# First Inertia switch



## dtbaker (Jan 5, 2008)

weird, I thought those switches would remain off until reset if they triggered. I have mine mounted horizontally, on a diagonal actually as I was thinking it would work to equalize operation from side or front/rear impact; and have not had any issues. But, quite honestly have only had maybe two 'panic stops' in the almost 11k miles as an EV....


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## piotrsko (Dec 9, 2007)

They don't normally reset like that as they are supposed to be one shot until manually reset. That is why they are usually semi-hidden: just enough to go out over extreme bumps, or during roll overs. Kinda annoying to the first responder to worry about fuel all over the place. Thinking you have something else going on. sounds like a re- precharge event to me


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## Ziggythewiz (May 16, 2010)

I'd try an experiment. Pull it out and get a continuity tester hooked up and whack it with a screwdriver to see how much force it takes to trigger, from what direction, and how long it triggers for. You may have a faulty switch if it's resetting, or it may have some in-between state.


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## tomofreno (Mar 3, 2009)

> weird, I thought those switches would remain off until reset if they triggered.


 Yeah, that's how they are supposed to operate. Wistar at kta-ev where I bought it says it is supposed to take 10g to trigger them, so the only way it could be the problem is if it is defective. I'm going to leave it bypassed for a while and keep testing it. If the problem doesn't recur I'll buy another one. Wistar offered to warranty it even though it is way out of warranty.

Btw, the paper work with the switch says to mount it vertically. Maybe it doesn't trigger in an impact if not mounted vertically.


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## dtbaker (Jan 5, 2008)

tomofreno said:


> Btw, the paper work with the switch says to mount it vertically. Maybe it doesn't trigger in an impact if not mounted vertically.



OH....
I didn;t get any paperwork.  i guess I'll have to re-mount at some point....


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## tomofreno (Mar 3, 2009)

> Pull it out and get a continuity tester hooked up and whack it with a screwdriver to see how much force it takes to trigger, from what direction, and how long it triggers for.


The paperwork does say after it is connected, turn on the engine and: "Holding the switch upright in one hand, strike it sharply with the other hand. The switch should operate and the engine should stop. Reset the switch and repeat the test." Maybe I'll try that too - I certainly will if I buy another one.


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## mk4gti (May 6, 2011)

I know, old thread.

Does anyone know the amperage that this baby can take?

I was thinking of "feeding ground" through it to ground the albrite SW200 main contactor (5 amp to activate I think). Do I need to provide a relay?


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## Ziggythewiz (May 16, 2010)

That's exactly what this is designed for, though usually on the power side.


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## piotrsko (Dec 9, 2007)

On my Ranger it controlled about 8 amps for the fuel pump. On the Ground side it might push another amp or two because the ground side seems to be "gentler" on switches.


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## Glider Rider (Jul 18, 2012)

Hi

I came across this thread whilst digging out some information for my CV.

Maybe I can help. I used to work for First Inertia Switch, calibrating the test equipment on which the fuel cut-off switches were tested.

The mechanical parts of the switch are all the same*, but the magnet, which in normal use, holds the ball at the bottom of a cone will differ in field strength depending upon the application. 

At the top, beneath the rubber boot, are two contacts joined by a couple of pieces of metal 'plates' in place by and over-centre spring. Beneath the 'plates' is a steel ball sitting in a cone. The ball is held in this cone by a magnet.

The idea is that in normal use the magnet will hold the ball in place if you drove over a pothole, hit a kerb or bumped a wall, but would release the ball in a real collision, which in turn hits the plates, and springs them into the open position, breaking the circuit. If it was false alarm, the switch can be reset by pressing the middle of the rubber boot.

Obviously the ride in a large car such as a Jaguar is very different to that in a small car such as a Fiat Cinquecento, so different magnet strengths are needed for different applications.

As an example, the Jaguar switch may be intended to withstand 6g, and open at 10g, whereas the Cinquecento switch might be intended to withstand 8g and open at 14g. It was a long time ago and these figures are just examples.

The switches differ by the amount of 'charge' or strength of the magnet. This is achieved in manufacture by the field strength generated in the magnet in manufacture. The magnets are all the same before this process. 

The magnet strength can only be determined by checking the switch serial number.

The switch is only intended to be mounted upright, and is intended to work at the same G force from all sides (not above or below). Each switch is functionally tested in manufacture eight times. Once from each direction in a 'no fire' test, and once in each direction for a 'fire' test.

It looks as if the company was taken ver by sensata around 1995. This is one of their data sheets for the First Inertia type of switches:

http://www.sensata.com/download/resettable-crash.pdf


* switches for Ford cars used different external mouldings and had a red plastic button instead of the rubber boot, but were essentially the same inside.


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## tomofreno (Mar 3, 2009)

Thanks for the description! I removed the switch and never had the problem again. I intended to replace it but forgot all about it.


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