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MG MGB Technical - Inertia Cut Off Switch Position

I'm intending to fit an Inertia Cut Off Switch to my 76 Roadster as an additional safety feature. The switch can be used to cut the power to the fuel pump or to the ignition circuit. I've decided to fit it to the ignition circuit and by doing so it would kill the engine as well as the fuel pump in the event of a shunt.

The problem I'm having is where to fit it? The instructions say that the switch must be mounted vertically onto a solid bulkhead and preferably within the cockpit, so it can easily be reset. There is little or no room under the dash, with the wiper motor etc. and wiring looms being in the way. As I have a remote servo fitted on the car, there is little or no room again to mount the switch on the front bulkhead / firewall. I could mount it on the bulkhead just above the steering column where it enters the engine bay. Has anybody fitted one of these and where was it fitted?

Thanks Andy




Andy Robinson

Side of the centre console?
paulh4

Andy on a TR6 it is mounted under the bonnet, nearside inner wing, near the bulkhead.

Colin
Colin Parkinson

I’ve put mine on the passengers side just below the dashboard, not generally seen but easy to access.

Tony Brough

On my car it only cuts the fuel pump and is mounted on the heel board behind the passenger seat where I can reach it easily. The switch I used can be turned off by pulling up the plunger, so is useful if you are doing electrical work when you need the ignition on but don't want the pump running.
Mike Howlett

Most of the new Fords here have them mounted behind the outer side panel in the footwell with a probably 25mm hole in the panel just level with the top of it so you can poke your finger in and reset it--------They have them on the passenger side but only because it's easy I reckon as the loom comes in that side
Probably easier on the driver side in a B


William Revit

From the factory on NA cars it was mounted behind the dash on the left side.

The blue device mounted under it is an electrical noise suppressing capacitor, also from the factory.


Steven Rechter

I believe that at one point there was a fuel pipe cut-off as well as that electrical cut-off ... but it leaked! Any idea where that was fitted?
paulh4

On the flat panel of the engine bay between the heater and pedal box on later US MGBs
Probably caused the under bonnet fires on all them MGBs that were lined up at the wreckers
I doubt there's any left in service, hope not anyway
William Revit

Thanks.
paulh4

Could these things be used as an immobiliser?
steve livesley

Yes, if you trip them.
paulh4

Steve
There are some very small/neat inline fuel cutoff solenoids available that are used in dual fuel lpg/petrol systems--Usually they're naturally shut and need powering to open so perfect as an imobilizer
Unfortunately with SU carbs the car will get almost 1/2 a mile down the road though with what's in the bowls,-unless you run it out when you park
William Revit

When hot maybe, considerably less than that when cold and choke is needed. Enough to get out into the road, and it would have to be a pretty brazen thief to start fiddling then.
paulh4

Just answering Steve's question Paul----

- A brazen thief, that would cover them all I would think-
They don't even try and start them here, just push em down the road with their 4x4's and load em on a trailer round the corner----------gone
William Revit

That carbs allow the car to start and drive a short distance is very helpful, as I suspect most thieves would just think unreliable old car.
Had a funny example here one with a midget which had the fuel pumovfused, and the fuse blew every time the indicators were operated.
It was stolen from the YMCA car park and stopped and abandoned 1/4 mile later after blowing said fuse. Which happened to be outside the main police station.
On another occasion, my own car was left for its WoF test, during which the tester turned all the switches off after doing the lights. Which included the fuel pump switch on the rally console. They spent 1/2 hour before grumpily phoning and telling me to bring a tow rope when I collected it
Paul Walbran

The only thing I see with Steve's question about adding a cutoff solenoid ,is why bother cluttering when you could do the same by having a hidden switch in the pump circuit

Your experience with your car Paul reminds me of a mate's Falcon ute he loaned me, I took it to the Mainland to pick up a load of parts and on the way back it wouldn't start at the wharf, tried everything but no power to the coil, -had to hotwire it to get aboard and get off at the other end---Rang him to find out if it had an imobilizer and he laughed his head off, -turn the rear demister on and she'll go he says and it did
Falcon utes back then didn't have a demister in the rear window but the switch was there for sedans and he had wired the ign. through it---cheap imobilizer, tricked me----but then that's not hard to do
William Revit

The primary reason for 'cluttering' with an inertia switch was for safety, as originally outlined. Mind you, that WAS nearly four weeks ago ...

