How to wire in a fuel inertia switch?

[MilesA]

Having come close to being taken out last weekend by someone who thought that as he was entering a roundabout he had right of way, got to thinking about safety and I thought I would install a fuel inertia cutoff switch in my 3A as it has a Facet cube fuel pump.

 

Having researched the Forum, I propose using the PBT-GF15 type switch used by other owners. However, there appear to be two schools of thought re how to wire in the inertia switch. As I am not too confident with things electrical, I seek guidance because I am sure someone here will have done this.

 

A. First suggestion is simply to break into the power line to the pump so that power to the pump is simply routed through the inertia switch. That I understand and is simple for me.

 

B. Second suggestion is to use a relay. High power in (30), then out to pump (87). The inertia switch should be fitted on the low power line in to the relay switch (86) with the earth coming off 85. This I also understand but, I am not sure where the power supply into the inertia switch should come from.

 

So which of the two methods is preferable and if the second, from where do I take the power supply into the inertia switch (and by that I would need to know which specific cable and colour!)?

 

Thanks - Miles

[john.r.davies]

Miles,

The usual recommendation if you are running Lucas Pi is to use a relay next to the fuel pump, as that takes a lot of amps (10-14?). A heavy duty power line is laid straight from the battery, or ignition switch, to the relay, and on to the pump. The OE lead to the pump from the ignition switch, is used to control the relay, so that need only carry a low current, and save its poor old wires.

 

But your Facet cube takes less than 2 amps to keep your carburettors topped up, so no relay required. I'd just put your inertia switch in that lead from ignition switch to pump.

 

I use the version of the PBT-GF15 that has three connectors, instead of the usual two. One is the supply from the battery, one is the feed to the relay, and the third goes to a warning light. When the inertia switch cuts off the relay and pump, it turns the light on to show that it has triggered. Helpful, if your engine suddenly cuts out for 'no reason'!

 

And ensure that the inertia swith you buy comes with the connector to the loom. The teminals are fine and unlike anything else on a TR, so it's easier to splice it in if you use the leads on the connector.

 

John

Edited October 11, 2012 by john.r.davies

[Guest ntc]

A Land rover one works fine

[Menno van Rij]

This one of the moments that it's a shame that a 2/3/3A doesn't have an oil pressure switch. In that case, installing an inertia switch between the oil pressure switch and the fuel pump would be very easy! Engine stops, no oil pressure, no petrol.

 

Menno

[billy l]

Hi Menno, You can fit an oil pressure switch, attached is a photo of one fitted to my TR3 and this is wired to a flashing/buzzing LED light on my dash. It takes the pressure (if you excuse the pun) of having to keep one eye on the gauge. Cheers Bill.

[MilesA]

John

Thanks for all that information (and good to know I can take the easier option for once!).

 

Neil

Thanks again and the switch I mentioned is used on Landrovers - so got that right.

 

Menno / Bill

One task at a time please - I am still learning!

 

Miles

[ianc]

Menno - a couple of points concerning your suggestion of using oil pressure to cut the electrical supply to the fuel pump:

1 - an over-ride would be required in order to get the engine started, since no oil pressure would mean that the fuel pump could not run;

2 - in the event of a severe shunt, the driver might be unconscious, the engine still running (possibly at high revs) and fuel could be leaking, but the oil pressure would keep the fuel pump running - not what one would want.

 

The inertia switch is fitted to the PI cars to cut the engine in the event of a shunt. For that reason, it should not be fitted to rally cars using an electric pump, as it might cut the engine when road conditions are severe!

 

Ian Cornish

[john.r.davies]

"the engine still running"

 

Anyone watch "The Plane Crash" last night Ch.4 I think?

http://www.channel4.com/programmes/the-plane-crash

Boeing 737, deliberately crashed into a Mexican desert, for 'scientific purposes' (I'll believe them)

The intent was to take it down at 1500ft/minute (compare landing 200ft/min) to simulate a survivable crash.

The nose was torn off, the landing gear came up through the wings, the interior looked ....... like a plane crash.

As the dust cleared - THE JET ENGINES WERE STILL RUNNING!!!!!!!!

 

The experts said that it might go on until the tanks were empty, so they sprayed fire hoses into the air intakes to stop them.

Don't jets have an impact cut-off?

 

John

Edited October 12, 2012 by john.r.davies

[Menno van Rij]

Menno - a couple of points concerning your suggestion of using oil pressure to cut the electrical supply to the fuel pump:

1 - an over-ride would be required in order to get the engine started, since no oil pressure would mean that the fuel pump could not run;

2 - in the event of a severe shunt, the driver might be unconscious, the engine still running (possibly at high revs) and fuel could be leaking, but the oil pressure would keep the fuel pump running - not what one would want.

 

The inertia switch is fitted to the PI cars to cut the engine in the event of a shunt. For that reason, it should not be fitted to rally cars using an electric pump, as it might cut the engine when road conditions are severe!

 

Ian Cornish

 

 

You're right. But this is what I had in mind (from the Dutch Saabforum - where a lot of guys are shifting to Facet pumps due to the 'battle' between old Saab fuel pumps and modern day fuels... There's a cut out relay and an inertia switch included (what I didn't mention though...).

 

Menno

 

 

Edited October 12, 2012 by Menno van Rij

[ianc]

Dear Menno,

Thanks for the diagram - but, although you state that it is there, the Inertia Switch is missing!

I imagine that it is in series with the connection from the fuse box to the solenoid of the cut-out relay (or from the solenoid of the cut-out relay to the oil pressure switch).

Ian Cornish