What I have:
FAST XFI 2.0 EFI controlling cooling fans. The EFI module provides a sink (negative) to ground output signal.
What I am working with:
Cooper Bussman Relay/Fuse module with five fused 35 Amp Relays and an additional 5 constant hot fuses.
The Problem:
The Relay/Fuse box is designed so the relays are triggered with a high/hot +12 volt signal, and the ground side is designed into the negative bus bar.
I think I could make the negative bus bar a positive bus bar and the relays will activate on the low side signal, but I also wanted to use the relays to drive the Low and High Beam head lights which would be high side triggered, so I need a way to invert the signal to the relay trigger?

I know of many way to do this in theory, like using another relay, or a transistor driver circuit, but is there a better / easier way?

Either split the buss bar or use a 2nd relay in line to change the signal to high drive from low drive.
Doug

Cut the bus bar. Divide it as you need. Solder a wire or stud to the cut off section, and power it from one of the fused side circuits.
There is a potential problem with doing this though. Some relays have an internally configured suppression diode. Since you will be firing the coil from a reversed polarity standpoint from which it was designed, the suppression diode wont do it’s job. (if it is so equipped)and will short circuit. Using a non suppressed relay will allow doing this.
The diode is there to prevent flyback voltage from making its way back to the control unit. You could easily add suppression diodes to the bar itself in the required polarity.
Google “suppression diode in relay” for more detail on why/ what/ how.

I found out about these things the hard way last year when wiring my coupe. After years of using relays that had no polarity preference, somehow I ended up with some of these diode relays. My fuel pumps didn't work because the polarity was backwards. Reverse polarity instantly burns up the relay. I burned up up about 3 relays before I figured it out. Beware of the diode relay!

Many relays with built-in suppression, use a 2K ohm resistor due to use in reverse polarity situations...some not so.

Mark

The relays do not have the suppression diodes, so no issue there.
The Relays are PICKER PC785-1C-12S-R-X
http://www.pickercomponents.com/pdf/Relays/PC785.pdf

The relay bus is molded into the block and connects to pin 86 of the relays, and there is no easy way to cut the bus. This is an EATON #15305-2-6-4
https://www.mouser.com/catalog/specsheets/eaton_01242018_1530X-X-X-X-SALES_RevA.pdf

I got this pre-built, wired and populated with the relays and fuses from Amazon
https://www.amazon.com/gp/product/B01FWILL20/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1

Link to the sellers description:
https://www.teamconcours.com/#!/Universal-Waterproof-Relay-Fuse-Distribution-Box-Cooper-Bussmann/p/65063101/category=13963121

Yeah I've run into this issue too. FAST grounds their fuel pump and fan signal while Holley sends out 12v to turn things on.

On the dyno we have to flip flop the wiring on the fuel pump relay depending if we're running a FAST system or a Holley system.

I do think the FAST method is smart, but a lot of pre-built relay kits ground the relay to the busbar so the FAST method doesn't work with them.

I think my antique Holley Commander has the option of programming the outputs to be either + or -. New and improved units don't have this option?

Kevin

Now I am thinking of just running the relay bus to the high side through one of the extra fuses, that way the EFI box can control the relay low sides for fans and A/C clutch, and I'll just do the Headlights with regular relay setup.
I am looking at running two puller cooling fans and and additional pusher fan for the A/C condenser. So with the A/C clutch too, 4 of the 5 relays are being used. I guess I could also use the fifth relay to replace the horn relay? Just trying to clean up the engine compartment wiring.
I may use some of the fused constant power circuits for the radio head unit constant power and the amplifier power?
Not sure what else to use the extra constant power fuses for (under hood?)

Interior wiring and EFI changes still in the works.

I heard the newer Holley untis can switch polarity which would be a nice feature. My mega squirt triggers the ground for everything so got an oversized relay panels that was half ground and half positive trigger relays because I didn’t know exactly what I would need since I was redoing almost the whole car.

It don't surprise me as I know when Mopar started using TIPM modules (Totoally Indegrated Power Module) they started using high side drivers which as a dealer tech I had not seen that from the factory before on factory cars. They would just ground the relay coil all the time and then the high side drivers send 12 volts to the other side of the relay coil to turn on that circuit. They started doing this on a few circuits instead of controlling the ground which I thought was the easier way to control a circuit. Ron