REFLEX+ Auxiliary PWM Fuel Pump Control (returnless)

The MOTIV REFLEX+ controller is equipped with 4 auxiliary outputs, they can be used to control a variety of systems and in this post we'll cover how to use one (or two) of these outputs to PWM control a second and/or third fuel pump and maintain a "RETURNLESS" fuel system configuration on your LT1, LT4, L83 or L86 GM Gen V engines.

Most vehicle manufacturers transitioned to a "RETURNLESS" style fuel system in the 90s. Benefits of this type of fuel system includes a reduction of evaporative emissions, reduced fuel pump power consumption, increased fuel pump life, and a reduction of mechanical components and fuel lines in the system to reduce cost and reduce chance of component failure. These systems proved their benefits in production and became the standard for fuel system today.

On the downside this fuel system is that originally it didn't lend itself easily to modification, so many times it would be replaced with an older cumbersome return-style fuel system adding components, lines, and weight to the vehicle. Adding an extra fuel pump in parallel to the factory pump can supplement the base system to achieve high horsepower on E85 without the additional complexity of a return style system.

Why do we want PWM fuel pump control for additional fuel pump?

Because an AUX or EXTRA fuel pump in a return-less system has no pressure control, it relies on the factory fuel pump and control system to manage the final ECU commanded pressure of the fuel system. By only adding the amount of additional volume needed with a PWM control of the extra fuel pump the overall system maintains a nice consistent low side fuel pressure across the engine operating range and eliminates the "pressure spike" seen with a simple off/on extra pump control system.

Fortunately if you're running a REFLEX+ controller you can easily add additional fuel pumps and control them to work in a RETURNLESS fuel system, let's get started!

Below is the wiring diagram for the extra fuel pump(s). Use the components shown below and you'll be able to drive the extra fuel pump with variable speed from the REFLEX+ controller. Remember that the extra inline fuel pump that you use with this system will need to have a check-valve to prevent backflow through the extra pump when it is off.  Most inline pumps include a check valve, but just double check to be sure.

Once you have completed the necessary connections shown above you'll need to setup your REFLEX+ software to control the output. We'll want to start by making sure that our Parameter Tree on the Left Side of TunerPro is set to Parameter Category. If your tree is not shown at all press F6 to turn on the Parameter Tree.

First we need to select the type of map we want to use for AUX 2 (or 3) to control our fuel pump. This is set using the "Aux2 Local Map Selector", for this example we are going to be using "MAP X / RPM Y Axis" which means we'll be controlling our fuel pump based off Manifold Pressure and Engine RPM. As you can see from the image below we also have controlled our pump based off Throttle Position, Ethanol Content, or Vehicle speed if desired.

Next set the frequency for the AUX 2 (or AUX 3) Output Frequency based off which output (or both) that you are using as shown below. Set this to 200Hz when using PWM fuel pump control.

Then you'll need to set your MAP bins (X-axis of your control table). These values are on PSIA so atmospheric pressure will be approximately 14.5 psia depending on your altitude. Normally we going to initiate the extra fuel pump once you transition into boost, so set these the values of this axis accordingly, remember the transition into boost happens at approximately 14.5 psia so in this case have scaled the axis from 10 psia to 32 psia (~17.5 psi boost).

Once we have our X Axis determined, we need to do the same to our Y Axis, you can see the example below for this setting.

Next we just need to configure our AUX2 (or 3) Output Duty Cycle Table. Make sure you choose the correct table - its named "AUX2 Output Duty Cycle", there are several options for AUX2 and you need this table for the correct control. Just as the name implies, this will be the duty cycle that drives the speed of the pump. If we need more volume to maintain pressure, we increase this value. The fuel pumps normally require about 15% duty cycle to start the extra pump running and then you can add additional fuel as needed with increasing boost

Finally log the "Aux2 Duty Cycle" along with Manifold Pressure and Engine RPM to make sure your software settings are correct and the additional fuel pump is receiving a signal to turn on as shown by the dark blue line below. Below we're  running a 400 l/hr AEM inline pump and you can see the gradual increase in duty cycle to supplement flow to the base fuel system. This data is from a 900 RWHP pull and you can see that I only needed to add 35% Duty cycle to the 400 l/hr pump to maintain my desired low side fuel pressure

That's it! We'll let your imagination run from here, if you need to run 2 pumps you could run them both from this single control system or you could duplicate it and run one pump off AUX2 and one pump off AUX3 and stage them as you see fit.