Car fire followup question, electric fuel pump cutoff

[Bruce B.]

After reading the comments in the "Car Fire" post, I realized that I need to install a cutoff switch on the fuel pump for the Hilborn setup in my 62.
The fuel pump is controlled through a relay board which supplies the 12 volt power.
The question is what is better, using the switch (30 amp rated) on the hot side or the ground side of the fuel pump?

Since the pump , a Walbro GL392 is capable of 255 LPH at 130 psi, I definately need a way to stop the pump besides using the ignition key. The actual fuel pressure at the injectors is 48 psi.

[Steven G.]

Re: Car fire followup question, electric fuel pump cutoff

Bruce, break the Hot, I can't imagine the other way, maybe someone can argue otherwise. Steve

[Steve V.]

Re: Car fire followup question, electric fuel pump cutoff

How about an low oil pressure cutoff switch? Motor stops running, no oil pressure, no power to the fuel pump.
Steve

[Steven G.]

Re: Car fire followup question, electric fuel pump cutoff

A roll over switch is another option. Steve

[Terry M.]

Re: Car fire followup question, electric fuel pump cutoff

How about an low oil pressure cutoff switch? Motor stops running, no oil pressure, no power to the fuel pump.
Steve

Interestingly enough this is how some "modern" fuel injected cars work.

[Ronald L.]

Re: Car fire followup question, electric fuel pump cutoff

Hot side, have to cut the juice and not pass it through the pump and expect the system to be fully shut off. Mid year wiring is not dead until you cut the positive for example.

[Clem Z.]

Re: Car fire followup question, electric fuel pump cutoff

How about an low oil pressure cutoff switch? Motor stops running, no oil pressure, no power to the fuel pump.
Steve

you will have to put in a bypass push button to allow the pump to run till the oil pressure come up or the car will not start. modern EFI the computer runs the fuel pump for several seconds to prime the system till the oil pressure comes up when the engine starts.

[John H.]

Re: Car fire followup question, electric fuel pump cutoff

Bruce -

The carbureted Vega engine used a tank-mounted electric fuel pump, although it only generated 5-6 psi; its power went through a 3-prong sending unit screwed into the main oil gallery so the pump only got current when the engine had oil pressure. The third prong got power from the "start" circuit during cranking so the pump got power before oil pressure built up.

You could use this type of switch to trigger a relay for your 30-amp pump circuit; engine stops, pump stops.

[Bruce B.]

Re: Car fire followup question, electric fuel pump cutoff

Thanks to everyone for the ideas and info.
Bruce B

[Joe R.]

Re: Car fire followup question, electric fuel pump cutoff

I vote for breaking the ground side of the circuit. The switch will last longer because there is less voltage to possibly arc.

Joe

[Steve V.]

Re: Car fire followup question, electric fuel pump cutoff

I vote for breaking the ground side of the circuit. The switch will last longer because there is less voltage to possibly arc.

Joe

Not a good idea. If you have a short to ground in the fuel pump, the pump keeps on running. If you have a short to ground in the hot circuit feeding the pump then you have sparks and a potential fire.

Steve

[Joe R.]

Re: Car fire followup question, electric fuel pump cutoff

Not a good idea. If you have a short to ground in the fuel pump, the pump keeps on running. If you have a short to ground in the hot circuit feeding the pump then you have sparks and a potential fire.

Steve

Steve,

The original question was which side to break the circuit, hot or ground. That tells me the fuel pump is not using its case to return current to the battery. Or in other words, the case is not grounded. So if the return is coming from the wiring, how can a short to the case complete the circuit? I don't see it. Now if the case of the fuel pump is grounded, it probably is better to switch the hot lead. I'm not sure what to make of your second sentence, but a fuse in series with the hot lead is for sure a good idea in case of a short to ground or just plain old overcurrent.

Joe

[Steve V.]

Re: Car fire followup question, electric fuel pump cutoff

Steve,

The original question was which side to break the circuit, hot or ground. That tells me the fuel pump is not using its case to return current to the battery. Or in other words, the case is not grounded. So if the return is coming from the wiring, how can a short to the case complete the circuit? I don't see it. Now if the case of the fuel pump is grounded, it probably is better to switch the hot lead. I'm not sure what to make of your second sentence, but a fuse in series with the hot lead is for sure a good idea in case of a short to ground or just plain old overcurrent.

Joe

Joe,

You are correct in stating the fuel pump is not grounded internally. Typically you would run a hot wire fed from the ignition switch run circuit through a fuse to one terminal on the fuel pump and then run a wire from the second terminal to the nearest ground point on the frame. If you chose to switch the ground circuit then you must run the second wire back to the dashboard and install a separate switch just for the fuel pump. There is no practical reason to do this because turning the ignition switch off removes the power feed to the pump. There is no way to switch the ground wire off with the ignition switch. Now since we agree that the case is not grounded internally then if the electric motor that runs the fuel pump was to develop a short in the internal wiring it would be possible for the pump to keep running if the ground circuit was switched off and the ignition switch was still on. In an automobile the ground circuit is always attached to the closest suitable ground point on the frame or block and the hot wire is always fused and switched. There is no reason to do otherwise that I am aware of. Also there is no less voltage or current on the ground wire than there is on the hot wire. There both the same potential all the time.
Steve

[Bruce B.]

Re: Car fire followup question, electric fuel pump cutoff

FYI;
The fuel pump is grounded internally as both the hot wire and the ground wire are insulated as they pass through the pump case.

My initial concern was prompted when someone mentioned that in the car fire video it appeared that the fuel pump was feeding the fire. We obviously do not know if the ignition switch was on or off and a short was possibly powering the fuel pump.

So I thought that in either situation (switch on or short) the best way to stop the pump was to remove the ground circuit by using a appropriate switch.

In reality I think the best way would to have a master cutoff switch in a remote location away from the engine or fire. This switch would stop the fuel pump, electric fans and eliminate any short circuits.

I appreciate the info everyone has provided.

Bruce B.

[Terry M.]

Re: Car fire followup question, electric fuel pump cutoff

Bruce, in an automobile fire that involvers the wiring it is common for the insulation to melt and the wires short together. A battery cut off or master switch is one solution, but if the engine continues to run the alternator will provide power to the shorted wires. I am not sure there is a solution that covers all potential (bad pun) situations.