Electric Fuel Pump, Safety Switch and Starter Circuit.....

[AndyMenon]

Hello Folks,

I've already wired my Holley electric fuel pump as shown in the wiring diagram attached. I've used a separate relay harness with the proper gauge wires, and an oil pressure safety switch in the circuit (because my motor has a carb). What remains is the purple "starter" wire leading out of the fuse box in the wiring harness. Where does this go to? I have a dotted line in there indicating a possibility. Can somebody help confirm the question I have on the sketch attached?

Thanks much!
Andy

 

[PotvinGuy]

Oops! The oil pressure safety switch can be used to kill the ignition if you lose oil pressure. It doesn't belong in the fuel pump wiring or the starter wiring. If you put it in the fuel pump wiring it would turn off the pump if oil pressure was lost, but the motor would keep running on the gas in the carb until the motor seized and ruined your day.

Whose harness? Sounds like they don't include good instructions.

I'd wire the fuel pump relay to run whenever the ignition is on. And the purple "starter" wire does go to the solenoid, and the blade connector should be the only one on it.
And if you want to use the safety switch, I'd use it to turn on a big red dash light to alert the driver to pull over fast. Killing the ignition on the freeway is dangerous.

Let me know if you'd like a wiring diagram for all this. Oh, the C on the switch stands for common, not closed.

 

[Wild Mango]

You can use the oil pressure switch to make sure the fuel pump only runs when the engine is running. This gives assurance that when things go accidentally awry, like the car ends up inverted, or a fuel line fails, the fuel pump will not continue to deliver fuel if the ignition circuit is still energised. Some rule books demand this, some also want a "tip" switch in the circuit as added protection.
Should you wire it this way, (I do it myself) you can use the NC contacts in the pressure switch to deliver power to the pump from the starter solenoid while she's cranking.
PotvinGuy is right on the money about relying on the switch to protect your engine from an oil pressure failure - you can't.

 

[AndyMenon]

Oops! The oil pressure safety switch can be used to kill the ignition if you lose oil pressure. It doesn't belong in the fuel pump wiring or the starter wiring. If you put it in the fuel pump wiring it would turn off the pump if oil pressure was lost, but the motor would keep running on the gas in the carb until the motor seized and ruined your day.

Whose harness? Sounds like they don't include good instructions.

I'd wire the fuel pump relay to run whenever the ignition is on. And the purple "starter" wire does go to the solenoid, and the blade connector should be the only one on it.
And if you want to use the safety switch, I'd use it to turn on a big red dash light to alert the driver to pull over fast. Killing the ignition on the freeway is dangerous.

Let me know if you'd like a wiring diagram for all this. Oh, the C on the switch stands for common, not closed.

I'm a bit confused.Maybe my fundamentals are goofed up? What is the situation under which the motor can remain running and still not have oil pressure? Other than a catastrophic accident, I was under the impression that the oil pressure would die when the engine is dead to begin with. For example, go too slow over a bump, and the motor stalls? Isn't the oil pressure safety switch named so because it is mounted at the same location as the oil pressure gauge sending unit? My thinking is that the oil pressure switch is actually sensing the vacuum pressure inside the motor. Also, is it not true that the switch is trigerred by the engine vacuum pressure, and not the other way round? No engine vacuum, switch shuts off pump because it connects ignition to starter. Engine fires up, and switch connects pump to power via the relay?

Thanks!
BTW, the relay wiring is from the Holley fuel pump instructions manual.

 

[AndyMenon]

Hi, about your last comment on using the switch to protect my engine from oil pressure failure, that is not the intent. The idea behind using the switch is to shut off the fuel pump when the engine does not run. The switch has no role in protecting the engine. If the engine vacuum dies (say that I tore the oil pan off), the switch is triggered to shut the fuel pump off. In other words, the switch is triggered by an event in the motor, and not the other way round.

Please let me know if this reasoning is correct.

