Here’s an interesting electrical question/poll.
 

Here’s an interesting electrical question:
Let’s say you have rolled your Cobra on a race track…
Your engine is racing, and fuel is beginning to leak from the fuel tank…
You are still inside the car fumbling with your harness and are moderately shaken up…
The emergency crew arrives and immediately flips your batteries emergency power switch to the off position…
*
Scenario A:
You have an electric fuel pump in your car.
*
Scenario B:
You have a mechanical fuel pump in your car.
*
What happens when the emergency crew flips the battery cut-off switch to the off position?
*
Curious to know what the general consensus is as to the answers for scenario's "A" & "B"…
Don’t research the answer, Just a quick poll to check what people assume the result will be…
No explainations now please, just 2 answers.
More on this later…thanks

If wired correctly it should do two things in either scenario,
1. disable the ignition system to stop the engine.
2. disconnect all electrical power coming from the battery.

A: The engine eventually stops running due to lack of fuel.

B: The engine may continue to run off of the alternator and the MFP will continue to pump fuel.

Bob

In either case the engine will stop and fuel will quit pumping UNLESS you are running a magneto (not very common today) in the ignition system. With a magneto providing ignition the MFP car could keep running.

A @ B the car keeps running until fuel pressure drops.........
when they turn the switch off car keeps running off the alternator.........
this is my guess.........

The engine stops because the battery switch should be wired to shut off all electricity to the car; ie: ignition. No igintion, no spark; no spark, no run.

HOWEVER, Blas never stated "where" exactly the fuel was leaking from the tank. If the tank is punctured, then fuel will leak regardless of whether there's battery power, or electric/mechanical pump, etc... And this can be problem.

In either case the engine will stop and fuel will quit pumping and the engine will shut down, unless it's an original 289 with a Generator. I believe a generator will continue to generate power with the battery turned off. The engine may continue to run off the generator.

Good question. -- Inertia kill switches anyone >?

This is a very good question. After thinking about it, I believe that the alternator will maintain power and the engine will continue running with either type of fuel pump until:

A. You switch off the key inside the car.
B. The High revving engine throws a rod or suffers other internal damage.

Scenerio C. If your car is like mine and it has electric fuel pumps and a carburetor. The ground connection of my fuel pump is connected to an inertia switch. If the impact was hard enough to trigger the switch, the engine will stop running when the fuel in the carburetor drops below the jets or it blows up. If the switch does not trigger (very likely in a soft rollover), it will keep running.

Scenerio D. If your car has elecric fuel injection with electric fuel pump, it probably has the Mustang's inertia switch, too. If the impact was hard enough to trigger the switch, the engine will stop running quickly when the fuel pressure drops. If not, it will keep running.

After considering this, I want to move the ground of my ignition system to my inertia switch along with the fuel pumps. Then, I will move the ground wire from the inertia switch to a separate connection on the external cutoff switch. If it doesn't have a second connection, I will use a relay to make sure that both circuits are turned off with the switch.

It is simple to test this. With the car idling in the driveway, turn off the external switch. Surprise!

Again, very good, thought provoking, question. :)

Oh, Yeah!
 

:) Nooch, I'm very fond of your perspective on such an event....would make even a dull day suddenly fascinating.

Alternators MUST have battery power to keep making electricity - the field windings cannot generate anything without it. Shutting off a disconnect removes that power, and everything then stops.

Generators do make their own power and could keep things running, but with the charging circuitry now facing an open circuit with the disconnect shut off it's hard to see how power will continue to reach the coil....with the result being that all stops.

I had my electric fuel pump wired through an oil pressure switch plus the Battery Cut off switch would stop all battery power. If the car is upside down any left over fuel in the Carb would go into the air cleaner if it leaked out and not into the cylinders. I also had an extra toggle switch wired to just the fuel pump which would cut it off alone. But this was so I could work on the car with the ignition on and the pump not running.

I think that in most cases everything would shut down quickly and if there was any fire it would come from the leaking fuel hitting a hot exhaust of something like that.

Ron

With what I believe is the “normal” way most people have their wiring setup:
Their cut-off switch wired breaking the “+” cable to the battery.
Their alternator/voltage regulator power out connection is wired to the hot side of the starter solenoid…or to be more generically said; power supply connected up-stream of their battery cut-off switch…

Scenario “A”: With an electric fuel pump…
Flipping the cut-off switch will not shut off the engine immediately.
The car will continue to run until the remaining fuel/fuel pressure is eliminated because the ignition system in powered “up stream” of the cut-off switch by the alternator.

