1989 allegro question about electric booster fuel pump oil pressure safety switch

[89 allegro]

hi , i have a 89 chevy allegro that has a fuel pump on the engine and it has a small booster electric fuel pump back at the tank.it has to be there or it will sometime vapor lock and when i bought this rig 6 years ago im not sure if it was working or not, but when i drove it one hot summer day i did research and found it had one that was actually coated all in foam from the factory.i bought a new one and it would only get power when i first turned the ignition switch on for 5 seconds or so and then it would stop.so i traced the wire and hooked up a rocker switch so when i need it ,i just flip the switch on and off we go,but i would like it to be the way it was meant to be again.is what i have found is it also has a oil pressure switch that has to power it and im not sure if its bad or not because it has that switch and the other one that shows the oil pressure.i do know only that one powers it for safety, is what i need to know is if someone can tell me how to test it to see if its bad or its something else and not the pressure switch.im gonna attach a picture of the switch and if someone can tell me the process to test it on the rv it would be great.i seen it was gm part#3986857 and im not sure if i can even find it anymore ,so i dont want to search for it , if its not the problem.any and all help would be great. .thanks for looking

 

[Krazeehorse33]

I googled gm 3 wire oil pressure switch wiring diagram. If your pump isn’t running after your engine starts (and you know the pump is good because it works for 5 seconds when you turn key on) then in my mind it has to be the pressure switch OR a relay between the switch and pump. I would be inclined to lean toward relay failure but I’m not a mechanic. Maybe the relay is in the heavy fuse box in the engine compartment. If it is then there may be a matching relay that you could temporarily swap to see if that fixes it. Good luck!

 

[89 allegro]

I googled gm 3 wire oil pressure switch wiring diagram. If your pump isn’t running after your engine starts (and you know the pump is good because it works for 5 seconds when you turn key on) then in my mind it has to be the pressure switch OR a relay between the switch and pump. I would be inclined to lean toward relay failure but I’m not a mechanic. Maybe the relay is in the heavy fuse box in the engine compartment. If it is then there may be a matching relay that you could temporarily swap to see if that fixes it. Good luck!

actually i had replaced the relay , but only one from a salvage yard and i tried like 3 of them all same results , because like most parts for this, nobody makes a replacement.and from what i seen the relay has to be good because thats what makes it work those first 5 seconds, and i have great oil pressure ,it only has 73K miles.im just not sure on those 3 terminals on that oil switch , what is supposed to have power when its running and from where it comes from. i did find a diagram thats close to mine, but not sure and it dont really say whats supposed to be hot when engine is running and whats not or how to try and bypass it to see if its something else ,or it. thanks

 

[HueyPilotVN]

You can bypass the low oil pressure shutoff by running a wire directly to the pump. I did that with a switch at the dash when I added a booster pump on mine years ago.

The main reason for the low pressure shutoff is in case of an accident it shuts off the fuel pump to avoid a fire.

 

[Krazeehorse33]

I’m not sure about terminals either. They may be all grounds. One going in to the switch. Then one terminal normally closed so it has continuity without pressure. That probably feeds the red light on your dash. Then the other terminal is normally open, the circuit doesn‘t close until there’s pressure and then grounds and closes the relay. Just a guess.

 

[89 allegro]

You can bypass the low oil pressure shutoff by running a wire directly to the pump. I did that with a switch at the dash when I added a booster pump on mine years ago.

The main reason for the low pressure shutoff is in case of an accident it shuts off the fuel pump to avoid a fire.

yes , i know that and i have it by passed now and it works fine ,i would like it the way it was supposed to be if thats possible, thanks

 

[89 allegro]

I’m not sure about terminals either. They may be all grounds. One going in to the switch. Then one terminal normally closed so it has continuity without pressure. That probably feeds the red light on your dash. Then the other terminal is normally open, the circuit doesn‘t close until there’s pressure and then grounds and closes the relay. Just a guess.

im not sure either , but im gonna try to figure it out . thanks

 

[HueyPilotVN]

I can understand wanting to retain the original safety feature of the low oil pressure disconnect for the fuel pump.

There is another option. My 32 year old Mustang and many other cars have addressed the issue with an inertia switch to disconnect the power in the event of an accident. Mine is located in the trunk.

The easiest solution if you can find the replacement parts is to go with original system, but if you cannot, then you have options.

 

[89 allegro]

I can understand wanting to retain the original safety feature of the low oil pressure disconnect for the fuel pump.

There is another option. My 32 year old Mustang and many other cars have addressed the issue with an inertia switch to disconnect the power in the event of an accident. Mine is located in the trunk.

The easiest solution if you can find the replacement parts is to go with original system, but if you cannot, then you have options.

yep, i found a switch from holley that you wire yourself to the same spot where that oil switch goes if worst comes to worst i may have to do that ,that switch is like $40 ,but it may be what i have to do to solve the problem.i was hoping to just be able to know how to test to see ,but that may not be possible. thanks for the suggestion.