GM Fuel Pump control scheme

[chetwagner at qwest.net]

I have done a few conversions on my vehicles and have not used an 
oil pressure switch in any of them.  I think the OP switch is just a 
back-up for the fuel pump relay.  Good thing to have in case the 
relay goes out while driving on the hi-way at high speeds.  I have 
had one relay go out on me when off road in the middle of nowhere. 
I had to wire the pump into a spare switch in the dash to be able to 
turn it on and off when needed.

On 28 Jan 2001, at 23:14, Keith D. Miller wrote:

> > -----Original Message-----
> > From: owner-gmecm at diy-efi.org 
> > [mailto:owner-gmecm at diy-efi.org]On Behalf
> > Of Bob Wooten
> > Sent: Sunday, January 28, 2001 10:37 PM
> > To: gmecm at diy-efi.org
> > Subject: Re: GM Fuel Pump control scheme
> > 
> > 
> > I have beenthining about this for a couple of days  & i am
> > still confused. if this were in fact to kill the engine if
> > the oil pressure dropped below the switch threshold & that
> > the redundant CPU relay was just for starting, then this
> > would make a lot of sense (kind of).  I would think that
> > putting in a switch to tell the cpu OP OK or OP BAD would
> > do the same thing.  the thing that i don't get is that my
> > car does not have a OP switch & it has never had a problem
> > with it running.  this tells me that the cpu must be
> > turning on the FPS relay & keeping it on (& it is
> > obviously not hot wired on as the CPU turns in on when i
> > turn the key on & then turns it off until the motor is
> > running).  this tells me that the OP switch is a
> >  completely redundant system.  the question is why.  
> > 
> > It does not make sense to me that the general would put
> > this in because the relays have a tendency to fail.  if
> > that were the case then good engineering says that you
> > spec a higher amp relay or put two of them in series with
> > each other so that the cheaper less amperage relay can
> > drive the coil of the power relay (kind of like a
> > darlington pair or a industrial controller).  one of the
> > gents on the list suggested that this had to do with limp
> > home mode & that does make some good sense.  If i am
> > right the resistors & "stuff" on the memcal does
> > something to the cpu to control injectors, the ign module
> > does spark & the other thing has to be fuel.  I would
> > think that there is a more high tech approach, but
> > sometime simplicity is the best way to go.  
> > 
> > Am i thinkin' to hard?  
> > thanks for listening. 
> > BW
> > 
> 
> Maybe, maybe not. We all know the ECM charges the rail fuel
> pressure at startup. Does it also provide power to the fuel
> pump once the motor has started? Or is that done only by the
> oil pressure switch after the motor has started?
> 
> The answers to those questions would likely show the
> original intent of the design.
> 
> Keith
> --
> 88 CSC (Down, but not out)
> kdmiller at westnet.com
> 
> 
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