Electric fuel pump

[Quikshft]

Wish my new L31 had provision for the mechanical fuel pump but alas - no.

I'm going to run the Holley Might Mite, and already have a relay and oil pressure safety switch like depicted in the diagram. I believe I can run the wires as the diagram shows, but as an electrically challenged individual I am not certain what is happening at the safety switch. I think the desire is for the fuel pump to run while the engine is cranking? So the wire from the "S" terminal on the starter solenoid will provide that correct? Then when the engine starts, the pressure switch closes and the solenoid quits feeding but a "Key hot" from the ignition switch takes over? Does that "Key hot" really need to come from the ignition switch or can I find something on the fuse block near the clutch/brake pedals under the dash?

For my own peace of mind let me know if I've got this right. Thanks!

[randy500]

Bottom right of the fuse box has 2 terminals that are unfused and hot when key is on.
Deceptively marked ign unfused....
The engine wiring harness depending on year, 71-2 for sure has a brown with white stripe wire that is hot when key in on also, I believe it’s unfused, small gauge so didn’t try to pull much from it, it would be fine to trigger the relay. This wire is hangs out just above the engine side harness connector, connected to TCS wiring harness if equipped.

[davepl]

Only extra thing I did on mine was ground the relay through the oil pressure sender and added a Duramax "lift pump" circuit. This gives me two major features:

1) The fuel pump runs for 5-6 seconds every time I turn the key on, but then stops running if it does not see oil pressure

2) The fuel pump stops in an accident or similar if the oil pressure drops (loses ground at the relay).

[Bigdav160]

I am not familiar with that 'Pressure Safety Switch'. Did it come with the fuel pump?

davepl said:

1) The fuel pump runs for 5-6 seconds every time I turn the key on, but then stops running if it does not see oil pressure

How is that controlled (timed)? Fuel injection controller?

[VetteVet]

Originally Posted by Bigdav160

I am not familiar with that 'Pressure Safety Switch'. Did it come with the fuel pump?

davepl said:



How is that controlled (timed)? Fuel injection controller?

It is my belief that the fuel pump will start and stop in cycles under normal operation because of a pressure switch built into it that shuts it off when the pressure reaches a certain level, about 40 psi. Or whatever the injecters are rated at. When you turn on the key the fuel pressure builds and shuts off when that is reached. Not because of the oil pressure switch. Otherwise the pump would not cycle on and off when the engine was running and oil pressure is constant. If you listen to the pump you can hear it cycling while the engine is running.
There may be relays that will shut off the pump if oil pressure is lost while the engine is running but I don't think that has anything to do with the normal operation of the pump. I'll have to look at the schematics to verify that.

[VetteVet]

Originally Posted by Quikshft

Wish my new L31 had provision for the mechanical fuel pump but alas - no.

I'm going to run the Holley Might Mite, and already have a relay and oil pressure safety switch like depicted in the diagram. I believe I can run the wires as the diagram shows, but as an electrically challenged individual I am not certain what is happening at the safety switch. I think the desire is for the fuel pump to run while the engine is cranking? So the wire from the "S" terminal on the starter solenoid will provide that correct? Then when the engine starts, the pressure switch closes and the solenoid quits feeding but a "Key hot" from the ignition switch takes over? Does that "Key hot" really need to come from the ignition switch or can I find something on the fuse block near the clutch/brake pedals under the dash?

For my own peace of mind let me know if I've got this right. Thanks!

Looking at the diagram I see that the N and C are connected but no power goes to them until the S terminal (starter solenoid) is energized. This closes the FP relay and the pump runs until the starter is de-energized. Then the oil pressure switch is closed from the ignition switch through NO to C which keeps the FP relay energized until the key is shut off or the oil pressure drops and re opens the connection between the NO and C connection.

The only thing missing is how the FP relay is closed when just the key is turned on initially. I believe there is a separate circuit which is the fuel pump prime circuit not shown on the diagram above which has a separate relay circuit. I seem to remember reading about something like that in the manual.

Some of our newer vehicle mechanics may be able to better explain how that works if they chime in.

[dmjlambert]

I think this is a much simpler way of showing the same wiring. It's easier to understand.



You need 3 types of 12V power: un-switched (battery), hot with key on (called ignition in this diagram), and hot during start. It allows the pump to get power during start when pressure switch is not yet closed, then after start the pump continues to get power until pressure switch loses pressure.

Adding impact and/or roll over switches would add even more safety.

[Quikshft]

Ah, this is simpler. I've spent a lot of time staring at that diagram I posted, wish I had found this one first. Thanks for adding that in here.

Originally Posted by dmjlambert

I think this is a much simpler way of showing the same wiring. It's easier to understand.



You need 3 types of 12V power: un-switched (battery), hot with key on (called ignition in this diagram), and hot during start. It allows the pump to get power during start when pressure switch is not yet closed, then after start the pump continues to get power until pressure switch loses pressure.

Adding impact and/or roll over switches would add even more safety.

[davepl]

Originally Posted by Bigdav160

I am not familiar with that 'Pressure Safety Switch'. Did it come with the fuel pump?

davepl said:

How is that controlled (timed)? Fuel injection controller?

No, it's a little black box that GM or someone makes for the Duramax trucks to prime the fuel system; it does essentially for the diesel lift pump what you want for a priming circuit: run for a few seconds when the key comes on, or when oil pressure is present.

I also added a Ford Explorer impact switch to all my fuel pump cars, and have never had a false alarm, so they're pretty robust.

[Quikshft]

Originally Posted by randy500

Bottom right of the fuse box has 2 terminals that are unfused and hot when key is on.
Deceptively marked ign unfused....
The engine wiring harness depending on year, 71-2 for sure has a brown with white stripe wire that is hot when key in on also, I believe it’s unfused, small gauge so didn’t try to pull much from it, it would be fine to trigger the relay. This wire is hangs out just above the engine side harness connector, connected to TCS wiring harness if equipped.

Got some garage time tonight. I did have the brown/white stripe wire dangling in the engine bay. Lights up when the key is turned to 'on' so I think I'll use it for the 'key hot' that runs to the oil pressure switch.

[Big Kev-O]

I used a Holley Mighty Mite for a couple years and it worked perfectly. Glad I found this post because I never thought about adding any form of safety relay to cut off the fuel.

May I ask what part number are you using for the oil pressure switch and where you place it on the engine?
Thank you.

Just FYI, This pump does not have an internal pressure switch, if it has power it is running.

[6T7 C10]

Along with a relay I also put in a kill switch for security and an inertia switch (part number FV7) for safety.

[Quikshft]

Originally Posted by Big Kev-O

I used a Holley Mighty Mite for a couple years and it worked perfectly. Glad I found this post because I never thought about adding any form of safety relay to cut off the fuel.

May I ask what part number are you using for the oil pressure switch and where you place it on the engine?
Thank you.

Just FYI, This pump does not have an internal pressure switch, if it has power it is running.

I'm using this switch from Jegs, the threads are 1/8" pipe. I was going to put it into the port that is just above the oil filter on the side of the block but wasn't really enthused about running the wires down to it there. So I got a 1/8" galvanized Tee and I installed it into the port on the back of the block near the distributor. This is the port that the line for the mechanical oil pressure gauge runs to. I put a nipple into the block and the Tee then installed on the nipple. Now I had two ports available one for the oil pressure gauge and one for the oil pressure safety switch.