I have read that if you pull fuel pump relay while engine is running, the oil pressure switch would act as a backup and keep it running if the oil pressure stayed above 5psi. If the engine died, this indicated that the oil pressure sensor/switch needed to be replaced.
The Haynes schematic seems to support that theory. I have tried on both a '99 and '00 in both crank and normal operation and they both die. Are the sensors/switches bad? If not what would the purpose of the oil switches be?

 

The Helms service manual says that the 'check gauges' lamp will illuminate if the oil pressure sensor reads too low oil pressure. The 'check gauges' lamp will also illuminate in case of excessive coolant temperature. These sensors feed the lamp via the VCM (vehicle control module). There is neither a description of fuel cut-off nor maintenance by the oil pressure sensor.
I'm not as familiar with Blazers but do know that a similar myth regarding Corvettes 'oil cut-off or sustaining the ignition' floats around. And the Corvette Helms service manual clearly shows its second oil pressure sensor feeding the 'check gauges' light via the computer (the other sensor feeds the gauges). I could not find a similar schematic for the 2000 Blazer, only the text description. That description mentions only one oil pressure sensor.
I have great trust in the Helms manuals, having inflicted permanent and expensive damage following the instructions in some less expensive manuals.

 

The Haynes doesn't show 2 sensors. It shows an oil pressure sensor and an oil pressure switch. The oil pressure switch is directly in parallel with the fuel pump relay. The oil sensor goes to the dash indicator and also controls the oil pressure switch. The schematic also implies that the sensor/switch is inside a single module.

I've never used a Helms, I would think it should have a schematic of the switch if it does exist for the '00 and '99.

Thanks