Fuel cut Inertia Switch.

PostPost by: Grizzly » Fri May 18, 2018 9:17 am

Hi all....

I'm in the process of fitting an Inertia Switch to my car to ease my paranoia of having an electric fuel pump wired from the ignition switch.

I've done a some reading up about them and i understand the weber type switch i have (same as the one Rover used) needs to be stood up so the button is at the top (which limits places i can put it inside the car without making it obvious), i was also told that it needs to be fixed to some thing metal as grp doesn't transfer shock in the same way?? (don't know if thats true as i thought Esprits had fuel cuts bolted to grp) but if that is true i guess it would have to be bolted to the chassis some where (maybe a front chassis rail??)

Interested to hear peoples opinions as to placement, from my past experience of these things placement is the key to it not constantly tripping on dodgy British roads.
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PostPost by: JonB » Fri May 18, 2018 10:14 am

A reply based on a bit of experience and a bit of reasoning...

I once built a kit car based on a Ford Sierra (Robin Hood 2B DOHC Lux), and I used the Sierra's cutoff switch. I fitted to a suitable place in the boot (the boot floor, which was plywood screwed directly into a chassis rail) and, like yours, it had to be oriented vertically. It never tripped accidentally on the bumpy South East roads, despite me driving very enthusiastically. Fortunately I had no cause to use it, either, although I did test it by shaking it in my hand, and it worked.

The Robin Hood was a clone of a Lotus 7 with a space frame chassis (albeit with a much wider body to accommodate the Sierra's rear suspension sub frame). The fuel pump never came on while the crank was stationary, so you might consider a crank speed or oil pressure sensor as well (which stops the pump when the engine stops). This may negate the reason for fitting it, of course (if it's there to also prime the carbs prior to starting).

I'm not sure about GRP shock transmission specifications, but if you think about it, wherever you are hit in an accident, some GRP will be at least partially absorbing the crash loads; so even if you mount the thing on the chassis, it still might not get a clean shock wave to trigger it. Same is true of metal bodywork, which absorbs crash loads, too (rightly so).

Perhaps it's fair to suppose the designers of the cutout switch have considered this and specified the trigger threshold accordingly? As your investigation is likely to get highly technical, I'd ask the supplier for advice as well.

Cheers
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PostPost by: john.p.clegg » Fri May 18, 2018 10:49 am

I used one out of a Mondeo ( if I remember correctly..) and all was well for years until after a little enthusiastic cornering the engine was very down on power , it seems the g-force tripped the H.P. pump ( for the EFI ) leaving the low pressure pump feeding the injectors...

Be aware

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PostPost by: billwill » Fri May 18, 2018 12:25 pm

Mine is inside the cabin, screwed to the metal horizontal bar that joins the two rear suspension turrets.

My theory being that if it tripped by mistake it would be fairly easy to reach without having to get out of the car, say on a motorway side strip.

Yes it is visible, but few people seem to know what it is anyway.
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PostPost by: jono » Fri May 18, 2018 1:36 pm

I had a Caterham with the K series engine and the switch was located on the bulkhead, passenger side, as you say with the button to the top.

The car used to get serious stick and I never once had a false actuation, in fact it was only shortly before I sold it that I discovered what it did :lol:
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PostPost by: billwill » Fri May 18, 2018 3:55 pm

As I mentioned in another topic on this matter, almost all the inertia switches look the same, but their shock-activate specifications inside can differ immensely and it is not known (?) what is a suitable rating for our cars.

