I know this has been convered but i am new and still confused. I am wrapping up my lt1 swap into a landcruiser and am wiring up a MSD inline pump. I have my schematics but a little unsure how this is actually working. From what i have read, the PCM will turn the fuel pump on for 2 seconds prior to starting. Once started if the pcm is getting ignition pulses it will continue to give the fuel pump relay 12v to operate the pump. Now, i have also read that if the oil pressure drops below 4psi the fuel pump should turn off as safety. Now, the oil pressure switch has 3 wires, a red that comes off of the pcm hot at all times 20 amp fuse, a yellow that spices into the power supply wire to the pump (after the relay) and i also have a wht/brn wire that is coming off of the oil switch but it isn't s hown in my schematic. I don't understand how low oil pressure could kill the fuel pump with this setup? It seems to me that if the relay went bad, i could still get power to the fuel pump through the oil switch, but its wires seem to small to handle the current the pump would draw. any help!! thanks a ton!

Oh, i also have just assumed that as long as my pump can deliever enough fuel the factory fuel pressure regulator that is on the fuel rail with work?

 

The two larger wires in the oil pressure switch (cavities C and D) are used to give power to the fuel pump when there is oil pressure. In addition, the PCM will provide power to the fuel pump relay when the engine is running. If the relay dies the pump will still run through the oil pressure switch.

I don't have any schematics handy, but the few LT1 harnesses I've worked with only have the tan wire on the oil pressure switch for the sending unit...the other two cavities are blocked with cavity plugs. I've kinda passed by the LT1 harnesses because it's not a very desirable engine to me...considering the LS type engines.

The MSD high pressure pump will work fine with your stock fuel pressure regulator.

 

thanks for the reply, that makes more sense. But, what is up with the oil pressure switch acting as a safety mechanism? If the relay is powering the fuel pump and oil pressure goes to zero how does that kill the engine? If the PCM knew what my oil pressure was i could see it not giving a signal for the relay but there are no sensors that send the oil pressure to the pcm. From what i just read, it sounds like the oil pressure switch circuit is just a backup.

 

GM never said it was a safety mechanism, but many have made that assumption. It's a redundant power source for the fuel pump. If the fuel pump relay dies, the pump still runs.

This topic has been discussed several times on this forum.

 

cool, thanks so much for the clarification. All i have ever heard is that it is a safety mechanism. Was it ever this way on any other GM engines? I.e Vortec?

 

No, the Vortec is the same.