Thanks

 

[AndyMenon]

Oops! The oil pressure safety switch can be used to kill the ignition if you lose oil pressure. It doesn't belong in the fuel pump wiring or the starter wiring. If you put it in the fuel pump wiring it would turn off the pump if oil pressure was lost, but the motor would keep running on the gas in the carb until the motor seized and ruined your day.

Whose harness? Sounds like they don't include good instructions.

I'd wire the fuel pump relay to run whenever the ignition is on. And the purple "starter" wire does go to the solenoid, and the blade connector should be the only one on it.
And if you want to use the safety switch, I'd use it to turn on a big red dash light to alert the driver to pull over fast. Killing the ignition on the freeway is dangerous.

Let me know if you'd like a wiring diagram for all this. Oh, the C on the switch stands for common, not closed.

Hi PotvinGuy,

Here are the instructions for the safety switch installation.

http://www.holley.com/data/Products/Technical/199R10367.pdf

To be more specific Figure 3 on Page 4 of this document.

Let me know if this makes sense.

thanks,
Andy

 

[PotvinGuy]

The Holley diagram is correct, but step 6 doesn't match it; you connect the relay, not the pump, to C.

A situation where the motor is running but oil pressure is lost would be an oil pump failure or oil level too low in the pan. And I had a real puzzler once; the distributor raised up just enough to disengage from the oil pump, but it still was being driven by the camshaft and the motor continued to run, but without oil pressure.

Don't understand your "vacuum pressure" comments; the safety switch senses oil pressure.

These safety switches are common on race cars, where they are often required by rule, and cut the fuel flow if the motor dies in an accident. Of course the car can crash and the motor stays running, so it isn't a foolproof deal. But if you have one, it can't hurt and might help some day.

 

[Wild Mango]

In a race engine it is common to use the oil pressure switch to kill the ignition as well. In the Midget the "on" power to the Motec unit goes through the NO contacts on the switch, when we push off the oil pressure comes up and switches on the ignition. If she tosses a belt on the pump stack, for instance, the motor stops.
As Potvinguy says, this is a very bad thing to have on a road car, in some traffic situations losing the engine suddenly will put you in danger big time. In this case, a 12V Piezo screamer from a home alarm system is a great warning device and impossible to ignore even over the wind blast and header song as you thunder down the freeway.

 

[grazza]

I would have thought having the fuel pump continue to work after the engine has shut down would be far more dangerous than having the engine fire go out anywhere, even on the freeway during peak rush hour. When I was Speedway racing, our electric fuel pumps had to shut off atomatically as soon as the engine stopped for whatever reason (stalled, roll over whatever). We used either the oil pressure switch as already mentioned or also a cut out switch from a LPG (I think you guys call it Propane ???) fuel system. Just remember to put an override system in place as well, so you can "prime" a dry carb before starting for the first time.

 

[AndyMenon]

Hi again,

Thanks for all the informative comments. Here is a very interesting article on wiring an electric fuel pump.

Electric Fuel Pump: How to Do It Right . http://www.how-to-build-hotrods.com/electric-fuel-pump.html
I had this article printed out, and looked it over once more, and I was able to see where I missed out . With that cleared up, I was able to wire up the circuit.

I was in the garage late last night and sort of turned the car into a high school electrical lab

Without turning on anything, I was able to test the circuits with a tester. And now, I see that the oil pressure switch turns on the relay trigger when I turn the ignition key to crank the starter (the relay "clicks", tester indicates full voltage to the end of the starter wire, and also at the (NC) terminal on the pressure switch). When the IGN returns back to RUN position, I see full voltage at the IGN terminal on the ignition switch.

To make sure that the fuel pump is turned on, I will temporarily wire up the electric fan to the fuel pump relay and see if it turns on on the "output" side of the relay.

Cheers,
Andy

 

[AndyMenon]

I would have thought having the fuel pump continue to work after the engine has shut down would be far more dangerous than having the engine fire go out anywhere, even on the freeway during peak rush hour. When I was Speedway racing, our electric fuel pumps had to shut off atomatically as soon as the engine stopped for whatever reason (stalled, roll over whatever). We used either the oil pressure switch as already mentioned or also a cut out switch from a LPG (I think you guys call it Propane ???) fuel system. Just remember to put an override system in place as well, so you can "prime" a dry carb before starting for the first time.

