"I think the eletric fuel pump power circut is wired so that oil pressure is required for the electric pump to run."
That's NOT true with those vehicles. The fuel pump relay (operated by the 'puter) and the fuel pump oil pressure switch are in PARALLEL.
The fuel pump will run as long as the 'puter keeps the relay energized 'cause it knows the engine is cranking or running OR as long as the oil pressure switch remains activated.
The oil pressure switch serves as a fail-safe for the relay, in the "ON" mode, meaning if the relay or relay circuit fails the fuel pump will still get energized, but after a longer cranking period 'til the oil pressure switch gets activated.
I know that flies in the face of conventional wisdom, but that IS how GM set these system up in that era. The oil pressure switch is redundant, and the vehicle will operate normally without it, AS LONG AS THE RELAY AND ASSOCIATED CIRCUITS ARE PROPERLY FUNCTIONING.
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Today's Featured Article - Field Modifications (Sins of the Farmer) - by Staff. Picture a new Chevrolet driving down the street without it's grill, right fender and trunk lid. Imagine a crude hole made in the hood to accommodate a new taller air cleaner, the fender wells cut away to make way for larger tires, and half of a sliding glass door used to replace the windshield. Top that off with an old set of '36 Ford headlight shells bolted to the hood. Pretty unlikely for a car... but for a tractor, this is pretty normal. It seems that more often than not they a
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