I was checking out the fuel delivery system on my "Supercab conversion" today and have a question.

The OE HD/L system uses dual pumps. I am under the impression that one runs all the time and the other kicks in later.

My question is, what is later? Under what conditions or what is the trigger? Is it boost pressure, throttle position, RPM or a combination.

Checking the wiring, at the pump plug, both of my pump leads are energized. Apparently my system or PCM thinks I am at WOT. I would like to troubleshoot but don't know where to start.

At this time I have a single 325LPH Bosch pump in my truck. It was more than sufficient for my Procharger setup (377 on the ground) but the jury is still out as to whether it will deliver enough for 450 from the new setup.

Once I get the bugs out I'll get it to a Dyno but first, I need to sort out the power feed as mentioned above.

 

I can't answer you question about the fuel system in your truck but I am sure somebody here knows how it works.

I'm interested in your "conversion". What did you change? What kind of a job was it? How much did it cost you? Do you have any pix of your truck?

Yeah, just nosey.

 

Both pumps are powered by the same power wire so they both must run at the same power all of the time. The system is a dual voltage system which is regulated by the pcm. More power =more pump speed. When the regulator sees less vacuum and more boost less fuel is allowed to go back to the tank so the fuel pressure is higher.
You can disable the dual voltage system by shorting the two wires at the resistor(located near the top of the intercooler pump.)

 

The fuel pumps work like this--

Both pumps get full battery "positive" power all the time. However, the the ground side of the pumps goes through a voltage dropping resistor, which is tied into the fuel pump high/low relay. When you are not at WOT (2.93 volts TP and lower), the pump ground goes through the resistor and you have reduced voltage. When you got WOT (over 2.93 volts TP), the pumps get full ground and thus full battery voltage.

Hope this helps.

 

Aha, comes the dawn. It sure does help. This explains the absence of any continuity on the ground leads for the pumps since I was not aware of the resistor in the circuit. I do not have complete schematics but now, knowing this, I think I can see reference to the resistor but full details are in a section I do not have (yet).

I guess great minds work in concert. With my 325 LPH pump I have always had high fuel pressure at idle, usually 42 psi due to the pump overpowering the flow capacity of the return line. It never seemed to cause any problem with the way my truck runs but I already have an adjustable resistor that I had planned to tie into the system, into the positive lead of what I thought was the primary pump, to slow the pump down. Then I was going to use the secondary pump circuit to bring the voltage up to full at the appropriate time.

Now, I see why that can't be done since the positive leads are both hot all the time. So, I'll simply go with the existing circuit, put my resistor in the ground lead, like the OE setup and achieve my same end result.

Thanks guys,

PS, just for ducks, can somebody post a pic of the OE resistor?