Inertia Switch

[jwebb]

What would or could cause the switch to trip? And what would be the symptoms of a tripped or open inertia switch?

[Papa]

The switch is designed to trip in a crash.

https://en.wikipedia.org/wiki/Inertial_switch

When tripped, it breaks the ground continuity to the fuel pump relay, thereby preventing the fuel pump from running.

Dave

[phileas_fogg]

Any hard jolt will trip the switch; a tripped inertia switch typically rattles when you shake it. Also, you'll hear it "click" into place when you press the plunger.

Also take a look at http://www.ffcars.com/forums/17-fact...-question.html describing a problem getting electricity to the fuel pump. Root cause turned out to be that the inertia switch was mis-wired.

My switch was mis-wired. After removing the switch from its connector, I used an ohm meter to verify that the switch required pins 1 and 3 to operate; it was wired to use pins 1 and 2 (or 2 and 3, I don’t really know which). In any event, I took the connector apart and moved the female pin from the center socket to the outboard socket so that the switch would operate correctly.

No pictures, as the referenced thread has all the help you need to determine if you have the problem and correct it if you do.


John

[jwebb]

No impact to a mechanical fuel pump vehicle, correct?

[Papa]

No impact to a mechanical fuel pump vehicle, correct?

Correct. If using a mechanical fuel pump on your roadster, there really isn't a need for the inertia switch at all as it only affects the fuel pump relay in the RF harness configuration.

Dave

[ironman16nc]

Mine was also miss-wired. Needed to move the connections to 1 and 3.

[weendoggy]

The switch is designed to trip in a crash.

https://en.wikipedia.org/wiki/Inertial_switch

When tripped, it breaks the ground continuity to the fuel pump relay, thereby preventing the fuel pump from running.

Dave

If this is a Ford wired circuit, it trips the Fuel Pump lead, not ground.

[skidd]

If this is a Ford wired circuit, it trips the Fuel Pump lead, not ground.

It can be wired on either side of the pump. Just so long as it opens the circuit. I wired mine to break the ground side of the circuit.

[Papa]

If this is a Ford wired circuit, it trips the Fuel Pump lead, not ground.

It can be wired on either side of the pump. Just so long as it opens the circuit. I wired mine to break the ground side of the circuit.

Like any switch, it can work as a positive or negative control of the circuit. For the Ron Francis harness, it is definitely on the ground side.

[TexasAviator]

It can be wired on either side of the pump. Just so long as it opens the circuit. I wired mine to break the ground side of the circuit.

Same here. Mount it straight up and inbetween the pump and ground. I used a small piece of powdercoated black steel. The button needs to be pointing up. When you flip, hit, roll, etc, it will trip the pump. I got mine from a 90 ford mustang. I also ran a fuel pump relay circuit and doubled up the wires to create redundancy in the circuit. The relay is further powered by my EFI system.

[AC Bill]

Mount it where you can easily reach it, but it's still out of the way..
IMG_2963.JPG

[Sigurd]

Correct. If using a mechanical fuel pump on your roadster, there really isn't a need for the inertia switch at all as it only affects the fuel pump relay in the RF harness configuration.

Dave

You can use it for cutting the ignition.

[i.e.427]

You can use it for cutting the ignition.

^ for sure. It comes in handy when you are diagnosing the car and don't want the fuel pump to run, especially on the 4.6 and Coyote crate motors with the Ford control pack (pump runs all the time when ignition is on).

[skidd]

I actually glued a small plastic nubb to the top of my inertia switch. So I can pop the trunk and pull it up to kill the fuel pump... if I felt the need to do so.

[TexasAviator]

I drilled a hole in the trunk floor just above where I mounted the switch in the above picture. Covered it with a rubber plug, it looks clean and doesnt protrude into my trunk area. Just pop the plug and reset the switch, easy access and flush mounted to the floor.

[rich grsc]

Ford wired that switch to the hot side of the pump for a reason. If there was an accident you should not have a hot wire connect to the fuel pump. A pinched wire could let the pump continue to run, or create a spark near leaking fuel. Ford spends million designing cars, it wasn't a random idea to interrupt the power lead to the pump.

[skidd]

That makes perfect sense.

[TexasAviator]

Ford wired that switch to the hot side of the pump for a reason. If there was an accident you should not have a hot wire connect to the fuel pump. A pinched wire could let the pump continue to run, or create a spark near leaking fuel. Ford spends million designing cars, it wasn't a random idea to interrupt the power lead to the pump.

True but Ford never built AC Cobras, in fact they did not have an inertia switch. Also to play the devils advocate, do you know how many carbed cars are out there that keep running after an accident? What about cars converted to EFI aftermarket like my Fitech. A quick google search shows that there are very few even considering it. It makes it a greater concern to the aftermarket that they shoud be included in the design of aftermarket systems. The truth is, if you have any kind of inertia switch in these cars, you are already well ahead of the status quo.

[Avalanche325]

Doing something like this can get it done.
BMW-3_0_CS_mp2_pic_54013.jpg

[Paul2STL]

Accidently pushing it while cleaning up around it or someone pushing it when they don't know what it is. It is a nice red button. Both have happen to me in the last two months while in go-cart.