Fuel inertia switches

[jmp]

Hi all advice needed please re- inertia fuel pump switches.
I had already decided that the next job on the S4 was to fit an electric fuel pump of sorts. However nearly having white van man run into me as I was turning right into our lane Yesterday I thought it expedient to fit a fuel inertia cut out switch. It's been a while since my heart has raced as much as it did.
My question is will any inertia switch be okay. I have 2 ford Ka's sitting here (ex kids cars who've now gone up market)would a switch out of one of these be suitable?
I did read the archives a few weeks ago and I think someone had used one out of a Puegeot?

Thanks in advance,

John

[billwill]

I expect that any inertia cut out switch will do, I've been looking through those on eBay & Amazon for one for my Elan and the most common one available seems to be for a Land Rover.

The specification I saw says it will cut out at 12G forces but not a 6G.

I expect it needs to be firmly fixed to the metal work of the car so that it can accept the full shock of an accident. No mounting on rubber bushes, but as yet I don't really know. I'm think of putting one on the steel bar that separates the cabin from the boot, so that the reset button on top can be reached, though I have a horrible suspicion that after a 12G shock crash there might be many holes in the bodywork to reach the switch.

[TroonSprint]

I have an inertia switch on my MGB V8 with fuel injection. It is from a Range Rover Classic as in the item here under ebay.co.uk item number 140687519222. I have it mounted it with the plunger uppermost in an inconspicuous place where I can easily reach it from the driving seat, and by pulling up the plunger it can act as an anti-theft device. I don't know how many G is necessary to activate it. All I can say is that it has never been activated while driving over more than 3 years, but that if I hit it with my fist it pops out immediately.

I intend to use an electric pump on my Sprint and will definitely fit one of these.

Mike

[john.p.clegg]

John
Fitted one as part of the EFI setup...donor vehicle-Ford Mondeo...sits under the bonnet and so far so good...

John

[ricarbo]

I've got an alternative, which is to wire the relay for the pump via the oil pressure switch. If there's no oil pressure, the pump doesn't run, but this gets overidden if the starter is operated. You can also add a push button to enable the pump without either oil pressure or starter, to fill empty carbs. I can find a schematic if you are intererested. Of course, not all elans have an oil pressure switch, but one can be added with a T piece on the block where the oil pressure gauge line is fitted.
regards
Richard

[john.p.clegg]

Richard

As most oil pressure switches "break" as pressure rises,wouldn't you need a relay as well?

John

[ricarbo]

Yes, you'd need a changeover type relay, but you'd probably want to fit a relay anyway so the fuel pump load isn't added to the ignition switch. I'll dig out the schematic.
regards
Richard

[billwill]

Yes, you'd need a changeover type relay, but you'd probably want to fit a relay anyway so the fuel pump load isn't added to the ignition switch. I'll dig out the schematic.
regards
Richard

You could possibly organise a relay anyway, to arrange for the fuel pump to run to prime the carbs, before there is any oil pressure, i.e before the engine starts.

[ricarbo]

Yes, that's possible, but the only way I know how to do that is to drive the relay from some electronics, acting as a timer which gives an output when the ignition is first turned on (with no oil pressure present), for, say, 30 seconds. Then it maintains that output as long as pressure is present, but immediately gives no output if the pressure is lost after the first 30 seconds. I've got a circuit for that somewhere, using a 555 integrated circuit, or possibly a 741 opamp, if I remember. I used it, inverted, to drive an alarm whenever oil pressure was lost and it revealed a loss of pressure on any roundabout taken at speed, when of course you are looking where you are going, rather than the oil pressure gauge.

[robertverhey]

Simpler solution surely is just to have a switching relay as suggested above with a separate spring loaded override switch to prime the carbs prior to firing up.

[ricarbo]

This is what i've got

fuel pump relays0001.pdf

(88.26 KiB) Downloaded 524 times

[elj221c]

If there's no oil pressure, the pump doesn't run,
Richard

A bit confused by this approach. The engine may continue to run after an accident.

What am I not seeing?

[billwill]

If there's no oil pressure, the pump doesn't run,
Richard

A bit confused by this approach. The engine may continue to run after an accident.

What am I not seeing?

True, but probably in many cases the engine will stop due to twisted metal or the car (and hence carbs) being upside down. Maybe both this oil pressure system and an inertia switch is the safest combination.

[kstrutt11]

I have a ka one on my +2, I screwed it to the inner sill panel on the drivers side down beside the seat in the same orientation as the ka (this is important), It seems pretty sensitive and a firm clout on the inner sill further forward trips it, of course the only way of truly finding out if you have it right is to crash the vehicle!!



kevin

[Esprit2]

A bit confused by this approach. The engine may continue to run after an accident. What am I not seeing?

The two approaches have different purposes.

The inertial switch is a safety item used to ensure the fuel pump is switched off in the event of an accident. The basic circuit simply switches off the electrical power to the pump and allows any latent pressure in the system to bleed off, eventually killing the engine as well. The next step up adds a solenoid controlled valve, such as the Flow-Lock Valve used by Lotus starting with the 1974 907 powered Elite, then the Eclat & Esprit. In that case, the inertial switch simultaneously switches off the fuel pump and closes the valve in the fuel line. All that is done to minimize the possibility of a fuel-fed fire after an accident.

The oil pressure switch is used to protect the engine in the event of a loss of oil pressure. It typically switches off the electrical feed to the ignition in order to instantly kill the engine. That on-off function can also be used via a relay to switch off the electric fuel pump. An oil pressure switch is great engine insurance in track day or motorsports scenarios where a loss of oil pressure might be experienced during sustained cornering G's... such as in a long sweeping curve. However, there is no guarantee that there will be a loss of oil pressure as the result of an accident since the engine may keep running, so the oil pressure switch isn't considered an adequate solution for a fuel system "safety switch" in the event of an accident.

Install an oil pressure switch (and an Accu-Sump) to protect your engine, but install an inertial switch (and Flow-Lock Valve) as a safety system to minimize fire risk after an accident. Both have value, but neither does the other's job reliably.

Note that an inertia switch combined with a Flow-Lock valve can also be an effective safety switch on cars equipped with mechanical fuel pumps. Closing the valve in the fuel line will starve the engine of fuel and cause it to stop, thereby stopping the mechanical fuel pump. The valve will also prevent any ongoing leakage of fuel via the hose.

Regards,
Tim Engel