GM Fuel Pump control scheme

[ae2598 at wayne.edu]

Since the Vega was brought up earlier, I figured I'd dig out my '71 shop
manual and take a peek at the theory of operation.  I quote:

"The fuel pump is energized when the ignition key is in the start
(cranking) position.  After the engine starts, the pump receives current
through the oil pressure safety switch as long as there is approximately 2
pounds of oil pressure.  If, for any reason, the oil pressure drops below
2 pounds, the contact is broken at the pressure switch and the pump is
de-activated."

The wiring diagram shows one terminal of the OP switch connected to the
starter, just like the old-school distributors.  This must be the cranking
bypass feature.

Now, they don't really define what they mean by "safety", but I'll bet
they're thinking about collision, fire, or some other catastrophe that
would cause the crank to stop turning.  Protection of the engine is
presumably left up to the operator.  The intent must be similar to Ford's
"slam switch" that they so cleverly hide in the trunk, which opens the 
fuel pump circuit upon impact.

On Mon, 29 Jan 2001 WEG1192 at aol.com wrote:

> In a message dated 01/29/2001 3:30:23 PM Eastern Standard Time, 
> ludis at cruzers.com writes:
> 
> << One problem with your theory - the fuel pump relay is controlled 100% of
>  the time by the limp home circuit.  The CPU (and PROM) need to do
>  nothing to operate the fuel pump relay.  In fact, they can't control the 
> relay.
>  
>  When initially powered up, the limp home circuit turns on the relay
>  briefly, and again whenever there is a distributor reference pulse. >>
> 
> Hmmm, this certainly blows a major hole in my theory. So the question 
> remains, exactly what is the OP switch for? If it were in series, I could 
> understand it as a safety from locking your engine, but in parallel, I for 
> one don't see the need for it. JW
> ----------------------------------------------------------------------------
> To unsubscribe from gmecm, send "unsubscribe gmecm" (without the quotes)
> in the body of a message (not the subject) to majordomo at lists.diy-efi.org
> 
> 


----------------------------------------------------------------------------
To unsubscribe from gmecm, send "unsubscribe gmecm" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org