This one http://www.evwest.com/catalog/product_i ... cts_id=281 has 8G to 30G options
http://www.evwest.com/support/Inertia%20Switch.pdf


~~~~~~~~~~~~~~~~~~
Interesting webpage about current Ford inertia crash detection etc.
https://rts.i-car.com/collision-repair- ... ch-go.html
Last edited by billwill on Fri May 18, 2018 4:18 pm, edited 1 time in total.
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PostPost by: Grizzly » Fri May 18, 2018 4:10 pm

Interesting...... i didn't realise there was different shock ratings.

I would really like to get it placed in the cabin but it would have to be hidden away as much as possible.
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PostPost by: billwill » Fri May 18, 2018 4:27 pm

Looking at the recovered inertia switches on eBay:
https://www.ebay.co.uk/sch/i.html?_from ... s&_sacat=0

I guess it would be best to buy one that came off a performance car such as a Jaguar rather than a mundane family car.
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PostPost by: jono » Fri May 18, 2018 4:29 pm

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PostPost by: Peter +2 » Fri May 18, 2018 4:48 pm

I recently have been caught out with safety cuts outs, for as mentioned earlier, they may not allow the pump to adequately prime the engine after a longish lay ups. I have been using the Electric type cut out which relies on the Coil signal for instruction. I hadn?t realised that it wouldn?t allow the pump to run for long enough on initial cranking, which sort of defeated the object of fitting an electric pump.

This may be worth considering even with an inertia triggered cut out.

I have yet to resolve this issue so would be interested in your thoughts.

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PostPost by: Grizzly » Fri May 18, 2018 10:38 pm

I did think about fitting one of these pressure switches https://www.ebay.co.uk/itm/Summit-Racin ... XQgb1ROH5o but the more i think about it the less sense it makes......... so the engine would have to be making 7psi of oil pressure before the pump came on (so a car that had been stood for a few weeks may still take some cranking to start which defeats the object??) if it had run recently the fuel bowls would be full of fuel so wouldn't make any difference to the starting, but once running even if the fuel line was severed the pump would keep going as the engine would be using the fuel in the bowls for a period of time.

The Inertia Switch would at least kill the fuel pump in a crash even if the engine was running from fuel in the bowls (assuming it's fitted correctly and actually trips in the event of a crash)
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PostPost by: nmauduit » Sat May 19, 2018 8:34 am

Grizzly wrote:I did think about fitting one of these pressure switches https://www.ebay.co.uk/itm/Summit-Racin ... XQgb1ROH5o but the more i think about it the less sense it makes.........

it's not a bullet proof option for all cases but the way I see it is like that : if somehow there is a leak on the fuel line and the electric pump is not turned off though the engine is not running anymore (failure/fire or a crash, driver passed out ...), then the pump is prevented to push more fuel forward thank's to this setup. One can also see interest in not continuously pressurising the fuel valves then bowls when the engine is not running, which may tend to accumulate fuel in the trumpets or below...

On my car the warning oil pressure switch is 20 psi, when I'll wire it in I will add a by-pass momentary switch for manually carbs bowls priming.
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PostPost by: Orsom Weels » Sat May 19, 2018 9:01 am

I suspect some experimentation with G ratings & location may be necessary to get a perfectly reliable operation, but I'm not sure even big manufacturers get it right, or at least I don't know what sort of impact they consider bad enough to trip a cut out. My daughter was recently hit hard from behind by a 40 ton lorry, shoved across a pavement, through a wooden picket fence, through a garden & came to rest when she finally hit the front wall of a house, (she had been stationary when the lorry hit her). The car, a 1.3 Ford Fiesta ghia, was understandably a mess, with serious damage both front & rear, the fuel pump hadn't cut out though ! Fireman sorted it when he put his bolt croppers through the battery leads :D.
Conversely, I remember back to when the first EFI Maestro's & Montego's came out, I worked in a local garage & we had 3 or 4 that tripped their inertia cut outs just going over a bump or hitting a pot hole.
Passengers footwell seems to be a common location for factory fit, I really don't know if there's any reason other than convenience for this, but could you not fabricate a bracket on to the steel reinforcing, mount the cut out behind the trim panel & cut a small hole for access ? It would be totally invisible except for the small hole, which probably wouldn't notice down there.

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PostPost by: Grizzly » Sat May 19, 2018 10:28 am

Tim.That sounds nasty, hope she was ok......... I worked in a Insurance Accident repair shop for a number of years and now i think about it i don't remember an Inertia Switch being tripped? but that doesn't mean some one hadn't reset it to move it or load it on a truck.


A few different people have told me that a Inertia Switch won't work on a GRP car due to how the Fibre glass absorbs and shatters rather than transmitting the shock (i don't know how true that is though but i have seen first hand how the body grp shell explodes in collisions considered minor by today's standards).

The crazy thing is i'm starting to have second thoughts about the electric pump, without any safety back up i do like how it starts from cold but apart from that it starts just the same day to day as the mechanical pump...... But if i fit a Oil switch to the Electric pump then i loose the advantage of being able to Prime the carbs before i start cranking, then there is the Inertia Switch which seems a bit of a unknown quantity? Will it cut out in a low speed collision? Do i need a sensitive Inertia Switch for a GRP car (as people are telling me) or do i need a less sensitive switch as it's a sports car?
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PostPost by: mbell » Sat May 19, 2018 12:00 pm

You could add a online switch that bypasses the oil pressure sensor for filling carbs.

Personally I have fired a cut off and also have it power the electric ignition module. So it will also kill the ignition/spark and engine.

The switch only needs to trigger in an event serious enough to potentially damage the fuel lines/carbs which would be a very heavy impact. The bigger concern for me is a roll over event where fuel could leak out of carbs around a hot engine and ignition system risking fire.

A cut of switch should give some protection against this situation and while not perfect I think it's a very sensible addition. If you concerned about it not activating you could mount it directly to one of the chassis mounts (like the rear ones in the boot) then you don't need to worry about the fiber glass absorbing the shock.
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