changed the trigger output between ground and 12V for different years/models or something.

I think this is an LQ9 ECU, which shows a constant ground, and 12V trigger.

http://lt1swap.com/pictures/6_bcm_fuelpump.gif

This is a '99 Corvette and shows constant ground and a 12V trigger.

http://i17.photobucket.com/albums/b8...I-IV/pcmFP.jpg

Edit: Both diagrams actually show a 12V trigger.

I see what you are saying about the ground in the diagrams.

Here is what I have for my Miata

This is the actual connector
Attachment 8470

This is the page of the Chilton manual showing the Miata wiring at the fuel pump and relay
Attachment 8471

Lastly, this is the page of the Mazda factory manual showing the Miata wiring at the fuel pump
Attachment 8472

On the last picture, I can see where the right side white/blue wire feeds the actual power thru the relay to give current to the red/blue wire going to the fuel pump. That is the wire I do NOT want to cut or ground.
The upper left white/blue wire that is opposite the light green wire appears to be the one that needs to ground to finish the light green circuit, correct?
It that is the case, how can I tell which white/blue wire on the connecter is the correct one to ground?

Edit: My guess is that I can plug the connector into the relay and test to see which white/blue wire closes the connection to the light green wire when I give the green wire 12 volts of current. Sound reasonable?

I am totally new to trying to figure out wiring diagrams and using a meter. My only previous experience has been following connection instructions for well marked car stereo wiring. Learning something new, that is for sure.

Yes, but the white/blue wire opposite the green wire does not need ground, it needs 12V, which is supplied by the ECU. The green wire needs ground. The pinout is shown at the bottom of the last picture. Pin A is labeled in the diagram as the coil excitation wire, and Pin E is labeled as the supply power for the fuel pump. So Pin A will go to the ECU, Pin E leave alone, red/blue leave alone, and green wire to ground.

Think I finally have it.
Appreciate the help.

You are saying I need to connect my gray ECU wire to the white/blue wire that connects thru the relay to the green wire and ground the green wire to finish the circuit. This in turn excites the relay to finish the connection between the powered white/blue wire that in turn finishes a connection to the red/blue wire that goes to the fuel pump.


That means that the relay is directional on the exciter side of the circuit, correct?
If I had connected the ECU to the green wire it would have been feeding the current backwards thru the relay and it would have not completed the exciter side of the circuit.

Edit: now I think I see where the "A" and ""E" connections are referenced on both the connecter and the relay.
Missed that the first ten times I looked at the diagram. This tells me I at least did find the W/B correct wire to snip and am leaving the correct W/B wire alone.

Let me confuse the issue even more. Many of us who have built V8 have had to upgrade the fuel relay system because of relay failures. The modification method requires direct wiring from the battery to the fuel pump relay with 12 gauge wiring to both the relay and from the relay to the fuel pump, At the fuel pump the 12 gauge gets spiced into the smaller fuel pump wire. The fuel pump remains tied in to the ECU for operation. Summit racing sells the kit that I used with my upgrade.

I had the same issue with my LS swap. A couple months after I finished the swap, I had an issue with the brake light switch. Reaching under the dash to get to the switch, I felt something burning my forearm. Turns out it was the fuel pump relay. I have no idea how it was still working as it was charred black. I installed a separate fuse block next to the battery with a new 30A relay and a short run of 12ga wire.

That is correct. That factory service manual was a savior for me, having all chassis, sensor, component pinouts is a so nice!

Good info from the others about the fuel pump relay being weak. I didn't think about that, since I'm using a Bussmann fuse/relay box with a 30A relay for the fuel pump.

This thread is a perfect example of why I feel I can depend on the V8 Miata community to make sure I do my conversion correctly and reliably.

Based on David Tibbits post, I took a hard look at the Fuel Pump relay wiring that is under the dash late last night.

Two things stand out.
First, to use the Miata relay you have to make a great big circle around the various wiring harnesses because of the routing of the Miata power wire and fuse. The relay is under the dash near the steering column while the power fuse is under the hood while the actual fuel pump is over the rear axle.
Second, if the Miata relay and/or power wire is too small (as Dave points out) it will destroy other wiring as it overheats and burns out.

Since the Miata battery sits in the trunk and the Painless Wiring Harness that I am using allows placement of the ECU module behind the passenger seat, it allows me to install a whole new relay and fuse and wiring to power the fuel pump and only run about 4 feet of heavy gauge wire which will all be located in the rear of the car and away from most of the rest of the wiring.

Bottom line is that in the interest of long term reliability I am going to run a whole new wiring circuit to power the upgraded fuel pump, similar to what Flyin Miata does by using the Chevy Performance harness.

I agree with your idea to run a new circuit and relay for the fuel pump. That's what I did on my car to eliminate the problematic stock fuel pump relay.