Engine/Transmission (1998.5 - 2002) A New Lift Pump Theory??

[jerryrigg]

Maybe some of you have thought of this but I did not find it posted in anything recent.



Note: I have not made these voltage measurements yet, maybe one of you have.



The lift pump, basically a standard 12V inline fuel pump. 90% of the electronics in the world are designed to operate at a certain voltage within say +/- 10%. That means the lift pump is designed to operate at 10. 8-13. 2volts.



The manual states

"Pressure Test: Because the transfer pump is operating

at two different pressure cycles (engine running

and engine cranking), two different pressure tests

will be performed. "



The minimum running pressure is 10psi, the minimum cranking pressure is 5-7 psi. The pump is controlled by the ECM, to achieve 10 psi we can assume the ECM is providing 12v, to achieve 5 psi I assume the ECM is only providing 6v, which would be out of "specs" for the pump design. Operating any electrically device under or over voltage specs will lead to early failure.



Possible solution: A 6 to 12 volt relay between ECM and pump that when closed supplies battery voltage to pump.



Your Thoughts?????

 

[WadePatton]

probe it

Probe the leads and tell us in fact if the voltage is varying.



I don't have access to good probes right now.



Otherwise I'll suspect that there's a different way of obtaining the two cycles.



I'm going to the Stealer Monday for low pressures. The local stealer is also a phord store--what a revoltin development!

 

[jerryrigg]

My Measurements

I just finished testing my voltage. The ECM was providing 12v while running (applying throttle) or 100% duty cycle as the manual states it, during starting it drops to 9v during "wait to start" and cycles between 12v and 9v at idle.



So what do you think??

 

[illflem]

What could you do short of a separate 9v pump just for start up since full pressure will cause hard starting? Think you may be onto something though.

 

[Gary - K7GLD]

I personally suspect your OTHER theory/post regarding cooling fuel flow requirements in the pump is more on target - the relatively short voltage cycles between 12-9 volts aren't likely to be significant over the lifespan of the pump - of course, the current lifespan for many isn't all that great either...

 

[Bigsaint]

I can tell ya that it would not do anything for the Mallorys I ran in the past, all but the first were wired through a relay, which uses the ecm wire as the trigger, meaning that they always saw 12V + when on ( yes, I checked it at the pump with a multi meter).



As to hard starts with high FP, never have seen it with what I'm running now. I was told that the BD 5's I have in now take an extra revolution before take off. That combined with an aftermarket pump which is either pushing 16-17 psi's at idle or off, still equate to a quick start for me.



Scott W.