MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - FIA Battery Cut-Off Switch

I have just replaced my cars FIA type Battery Cut-Off switch with a new one that is identical to the original one.
My car was a race car before I got it which explains why the FIA type was fitted.

It has a 3 ohm 11 watt resistor fitted between one of the spade connectors and earth.
I had noticed this a few years ago and at first I wondered if maybe it was set up as a ballast resistor for the ignition system.
I now think it is acting as some kind of anti-surge protector.
Does anyone know how these work?
I have attached a pic of the old switch and another of the connectors diagram.
Cheers
Colyn



Colyn Firth

This is a pic of the connectors diagram
Cheers


Colyn Firth

Hi Colyn
According to this website www.pegasusautoracing.com/document.asp?DocID=TECH00109
it is to protect the alternator if the battery is cut off whilst the engine is still running. Hope that helps.

I have a basic on/off cut off switch and was considering fitting a polyfuse. You probably have heard of them, but I hadnt. Apparently they can be used to bridge the switch when it is open, so allowing clock, radio memory etc to continue to run, but if someone were to try to start the car, the fuse would blow and so help protect the car from theft. The clever bit is when they blow, they are self repairing, so don't need to be replaced if it blows. And only a couple of pounds on ebay.
Graham V

What Graham says.

It is to protect the alternator when the battery is disconnected. It basically connects the alternator output (brown circuit) to ground.

If you have a dynamo, it isn't necessary to connect that side of the switch.
Dave O'Neill 2

Thanks Graham and Dave,
when I first got my MGA it already had this FIA type cut-off switch fitted with the resistor installed, even though the engine had a generator fitted back then.

So I am at a loss to understand why the PO fitted this type of cut-off switch. (I wouldn't refer to him as a DPO because I fell in love with the car as soon as I saw it and I wouldn't change how he made it look)

I actually converted from a generator to a Lucas alternator myself a few years ago when I fitted the twin-electric fans to make sure there was enough power available to keep the battery fully charged.

So, somehow I have ended up with the ideal cut-off switch, but it is still a bit of a puzzle as to why.

Cheers
Colyn
Colyn Firth

As well as protecting the alternator if the switch is turned off while the engine is running,-

It is a FIA requirement that the cut off switch kills a running engine when turned off

If the resistor "isn't" there, when the switch is turned off with the engine running it will continue running, being powered from the alternator/generator ---until the ign switch is turned off.
William Revit

With a dynamo (generator) you would still have the ignition wired through the other set of contacts, so the engine will stop running.
Dave O'Neill 2

...unless, of course, you are just using a battery master switch, which doesn’t have the extra two sets of contacts - NO & NC.
Dave O'Neill 2

Thinking, thinking, -yes you are quite right
I can't remember how many contacts are on the switch we use, but I can remember the issue of running on without the resistor fitted-
Must have a look at that next time I'm in there-just to refresh the memory of what goes on there
Cheers
willy
William Revit

Just an update on this.

I didn't explain why I decided to replace my battery isolator switch.

I have been experimenting with fitting different types of High Torque starter motor to my MGA to see if I could get one to work.

I bought a new one from the MGOC last year but although it started the engine ok, it wouldn't disengage from the started ring when I released the starter button and I could hear that the motor was still being spun by the engine.
So that motor is now going to a new home.

I recently was loaned three similar high-torque starter motor by Bob West to see if I
could get one of them to work.

I carefully measured the three motors and selected the one most likely to fit my 18v MGB engine/5-speed box.
However, when I pushed the starter button the pinion wouldn't engage the starter ring gear.

So then I fitted the second motor and pushed the button, the motor didn't operate. But there was just a faint whining noise quickly followed by a hot insulation smell!
I very quickly turned the battery isolator switch off and checked the starter circuit.
The battery cables were very warm but the starter solenoid was very hot indeed!.( my car has a push-button starter and a solenoid)

There had obviously been a dead short at the starter motor!

Fortunately the only casualty was the battery isolator switch which had developed a rather squishy-soft feel.

I have opened this switch up and you can see in the attached picture that it had begun to melt. This was after about 5 second and so there must have been a scary amount of amps going through those contacts.

So I have re-fitted the original inertia starter motor and to be honest it does seem to operate in a much livelier fashion than it did before.
So I presume that the original isolator switch must have been suspect.

Hopefully, this will stop those mysterious occasional sluggish starter issues that I have had since I first bought this car.

Cheers
Colyn



Colyn Firth

2nd pic of the isolator switch.


Colyn Firth

This thread was discussed between 08/03/2019 and 03/04/2019

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.