fuel cutoff inertia switch

JD1964

there is enough sticking out to grab on to
15 Year Member
Jun 28, 2013
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I've never had problems with the inertia switch on either of my Foxbody cars. I assume they are pretty reliable. Are they as reliable as I think?

I'm asking because I might want to use one on another vehicle I'm working on. It's an LT1 - 4L60e from a 94 Z28 in a 79 Malibu wagon. The project is going well.

How well does the inertia switch provide the safety feature for which its intended? What type of impacts will it detect and react to? I'm particularly interested if it will react to a broad side impact to the drives side since that might knock the driver unconscious which would leave the key on and have potential for big fire if theres a fuel leak.

Any input on these inertia switches and how they work is much appreciated.
 
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Years ago I spun my 4 cylinder Mustang around on the ice. I did a 270* rotation and took out 4 guard posts. First hit just in front of driver door, popped the next two out of the ground as the guard cable rode up and ground half way through the A-pillar, further spinning me clockwise where the 4th post hit passenger door, I then slid straight backwards down embankment (approx 50-60* incline) where I came to rest against a farmers fence.

Next day needed to clean the frozen mud from tailpipe and reset inertia switch to start and run the car.
 
No problem. In short it worked. I had no injuries. For fun I just looked up where it happened on Google Maps and here is a recent StreetView pic of the area.

I went down near the Narrow Lane sign.

Ditch.jpg
 
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Yes I could have. It was the wood posts with 3 cables back then (93/94). Basically when I was popping the posts out, the car went under the cables. They needed 2 tow trucks to get the car up. One to lift the cables, one to pull up the car.
 
Any input on these inertia switches and how they work is much appreciated.

Unsure how many G's sets the Mustang one off. They do require a hard impact to trip though, and since they work on inertia, they need to be oriented properly.

There are aftermarket switches available, which do give some specs

 
I'll look for one at a junkyard when one comes available. Probably get it for under $10 including the pigtail and a enough wire to make some good splices
 
I got an inertia switch from ebay. Have any of you ever held one in your hand and gave it a jolt? It's pretty cool. When you bump it hard enough you can feel a counter weight spinning inside it. If it spins enough, the button pops up and trips the circuit. Anyway, I did not know how exactly they work but you can feel something spinning in side when you give it a lateral hit. Hit it on top and it wont do it. Gotta be front to back or sideways

I stand corrected. It's actually a ball on a magnet. Explained in this vid


View: https://www.youtube.com/watch?v=MTwkQYC1viY
 
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Put it this way, it's based on the exact same technology as airbag switches. When have you heard of an air bag not going off when it was supposed to?

Kurt
 
Put it this way, it's based on the exact same technology as airbag switches. When have you heard of an air bag not going off when it was supposed to?

Kurt
Hmmm, not sure if this is sarcasm or not. We had a 98 jeep cherokee that had some pretty good jolts in its life and it never deployed the airbag. On the other hand, I've been hearing stories of airbags deploying when their not suppose to, but I think that was corrosion related.

Has anyone had this Ford style inertia switch trip when is shouldn't?
 
Just 2 weeks ago I was driving home, took a hard right turn. The case of drinking water I had just purchased slid across the trunk into the cover over the inertia switch, tripped it.
I spent $100 on a tow home and several hours finding out why it suddenly stopped running. It was a valuable learning exercise.
 
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We used to take a wrench and tap them on every guys ford car or truck we knew. The ranger has it behind the passenger kick panel. Most of the other ford cars have them in the trunk. All you have to do is tap that thing and it'll kill power to the pump. It's also a very good theft deterrent. No need for a kill switch...just tap the inertia switch. No thief is going to go back there and push the button.
 
We used to take a wrench and tap them on every guys ford car or truck we knew. The ranger has it behind the passenger kick panel. Most of the other ford cars have them in the trunk. All you have to do is tap that thing and it'll kill power to the pump. It's also a very good theft deterrent. No need for a kill switch...just tap the inertia switch. No thief is going to go back there and push the button.
nowadays all you need is a stick shift for theft deterrent.
 
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Hmmm, not sure if this is sarcasm or not. We had a 98 jeep cherokee that had some pretty good jolts in its life and it never deployed the airbag. On the other hand, I've been hearing stories of airbags deploying when their not suppose to, but I think that was corrosion related.
The airbag should not deploy due to a jolt.

Kurt
 
The airbag should not deploy due to a jolt.

Kurt
Agreed. Maybe I was lacking clarity but the jolts I meant to describe also included impacts.

I live on a fairly busy road with a traffic light maybe 100 ft down the road from my driveway. It's not uncommon for us to hear the chirp chirp of antilock brakes followed by the pop of airbags. A quick look out the window and see the discharge vapors rolling out around the car. The front end damage many times appears minimal.
 
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About the location of the inertia switch. Obviously accessibility is important so I can see why Foxbody has it in the rear hatch panel or trunk panel so you can get to it. Also, its protected from weather in that area. Also, it's closer to the fuel pump helping minimize current load on the switch components. Is there any other reasons Ford mounted it back there?

I ask because on this swap project I want to mount this inertia switch in the engine compartment close to the PCM on the fender well. It will be accessible and protected from the weather there. I realize it wont be close to the pump but I plan to use it to interrupt the fuel pump relay trigger wire that comes out of the PCM. In the event of a hard enough hit, the inertia switch will trip and interrupt the relay trigger wire and cut the power off at the relay. I realize the relay can get stuck closed and the pump might still run but I'm willing to live with that low probability in order to simplify the installation.

Is there any reason its a bad idea to mount the switch in the engine compartment and have it configured to interrupt the relay trigger wire coming out of the PCM?

Thanks
 
Should be no problem mounting it in the engine bay as long as it's mounted properly and out of the weather. I personally don't see an issue wiring it into the relay trigger as it will accomplish the same job. For reference, I attached the instruction sheet for the inertia switch I used on my E30 "race" car.
 

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