Lexus AEM Variable Voltage Fuel Pump Control


With a few wiring and AEM configuration changes, I have been able to implement the variable voltage control that the OEM Supra ECU provides for the fuel pump. So what’s the advantage in this? Automotive OEM’s and racers know that excessive fuel circulation at idle and low engine load causes heating of the fuel, loss of engine power, and more fuel loss due to evaporation/vaporization,  and none of these are beneficial to the engine, or the environment.

In the early days of EFI, the OEM’s addressed the excessive fuel pump delivery at idle and low load by decreasing the voltage to the fuel pump with a fixed resistor and a relay. This was a makeshift solution and it proved to be unreliable (at least for the domestic carmakers), then in the early 90’s Toyota introduced the concept of variable voltage control with PWM technology, which was more reliable than the resistor, and enabled better control over the fuel pump delivery.

Just a quick review of how the OEM system works; the MKIV Supra ECU outputs a +5v PWM signal on the FPC line, and the fuel pump ECU responds to that signal by switching a +12v PWM output to the fuel pump. Although PWM technology provides nearly infinite voltage control between 0 and 12v, according to the Toyota documentation for the Supra, only two levels are output: Low & High, with low being in the range of 9-9.5v and high being 12v.

With the standard AEM MKIV Supra setup, Coil4 is configured as the FPC signal, but with the limited functionality of the Fuel Pump Option, it is only an On/Off signal, not PWM, so the AEM only sends either 0v or +5v to the FP ECU, and the FP ECU puts out only 0v or +12v to the fuel pump. The “Low” output level of 9v is not available.

My setup consists of the AEM, a single Supra fuel pump, a Kenne Bell Boost-a-Pump, and the OEM FP ECU has been replaced by a simple SSR (Solid State Relay). Why only a single fuel pump, and why the BAP? A single OEM fuel pump delivers more than enough fuel to the engine for all operating conditions except higher boost levels. Under these high boost conditions, the Kenne Bell BAP “boosts” the voltage up to 16v to the pump, which then increases its output to meet these conditions.

At 16v, the Supra pump has been tested to flow 511 Lbs/Hr @ 60 PSI, or 480 Lbs/Hr @ 70 PSI. See this link for test results:

These flow rates are sufficient to support 511 ÷ 0.55 = 929 BHP @ 25 psi boost, or 480 ÷ 0.55 = 872 BHP @ 35 psi boost.

CAUTION: Do not take these BHP capability figures as absolutes; always ensure that your fuel pressure and AFR are sufficient at higher engine output levels.

Some may question whether higher voltage will diminish the life of the fuel pump, however it is actually prolonged overcurrent (heating) that diminishes motor life, not overvoltage. In addition, realistically how long can anyone keep an engine on prolonged high boost for a street driven car? Finally, some may challenge this setup as being less reliable than a twin pump setup, however I believe it is more, not less, reliable.

With twin pumps, there are twice as many electromechanical devices to fail as with a single pump, and if one of the two pumps fails in a high boost situation, the engine will still receive “some” fuel, but will instantly go lean, and probably suffer severe damage. With a single pump setup, if the pump fails, the engine will simply stop running due to “no” fuel delivery. Kenne Bell have some excellent reading on their site concerning the merits of their BAP (of course), but also the merits of not overpumping fuel to the engine when it doesn’t require it.

Because my car was originally a GE (non turbo) car, I did not have a separate EFI #2 relay, nor a separate circuit for the fuel pump, so I ran a separate 30a fused, #10 wire to the hatch of the car, and installed a 75a heavy duty Bosch relay which is triggered by the EFI #1 relay signal from the old FP ECU harness connector.

The output of the Bosch relay goes to the Kenne Bell BAP, the output of the BAP feeds the SSR, and the output of the SSR feeds the fuel pump. I also have the AEM setup with LS8 triggering a relay to give the BAP a signal to increase its voltage to 16v whenever boost is 15psi, engine rpm is 3000, etc.

I kept Coil4 configured as “Fuel Pump” in the Options, and its output goes to the plus control side of the SSR via the existing FPC wire. I then configured Injector 9 as the User #1 PW output, and ran it to the minus control side of the SSR via the old “DI” signal wire. To do this it was necessary to move this green DI signal wire from ECU harness pin 21A to 74B. Many of you will recognize this as similar to the way Derek (AnArKey) controls his MW pump, however I decided to use the User #1 function where he elected to keep the injector configured as an injector.

The User #1 PW Options were then setup as follows:

User #1 Analog In:                   Engine Load

User #1 Period:                        50 msec

User #1 PWM Output: Injector 9

User #1 Period Hi:                    0

User #1 Period M:                    0

The User #1 output map (see screen shot below) starts at 60% for 0% Load, and increases linearly up to 100% at 31.25%, then stays at 100% for the remainder of engine load. In this way, the fuel pump receives about 8-8.5v at idle, and this increases to 12-13v at 31%.  Then when the engine hits 15psi boost, and 3000 RPM, I have the BAP configured to kick the voltage up to 16v, just to be sure there’s no fuel starvation at higher boost.

While I’ve not yet been on the dyno with this setup, I’ve confirmed the AFR stays steady up to boost levels of 1.2 kg/cm2 in 3rd and 4th gears. Driveability is good, and best of all, the excessive fumes from the charcoal canister, and pressure release from the fuel tank are gone.


AnArKey (Derek O.) for providing a practical application with both a HS and LS PWM driver, which I shamelessly adapted for my own purpose.

Hau Do, and BLKMGK (John D.) for advice and general hand-holding, as I worked the bugs (caused by me) out of this system.

Do you have any questions? If so, please head over to the forums to get a quick answer or share your experience!

This is a freelance site with no support by huge companies.  I have been doing most of the R&D and technical write-ups by myself with my personal money and literally thousands of hours of my time.  I have taken extra steps to demonstrate in details how things are done.  Currently I am one of the few people doing Lexus V8 research and performance enhancement.  This effort comes from my personal love for this wonderful engine.  Most of the modifications are from trill and errors.  There is no cookbook for the 1UZFE modifications and its a virgin territory for performance.  The parts, labor, web development and site hosting are from my personal hobby money.  If you feel my efforts help you in any form, please do not hesitate to donate any amount of money to support this site. You have no idea how much I appreciate it!

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