350 starting without fuel pump relay.

[87silveradok20]

So today I was working on trying to figure out why my 1990 r3500 wouldn't start and I decided to switch the fuel pump relay into my 87 k20 to see if it was bad. Well the k20 fired right up. So I decided to try it without the really and I fired right up again. So I unplugged the relay that is next to it and again it fired right up. So my questions are 1 is this normal and if not what is making it do this. And 2 what is the other relay for.

 

[Georgeb]

hot fuel handling module?

 

[foamypirate]

TBI trucks have an Oil Pressure switch backup to the fuel pump relay.

 

[87silveradok20]

TBI trucks have an Oil Pressure switch backup to the fuel pump relay.

So you are saying that in the cranking it built enough oil pressure to flip the switch.

 

[Georgeb]

So you are saying that in the cranking it built enough oil pressure to flip the switch.

It could have.

 

[87silveradok20]

Alright. Thanks

 

[bucket]

TBI trucks have an Oil Pressure switch backup to the fuel pump relay.

Right. Common belief is that it kills the fuel pump if the oil pressure gets too low, but common belief is wrong.

 

[Jims86]

The fuel pump relay is only for the 2 second prime shot in these vehicles. If your oil pressure comes up quick enough, it will activate the pump. If oil pressure goes dangerously low, it will kill the power to the pump...unless the wiring has been dicked with.