GM Fuel Pump control scheme

[Louis Baiani]

Hey guys,

I have a an 84 Buick Century (closed loop carburator control), which has a fuel
pump as well, and it takes a while to crank (especially in the cold) when the
engine is cold before it starts.  I know i'm getting power to the relay, probably
from the ECM...it could be the relay is bad.  Now, that relay must have a contact
for the ECM, and one from the block to power the fuel pump?  I remember one day,
the oil switch did stick, and the engine started with the remaining fuel
pressure, but unfortunately when I touched the gas, it stalled.  To move the car,
I had to bypass some power directly to the pump.  The next day, the oil switch
kicked in again, and everything was fine again.  So from way back when they
started this, they always had the ECM's driving the fuel pump as well?

So can anyone give a simple schematic for this?  How long is "too long" regarding
slow starts?  In cold weather, the Buick takes like 4 seconds to kick off.

Thanks,

Louis

Wes Branchflower wrote:

> I thought the ECM would run the fuel pump for approx 10 seconds, then stop.
> My brother used to have a VL commodore, when he turned the ignition to ON
> (not start) the pump would go on for about 10 seconds, then stop. (I know
> this cause his fuel pump was REAL noisy ;)
>
> This leads me to think that the fuel pump is activated by the ignition, for
> a brief 'turn on' time, after which, is turned off. When the engine starts
> (off the pressure already in the fuel rail), the engine's oil pressure
> switch would take over the control of the fuel pump. This makes it so if
> ever you loose oil pressure, your engine will shut off. (saving the motor)
>
> Does it seem right?
>
> remember this is a nissan S6 found in the VL's. not a GM.
> But i think it could be a very similar design.
>
> If it doesn't already do this, i'm definetly making this modification on my
> engine.
>
> >From: ae2598 at wayne.edu
> >Reply-To: gmecm at diy-efi.org
> >To: gmecm at diy-efi.org
> >Subject: Re: GM Fuel Pump control scheme
> >Date: Sat, 27 Jan 2001 08:49:14 -0500 (EST)
> >
> >I guess that's why the car will still run if you disconnect the oil
> >pressure switch!  I've seen more than one S-10 4-cyl. switch fail and leak
> >profusely, it happened to mine a few years ago and it was late at night so
> >all I could find was a pipe plug to plug the oil passage.  truck ran fine
> >with the wiring harness dangling..
> >
> >  On Sat, 27 Jan 2001, Shannen Durphey wrote:
> >
> > > Another advantage to having a parallel control scheme is redundancy.
> > > Relays also tend to get "burned" due to arcing across the contact points
> > > as they open and close.   This causes a poor electrical connection.  The
> > > oil pressure switch will still allow the pump to operate if this
> >happens.
> > >
> > > In some trucks, the relay is used to circulate fuel for an extended time
> > > after the vehicle is shut off.  This helps prevent vapor formation in
> >the
> > > lines as the engine hot soaks, and aids in hot restarts.
> > >
> > > Throttle body systems don't often show signs of bad relays, in my
> > > experience.  High pressure port injected fuel systems are more likely
> >to.
> > > In practice, the o.p. switch is not necessary.  I've run my converted 55
> > > chevy since 95 or 96 without an oil pressure switch.  But there's a wire
> > > to the pump conveniently located next to a wire from the battery, just
> >in
> > > case something happens.
> > > Shannen
> > >
> > > Bob Wooten wrote:
> > > >
> > > > This is what i thought but then it can be said that if the CPU is
> > > > controlling the fuel pump, it certainly knows if the engine is running
> >or
> > > > not & can turn off the pump.  I always thought that the oil pressure
> >switch
> > > > was in series with the fuel pump relay for the shut down & that the
> >relay
> > > > gets its power from the Ign. switch but i just checked the diagram in
> >my
> > > > manual & that is not the case & they are certainly in parallel.
> > > >
> > > > limp home mode sounds most plausible to me, but how does it get the
> >motor
> > > > started?  no oil pressure no fuel, no fuel no oil pressure.
> > > >
> > > > BW
> > >
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> > >
> >
> >
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