Scenario “B”: With a mechanical fuel pump.
Flipping the cut-off switch will not shut off the engine…
The car will continue to run because the fuel pump is mechanical activated by the running engine, and the ignition system in powered “up stream” of the cut-off switch by the alternator.

The Fix:
You need to run the alternator charge wire to the battery side of the cut-off switch or directly to the positive terminal of the battery. When wired properly, the car should turn off when the switch is turned off. If the alternator is feeding between the switch and the engine, it will still run even if the switch is turned off.
Note: You must run the positive (+) cable to the cut-off switch, not the negative.

Thought this might be of interest to some of you. Came across it purely by chance when a local owner got curious and flipped his cut-off switch with the engine running…car kept running for a while…SURPRISE :confused:

It does not matter what type of fuel pump you have.

The engine will stop running if the switch is turned off ( providing it is wired correctly ). Some engines have problems running up-side-down as it is.

If gas is leaking, nothing short of absorant material is going to stop that.

:D :D

The BIG problem is the BIG wire that has to carry all the current from the alternator to all the circuits of the car and to the battery to charge it. You cannot cut that off without a massive (and totally impractical) relay.

You have to focus on cutting power to the ignition system and fuel pumps (if they are electric). It does not matter if you do that on the hot side or the ground side. It is important that both the emergency cutoff switch and the inertia switch break the connection to both of these circuits. Once you cut ignition, the engine will stop and the alternator will make no more power. Then, there is very little danger of a spark igniting all that spilt fuel.

If you don't have an inertia switch, you can get one out of a junkyard Mustang for almost nothing. Odds are, it has only been used once!

RallySnake

The seneraio described below was just to set the sceen, the important part was understanding the limitations of the cut-off switch due to the way it is installed on your car. Heck, I make the custom wiring diagrams and I never really analyzed how the cut-off switch operated beyond knowing that it isolates the power so you could work on the wiring safely. Had no real reason to look at the installation that closely...
And I agree that making the system work properly would require some fancy wiring, especially with an AMP meter installed in the dash and battery installed in the back...Even the red plastic key won't pass NHRA inspection..
Food for thought...
How many people have an inertia switch installed? Ever any problems with it under hard acceleration or deceleration? Maybe a new addition for the wiring diagrams...Maybe someone has a photo of their inertia switch installation handy?

inertia electric switch
 

I have a "inertia" activated switch mounted on the chassis upright just in front of the drivers door.

Wired it to shut off the power to the Holley electric pump in the left rear wheel well. Activated by impact ( inversion also I think). They must be mounted to something solid, like the frame...not something that can move and absorb impact before the switch gets tripped.

Installed it where I could reach to reset it (push a button on the top) as I had no idea how sensitive it would be in the car. It has never tripped. ( has been installed for 13000 miles and 10 or more track events).

This switch is from Pegasus Auto Racing Supplies, part #1108 about $65~. It comes with electrical plug, wiring, terminals and bracket.

You can get them for much less money, but then you have to have the "plug" to finish the wiring. Almost got a Ford Tempo switch for like $15? and then realized I couldn't easily get the electrical plug as it is part of the wiring harness in the car. If you took one out of a junkyard car you would have the plug I guess.

My "main kill switch" shuts off EVERYTHING. Nothing stays as a "hot" circuit.

The main-kill switch is behind the passenger seat. Good for security, not so good for the track.

Pete

I agree with Blas's post on the correct (and incorrect) way to install a battery disconnect switch. It's probable most switches are installed as a theft deterrent and not as a true emergency cutoff device.

Since we are also discussing electric fuel pump wiring, I have an engine oil pressure switch wired in series with the fuel pump toggle switch and fuel pump relay. The toggle switch is 3-position (momentary-on/off/maintained-on). My electric fuel pump feeds a by-pass regulator with a return back to the tank. The momentary-on toggle switch position allows me to circulate fuel (and fill the carb fuel bowls) without cranking the engine.

Given that I have a standard transmission, and if I'm in an accident the engine will stall (unless of course, the clutch is pushed in or I'm in neutral), is there a safety advantage of an inertia switch over my present installation?

Rodger

Interesting question. Is this you in the video? **)

http://www.youtube.com/watch?v=sYRN7...layer_embedded

Just following this potential learning thread, but for what it is worth, every Cobra/Street Rod should have an INERTIA Switch (cut-off switch) built into the system.
DV