While the original system used four fuse/relay combinations, one of these was for powering the fuel pumps off of a fuel pump trigger signal. In other words, the ECU would energize a relay coil which would then allow higher-current fused power to go to the fuel pump when it decided the pump should be running.
As with all fabrication work, test fitting and checking multiple times ensures that you don't get too far if you have made a mistake.
It should be noted that Haltech used both single- and double-circuit relays in their original harness design. Both of these types of relays use the same receptacle. All of the relays, regardless of their original function, were replaced with double-circuit relays. Since the single-circuit relays had no receiving pins in the receptacle and were not connected to anything, it didn’t matter that a double-circuit relay replaced it. This also simplifies what spare parts need to be kept laying around. No matter what happens, just replace with another Hella 4RA 933 791 (or similar) double-circuit relay with a mounting tab and off you go- hopefully.
Since we had previously installed the Radium / Fuel Safe fuel cell with FCST that has the dual fuel pumps and surge tank, we now had two pumps to control and power. The factory Haltech fuse and relay circuit was eliminated and the Haltech ECU’s fuel pump trigger signal will be fed directly into a Racepak Smartwire input. When the ECU turns on the fuel pump signal, it will cause the Smartwire to energize two 20-amp circuits: one for the lift pump and one for the high pressure pump.
The load on a fuel pump increases with fuel pressure. Essentially the fuel pressure is a form of resistance. As the pressure increases, the resistance increases, which means the pump is working harder, which means the pump is going to draw more current.
AEM’s 50-1000 fuel pumps draw 12 amps at 40psi. Since we are likely going to run northwards of 40psi of fuel pressure, it was logical to provide a 20 amp circuit for the high-pressure pump that in the Radium/Fuel Safe cell.
While the lift fuel pump that provides fuel to the surge tank likely sees little pressure (resistance) and therefore is likely to draw little current, we are opting to dedicate a 20-amp circuit and appropriate wiring to the lift pump anyway, just to be safe. Once the car is set up and running, the Racepak Smartwire will be able to provide data on the current usage of all the circuits – the fuel pumps included. If it appears that the lift pump current draw is well under 10 amps, a simple re-pin of the connector at the Racepak end will switch us from a 20-amp to 10-amp output. Easy peasy.
The Haltech 2JZ plug-and-play harness is still a mess because everything isn’t finalized. We still have many wires to add to the Haltech ECU-end connectors, and we still need to block off the holes through the firewall. Why? Minimizing the number of holes in the firewall provides two forms of safety.
Firstly, it reduces the ability of water and/or debris to pass from the engine bay into the cabin.
Secondly, and more importantly, in the case of a fire in the engine bay it helps to reduce the chances that the fire will pass into the cabin and ignite something inside the car — like the driver!
While not a safety measure, it can also reduce noise passing into the cabin. Running a turbo 1.2JZ car with a cat makes it relatively quiet, but it will still be a racecar running at full tilt, which is inherently noisy.