GM Fuel Pump control scheme

[Bob Valentine]

Yikes, I started a monster with this.    My Cutlass doesn't run a OP
switch, and I've never had a problem either.   I ran a few miles in limp
home, no problem with FP tho...  This was with a '730, V8 memcal.   

Limp home mode was caused due to flashlight falling off seat onto ECM thus
knocking exposed eprom in ZIFF socket out of socket, while spilling cup of
coffee into ECM in attempt to grab said flashlight .     Hope someone gets
a laugh out of this.  Wasn't very funny in East Nowheresville, PA at 1am,
on the way to the efi pow-wow last year.  Spare ECM got me out of that jam.   

-> Bob Valentine
-> bob at tecmark.com


   
(on the way to the efi pow-wow no less.... 
At 09:35 PM 1/28/2001 -0700, you wrote:
>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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>
>
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