You're right. I looked at an instruction manual for a pressure switch that SUMMIT Racing is selling, and it talks about having a bypass circuit to override the pressure switch. But it states that it is for pumps that draw more than 5 AMPS. If memory serves me right, Holley pumps do not pull more than 4 AMPs. Or do they?

As far as the carb is concerned, I would pour in a pre-defined quantity of fuel directly into the bowl before I fire the motor up. I think we have to do this anyways regardless of whether or not we use an electric fuel pump.

thanks,
Andy

 

[AndyMenon]

The Holley diagram is correct, but step 6 doesn't match it; you connect the relay, not the pump, to C.

A situation where the motor is running but oil pressure is lost would be an oil pump failure or oil level too low in the pan. And I had a real puzzler once; the distributor raised up just enough to disengage from the oil pump, but it still was being driven by the camshaft and the motor continued to run, but without oil pressure.

Don't understand your "vacuum pressure" comments; the safety switch senses oil pressure.

These safety switches are common on race cars, where they are often required by rule, and cut the fuel flow if the motor dies in an accident. Of course the car can crash and the motor stays running, so it isn't a foolproof deal. But if you have one, it can't hurt and might help some day.

Hi PotvinGuy,

Right, I understand now. Yes, the relay trigger (86) wire from the relay hooks up to (C) as show in my diagram. The fuel pump was the second appliance I wired on the car about a month ago. At that time I was able to test the fuel pump relay by wiring the electric fan to it (obviously because I cannot dry-run the pump). Finally, when I inserted the safety switch, I wrestled with the starter wire issue. I've cleared that hurdle (as mentioned in my post elsewhere here).

Thanks,
Andy

 

[Wild Mango]

Hi again,

Thanks for all the informative comments. Here is a very interesting article on wiring an electric fuel pump.

Electric Fuel Pump: How to Do It Right . http://www.how-to-build-hotrods.com/electric-fuel-pump.html
I had this article printed out, and looked it over once more, and I was able to see where I missed out . With that cleared up, I was able to wire up the circuit.

I was in the garage late last night and sort of turned the car into a high school electrical lab

Without turning on anything, I was able to test the circuits with a tester. And now, I see that the oil pressure switch turns on the relay trigger when I turn the ignition key to crank the starter (the relay "clicks", tester indicates full voltage to the end of the starter wire, and also at the (NC) terminal on the pressure switch). When the IGN returns back to RUN position, I see full voltage at the IGN terminal on the ignition switch.

To make sure that the fuel pump is turned on, I will temporarily wire up the electric fan to the fuel pump relay and see if it turns on on the "output" side of the relay.

Cheers,
Andy

Andy, using your tester or multimeter to check out your circuits is exactly the right way, and the way all professionals do it too. Using your instruments correctly will never blow anything up or burn out any wiring. Drawing diagrams as you go is something we all do as well. Learning new skills is always fun, and learning by doing is a good way, we can see thats what you are doing here. Happy tinkering!!!!!!

 

[AndyMenon]

Andy, using your tester or multimeter to check out your circuits is exactly the right way, and the way all professionals do it too. Using your instruments correctly will never blow anything up or burn out any wiring. Drawing diagrams as you go is something we all do as well. Learning new skills is always fun, and learning by doing is a good way, we can see thats what you are doing here. Happy tinkering!!!!!!

Thanks WildMango!

This has been a very pleasant experience indeed because I've been doing a lot of reading over the past winter to refresh my electrical lessons that I went thro' in high school. I can't agree with you any less about using a multi-meter (I stand corrected. The use of the term "Tester" is an old habit of mine, that I need to change).

Once I've wired up a circuit, I check the end of each wire to make sure that it's getting power even before I decide to plug in the appliance on that circuit. Infact, all high-amp consumers have been wired using relays. Someday when I'm done with this, I will post all of my drawings here so that someone doing this for the first time can benefit from this exercise !

Happy tinkering to you too!

Andy