Car fire followup question, electric fuel pump cutoff

[Joe R.]

Re: Car fire followup question, electric fuel pump cutoff

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

Steve,

Here's an excerpt from what's in the original post:

"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."

Bruce wants a separate shutoff switch, not the ignition switch. If you did want to use the ignition switch and you wanted to switch the ground, you could use a relay to do that. I don't believe Bruce mentioned where he wanted to mount the switch, so I don't know that we're running any wiring through the firewall back to the dashboard.

"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."

What???? The hot wire is at nominally 12 Volts DC, the ground wire is at, well, ground (0 Vdc). Now, as far as current goes, assuming that the only load on the two wires is the fuel pump, they will both carry the same amount of current.

Joe

[Bruce B.]

Re: Car fire followup question, electric fuel pump cutoff

Terry,
Thats a very interesting point.
After I retired from my real job, I worked at a body shop for 4 years.
One of the most aggrevating things we ran into (and still do) was when there was a car accident, if the fire department was on the scene they would cut the power and ground wires at the battery if possible. Since batteries are in some hidden places on some cars they would just cut lots of wires. Cost the owner or the insurance company lots of money.
I fully understand why the did it but it was still a PITA.