fuel pump circuit LT1

[JD1964]

94 Z28 LT1 swap into 79 Malibu.

I'm planning out the fuel pump and relay wiring. In the 79 Malibu, I'll be using the under hood power dist block from the Z28 including the cooling fan relays and related wiring. I'll also be powering the PCM from fuses in this block.

The block has positions for 6 relays, three of which will be for fan relays. Now, the Z28 factory setup for fuel pump circuit wiring has the FP relay under the dash and I have all that including the wiring on hand. However, I'm thinking of just using one of the available relay positions in the main dist block for a FP relay and wiring.

The stock Z28 FP relay that was put under the dash by GM is a physically larger relay manufactured by Hella. The relays that fit in the dist block under the hood are physically smaller and manufactured by Potter & Brumfield. I know that both of these relay types can perform the same function if properly wired.


Here's my question: Can I get reliable function as FP relay from the physically smaller Potter & Brumfield that I would locate in the dist block, or, is there a reason I should stick with the physically larger Hella relay and mount it somewhere outside of the dist block. (larger Hella relay case design will not plug into the dist box).

[ScottP]

I would not hesitate for a moment to use the Potter & Brumfield relays for your fuel pump. The
style used in the F-body LT1 fusebox are commonly known as 1 Form C relays. This style is rated
at up to 40A, which is more than enough for your fuel pump. You can check out the datasheet
here:

https://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=VF4-15F11&DocType=Customer+Drawing&DocLang=English

What you really want to focus on is the wiring from the relay to the fuel pump. Minimizing the
voltage drop to the pump by using no less than 12AWG would be my recommendation. 10AWG
is my personal preference, even for a stock to mild build, and is what I use on my own EFI projects.
My motto here is, wire is cheap - your time isn't. 10AWG gives you room to grow when you
eventually go to a much larger pump to support the twin turbos you decide have to go under the
hood in a few years!<g>

-Scott

[JD1964]

Thanks for the reply Scott. Potter & Brumfield relay in the Z28 dist block it will be. I understand the preference for adequate gauge wiring to the pump. I've got plenty various gauges of wire available to use. I just measured the stock Z28 wire to the pump and its a measly 16g at best. Wow!

[JD1964]

It looks like the ABS relay position is a good choice for the FP relay. It's already got nice doubled #10 from batt coming in and going back out in a single #10. Fuseable link in the end where it hooks to the terminal block near the battery.

16619

[JD1964]

I clipped out all the stuff not needed but left enough wire to splice back in if anything needs to be added in the future. Here's what I'm left with

16624

[ScottP]

Looks like you have a good workable solution there. Have you sourced replacement terminals yet
for the fuse/relay center so that you can avoid having to splice into the existing old wires? Mouser
is a great source for all of the Packard/Delphi terminals and connectors you will likely use during
this build.

-Scott

[ScottP]

For that generation of relay center, I believe you will require the Metripack 630 series
unsealed terminals if you decide to rewire without splices. They are available under
the following part numbers:

5.0mm^2 - 12084588
3.0mm^2 - 12052227
2.0 - 1.0mm^2 - 12015865
1.0 - 0.80mm^2 - 12015864
0.50 - 0.35mm^2 - 12052225

https://www.mouser.com/ProductDetail/Aptiv-formerly-Delphi/12084588-L?qs=DlE0rdXB%2FL2snBiQS%2FDFkQ%3D%3D
https://www.mouser.com/ProductDetail/Aptiv-formerly-Delphi/12052227-L?qs=DlE0rdXB%2FL21oKhGD7ojbA%3D%3D
https://www.mouser.com/ProductDetail/Aptiv-formerly-Delphi/12015865-L?qs=DlE0rdXB%2FL3CPCPt0GF9NQ%3D%3D
https://www.mouser.com/ProductDetail/Aptiv-formerly-Delphi/12015864-L?qs=DlE0rdXB%2FL2MQRqiSbovyQ%3D%3D
https://www.mouser.com/ProductDetail/Aptiv-formerly-Delphi/12052225-L?qs=0q6jlcGHEi3LTyF5DPpSww%3D%3D

-Scott

[JD1964]

Thanks for that extra info Scott. When I took this relay center out of the Camaro I kept the wiring intact in the longest lengths practically possible. I can now make most of the wiring tasks with minimal splicing. When I do splice I'll be soldering.

[JD1964]

I need to make sure I understand the PCM's function in controlling the cooling fan relays and the fuel pump relay.

In looking at the fuel pump relay schematic and I see that the fuel pump relay triggering function has one of the wires going directly to ground such as in this schematic > fuel pump relay trigger grounded (https://www.lt1swap.com/pictures/100_1775.jpg)

In looking at the cooling fan relay schematic I see that the triggering function does not have a wire running to ground but instead to ignition, such as in this schematic > fan relay trigger to ignition
(https://www.lt1swap.com/pictures/100_1778.jpg)

So, am I correct in assuming that the pcm pin A7 (fuel pump relay control) shoots a 12v signal to the FP relay for triggering

....and, the pcm pins A10 & A11 (fan relay controls) create a ground for triggering the cooling fan relays?

[JD1964]

I actually found my answer in the text near the bottom of this page.

https://lt1swap.com/wiringharness.htm

Yes, pcm trigger for FP relay is a 12v +, and pcm triggers for cooling fan relays are negative ground (-)