Has Anyone Gotten Successful Control Over the GMT-900 Truck FPCM?

[cmitchell17]

So the CTSV and Camaro people are able to control their FPCM's so they can get the maximum duty cycle and flow out of their pumps.

However, the truck FPCM (20898936), which has included stuff (not sure if its hardware or software or both, but likely both since the trucks without RPO JL1 (integrated brake control) share the same PN) for the trailer brake control has some special stupid limit to pump duty cycle so I can't get the entire flow out of the pump like I should.

Now I know this can be debated on weather you are supposed to run the pump beyond about 90% or if it is even beneficial at all, but I think it is, even if it was just shorted to hold at 100% DC all the time, I think that would help a very significant amount.

So I have tired all the GEN IV FPCM (FSCM) tricks, max fuel pressure table, upping open loop DC, changing high fuel flow thresholds, etc. nothing has helped at all. If I just try to set the minimum DC table to 100% at only the higher pressure (high flow mode) the FPCM still gets stuck maxed out at about 90-91% DC, it will go to 100% DC once it gets to high flow mode, but then goes to its stupid limit at about 90%.

The even weirder thing is that I can up on the fuel system minimum voltage (ECM 18102) to say 16 or 17 volts (CTSV uses 16 volts at the higher y axis values (y-axis has no units on this table)), and the altenator will respond and kick voltage up, but it will only go up to about 15 volts max since there appears to be a limiter elsewhere in the ECM, but the the fuel pump duty cycle at WOT falls even more to about 86% from the 90-91%? So this tells me the voltage boost thing is at least working, but the end result is that even if I boost system voltage for the FPCM driver, the control strategy brings the duty cycle down even more. This tells me the limiter is not just a simple limit on commanded duty cycle, when it hits its limit it also bounces around a couple .1% or .2% which proves even more its not a simple limit.

I am really hoping someone knows how to get around this stupid limit? I know these pumps even stock have a lot of potential and I really like keeping the stock functionality and hate ripping out everything and spending 3 thousand dollars on stainless, braided AN lines and fittings and going to a return regulator setup and lose all kinds of control and all kinds of money as well.

[cmitchell17]

Does anyone know how the fuel trims work on the FSCM control loop? They appear to just be multipliers as both short and long term trims I usually see in the 0.89-1.01 range. But they just don't make sense, if I see a higher fuel pressure than commanded the trims don't appear to change and the short term will still be under 1.

I also replaced my stock pump with a Walbro 450, then a Walbro 525 (F90000295), but what makes no sense again is with a pump that flows supposedly about 1.34 times as much as the stock pump, the open loop PWM control table still works perfectly fine. It makes no difference if you scale the table up. I am beginning to think the open loop table is only referenced in the event of a total sensor feedback failure and it has some other control strategy when the fuel pressure sensor and everything else is working correctly. Everyone else from what I read seems like they have the same problem, you can't just go to open loop mode since there is no way to "fail" the sensor and force it to open loop, you would also lose your sensor reading unless you tapped into it and ran the signal out or had some other fuel gauge.

[hjtrbo]

Disclaimer, I have no idea.

What I have seen in other non related strategies is the open loop dc forms the base of the control output signal and is then modified by the closed loop control system to achieve the target value.

[cmitchell17]

Yes, thanks, it seems this is the way it would work too, but with mine you can butcher it pretty bad and it still works, now if you zero it out or max it out, then yes it will throw a fuel pump flow insufficient code.

I am getting barely enough flow from the "525" so I guess it will have to do for now, unless I just want to do the pressure switch and manually force it on. I bet if I did that I would find it wouldn't even flow much more than it does at 91% DC.