I understand at one time some cars in America - don't know if it was mandated or not - had a 'random' switch (that had its own function) designated as an imobiliser, and that had to be turned on before the engine would start, then turned off if not needed. Caught me out when borrowing a car from daughter's friend. She knew about it, but not which switch, so I had to try each in turn. Got there in the end.
paulh4

We answered Andy's question earlier as you say Paul but
Again Paul--I'm just responding to Steve's question on using a cutoff as an imobilizer which in my view is just adding something else that can fail
Keep it simple--------?
William Revit

Oh yeah, it's not a 'solenoid' either ...
paulh4

What's not a solenoid----
There are two seperate questions going here
Initially about the safety inertia switch--Andy
And then about the fuel line cutoff solenoid--Steve

This type of solenoid is what I mentioned to Steve he could use as an antitheft device instead of the leaky MG valve

https://www.valvesonline.com.au/stainless-steel-general-purpose-direct-acting-norm?gclid=Cj0KCQjwu6fzBRC6ARIsAJUwa2So_rsyRxNHP9U_UmJRejj4BEmoJWxtsfma6gr_er21epy3KRku6_YaAiqiEALw_wcB

Paul--If you have a problem with me you better spit it out--what is it that makes you want to pick sh*t out of everything I say, If you read back to the start of this thread you didn't know where the cutout solenoids were on the car anyway and now you're the expert
What's going on---------------

William Revit

Thanks everybody for your input. As previously stated I plan fitting a Inertia Cut Off Switch to my 76 Roadster as an additional safety feature. The whole idea was that if the switch was used to just cut off the power supply to the petrol pump, then the engine would still run until the fuel is exhausted in the carbs. It seemed to me to be a much better idea to use the switch (as a safety feature in the event of a shunt) to cut the ignition circuit, thereby stopping the engine as well as the fuel pump.

To use the switch as a immobiliser, would require you to give it a whack to disengage the switch - not ideal. Maybe a better way to immobilise the car would be a battery cut off switch and key or a separate ignition cut off switch as previously suggested.

Andy
Andy Robinson

Cutting the power to the coil is one of the first things anyone wanting to take a classic car would look for and is very easily bypassed. A sneakier way - if you have the RVC tach - is to use a switch to earth the signal wire at the tach which shorts out the points. Some electronic ignition systems may not like that though.
paulh4

IIRC the TR6 switch could be operated manually, by pulling the top of the switch up. Although it was early ‘80s when I owned one.

I also think I have a Rover(?) cut-off switch somewhere, which could also be operated manually. It was on my list of things to fit, but I’m not sure where it is now.
Dave O'Neill 2

I already have a battery immobiliser ( which would be quite easy to bypass) and a fuel pump switch which I was thinking of swopping for an inertia switch. This would then serve 2 functions - to cut the fuel pump in case of an accident and to act as an immobiliser if positioned in a convenient place. I think I would want to locate it in a place firmly fixed to the bodywork but easily accessible whilst driving in case the thing cut the fuel say if i hit a pot hole whilst driving. I had a Montego estate in the 90s that "tripped" a couple of times during about 7 years of ownership for no apparent reason - it may just have been faulty.
I shall take a good look around the drivers sitting area for a good place as per the suggestions above.
steve livesley

The ones commonly used by MG/Rover, JLR and probably others have a rubber boot on the top and the button is recessed into a harder surround. You would probably have to screw something into the top of the button to be able to lift it, and even then it may not trip the switch.
paulh4

Paul the type I was thinking of using was the same as I had on my Montego - although hopefully more reliable. I remember you can easily pull the plunger up to activate and press to reset.
I'll put one on my autojumble list.


steve livesley

Instantly distinguishable from the type I mentioned on my ZS, but probably means you would have to look for something at least 20 years old. I see ones like yours at £55 new as opposed to the Rover one at £15!


paulh4

Serve

That switch is the same as the one I have...somewhere
Dave O'Neill 2

I've got hold of a rover one as in Paul's picture just above. Do they have to be mounted in any particular way up? Thinking of mounting it on the rear bulkhead within reaching distance of drivers seat - just in case.
steve livesley

Steve, the cut off switch should be mounted vertically (i.e. button upwards) and on a metal bulkhead.

Andy
Andy Robinson

Can you pull the plunger up on those, as supplied?
paulh4

Paul, there is no plunger on the switch to pull up. To reset the switch you push down on the top. I assume there is a ball bearing and magnet inside, as when it is disconnect you can feel the ball moving about.

Here's a link to some fitting instructions

https://www.dellorto.co.uk/wp-content/uploads/2016/09/9990205700.pdf

Andy
Andy Robinson

That's how it seems to me. But you said you wanted to use it as an immobiliser, and were mounting it where you could reach it. If it's only as a safety feature and it trips in a collision I don't think resetting it would be my first priority :o)

Otherwise it would have to be a helluva a pothole.
paulh4

Paul I won't be using it as an immobiliser but as previously said years ago my Montego fuel cut out - cut out too easily. So just want to be able to reset it on the go. Hopefully like you say it would have to be a heluva pothole - but for just peace off mind I don't want it cutting out in an awkward or unsafe situation.
steve livesley

This thread was discussed between 17/02/2020 and 27/03/2020

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