MKIV Supra 12Volt Mod
This information was obtained from MKIV.com, and the original write-up was made by ‘Brian B’. Credits to the original writer were due.
The stock fuel pump on the twin turbo Supra has two modes of operation. There is a high speed mode for high engine demand, and a low speed mode for cruising. The fuel pump ECU(separate from the main ECU) receives information from the main ECU which determines which mode the fuel pump should be operating in. Various sensors come into play to determine when high and low speed operation should be used.
Under idle conditions and cruising, the fuel pump ECU sends a reduced voltage output to the fuel pump(9 volts), and the fuel pump operates in “low speed”. When engine sensors determine a high engine load, the fuel pump ECU will send a full 12 volt signal to the fuel pump, kicking it into “high speed” so that it will supply more fuel.
When modifications are made to the car to increase boost pressure and engine breathing ability, some owners have experienced detonation around 4000 rpm, as the 2nd turbo builds boost and comes on line. Usually, this detonation disappears above 5000 rpm. The problem is, the fuel pump is still in “low speed” mode around 4000 rpm’s, but the performance modifications have increased fuel demand to the point of “outrunning” the low speed operation. By 5000 rpm, the fuel pump ECU has caught up and switches to high speed operation and all detonation disappears.
There is an easy way around this. You can wire a 12 volt signal directly to the fuel pump, effectively bypassing the fuel pump ECU so that the fuel pump is always in high speed operation. The TT Supra is one of only a few cars that has this dual mode of fuel pump operation, whereas most cars always have 12 volts to the fuel pump. This mod is completely safe. You are basically just turning your complicated fuel system into a basic, Camry style fuel system. You may shorten the life of your fuel pump from say 20 years down to 18 years, but nothing to worry about in the short term. I’ve personally had mine hooked up this way for over 2 years now without any troubles.
The fuel pump ECU is located in the trunk area(Fig 1). Pop your hatch, and look for the trunk courtesy light on the driver side plastic paneling by the rear shock tower. The fuel pump ECU is bolted to the body, under this panel. To access, pull back the carpet. You will expose a black piece of styrofoam on the left side of the spare tire. Remove the spare and pull out this black styrofoam, it is only velcroed in place. Once the styrofoam is out, you can reach up under the plastic and feel around for the ECU. It is about 4″ x 6″ x 4″. There will be an electrical plug on the side of the ECU closest to the tail light. Unplug this plug. There will be enough slack in the wires to pull the plug below the plastic paneling for easy viewing. There should be a blue wire with an orange stripe. Note that for 1997 this wire is solid blue, on 1998 this wire is black with a red stripe. – verify which wire applies to you before you proceed. This is the 12 volt power supply for the fuel pump ECU. It is only 12 volts when the ignition is in the “on” position and has a fuse in the closer to the battery. This will be the new supply for the fuel pump. There should be a blue wire with a red stripe. This is the output signal from the fuel pump ECU to the fuel pump. Cut the blue/orange wire and the blue/red wire and connect these two together. You now have 12 volts wired directly to the fuel pump. use electrical tape on the two wire stubs still connected to the fuel pump ECU so they can’t short out on body ground. Make sure you use good quality electrical connectors so the fuel pump doesn’t see an intermittent signal or short out on body ground. Plug the connector back into the Fuel Pump ECU.
Note: Verify which wire applies to you before you proceed.
For 1997-98 the 12 volt power supply wire is solid blue
For 1993-96 the 12 volt power supply wire is solid blue with an orange stripe