Fuel Pump Inertia Switch

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Tom Brearley
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Fuel Pump Inertia Switch

Post by Tom Brearley » Sun May 10, 2020 9:20 am

I have been meaning to fit a fuel pump inertia switch for some time. The purpose of the switch is to break the circuit to the electric pump in the event of an accident. Such switches were fitted to late North American Bs.

There's a good Moss video that explains inertia switches here https://www.youtube.com/watch?v=o_K1h4PoMeQ

Once I had overcome my own intertia to get the job done, I sourced an OE switch from the States and fitted it yesterday. It is positioned behind the dash on the bulkhead, above the driver's left knee.

It seems like a worthwhile addition and one that might, conceivably, have been made 'in period' during the late 70s. I must warn anyone trying anything similar that access is awkward and will involve some swearing.

Tom
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1973 MGB GT (Exeter) - currently shaking down after 3 years off the road

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Tom Brearley
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Re: Fuel Pump Inertia Switch

Post by Tom Brearley » Sun May 10, 2020 9:22 am

The pump circuit is interupted just behind the ignition switch.
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Tom Brearley
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Re: Fuel Pump Inertia Switch

Post by Tom Brearley » Sun May 10, 2020 10:55 am

Conviently(?) one also gets a warning when the switch has tripped as the indicators don't work! This could perhaps be useful in the event of a very minor shunt or a bad pothole causing the float bowls to empty a few minutes further down the road.

(The white wire going to the fuse box is a spur off the fuel pump supply.)
1973 MGB GT (Exeter) - currently shaking down after 3 years off the road

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Tom Brearley
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Re: Fuel Pump Inertia Switch

Post by Tom Brearley » Sun Jul 12, 2020 9:59 pm

My last post above undersold it rather! Because the wire which is interrupted by the switch also supplies the coil, the ignition is switched off if the inertia switch is tripped.

This actually makes it a handy immobiliser since the 'tit' on the OE switch can be pulled up by hand thus mimicking activation by an impact. The tit can then be pressed back to restore the circuit.

I'm not sure why it took me so long to work this out. :idea:
1973 MGB GT (Exeter) - currently shaking down after 3 years off the road

Not_Anumber
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Re: Fuel Pump Inertia Switch

Post by Not_Anumber » Thu Oct 01, 2020 9:34 pm

Doesnt the fuel pump relay rely on a constant pulse from the ignition whilst the engine is running so hence the power to the fuel pump is cut immediately when the engine stops anyway. Or was this only standard on later B's ?

Vic Butler
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Re: Fuel Pump Inertia Switch

Post by Vic Butler » Thu Oct 01, 2020 10:06 pm

As long as the ignition is on the pump will operate. If the fuel line is ruptured the pump will keep pumping until the ignition is switched off. It will stop pumping when the float chambers are full if the engine isn't running provided there are no leaks anywhere. To test, switch the ignition on. The pump will tick rapidly, then gradually slower until it stops. If it doesn't stop, there's a leak.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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