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MG MGB Technical - Battery Master Switch

I am planning to fit a battery master switch with one of those red keys.
The more expensive ones have something that looks like a relay stuck to the bottom, like the one in the link:-

http://www.demon-tweeks.co.uk/Motorsport/Electrical_Components/Battery_Isolators_&_Master_Switches/Autolec_FIA_Battery_Master_CutOut_Switch/1594/0/14928

Can someone tell me what the extra bit does and how it is wired. I have bought one second hand and don't have any instructions for it.

Thanks,

David
David Witham

If you just want to use it to isolate the battery, you don't need the expensive one.

The expensive one has two extra switches built into it - one normally open and one normally closed. This version is used as a 'kill' switch on race cars, normally. One switch cuts the ignition circuit in order to stop the engine, while the other grounds the alternator output through a resistor to prevent damage to the alternator through disconnecting the battery.

HTH
Dave O'Neill 2

Dave is right, you shouldn't need the fancy switch - it is like that so the engine can be cut by turning the key in an emergency without damaging anything electrical. If you crash in a motor race a marshall can then switch off everything safely.

For simplicities sake you can just use the two large terminals for your cut battery lead. But you must be careful never to turn off the key with the engine running. If you do, for a start the engine won't stop, and secondly you will almost certainly fry the alternator as you have removed the battery from the circuit.

I wouldn't be without a battery isolator. Its a great anti-theft device for a start, but it is also so useful when you are working on the car. Down on the heel board behind the seats it is almost unnoticed once the key is removed.
Mike Howlett

I've fitted one to all three of my cars, originally the V8 when I stopped using it every day to prevent the alarm draining the battery, then to the roadster as primarily a safety device but also as an additional anti-theft device, then I found I needed one on my ZS as well as that is primarily a winter car and the electronics were draining the battery on that.

I've fitted mine in the 12v cable as it conveniently runs down the heelboard behind the drivers seat, but there are marginal benefits in fitting it in the earth cable.

As well as the one with the additional contacts for race cars don't get the one with the bypass fuse (or don't fit the fuse) as that won't stop the battery draining while parked, although it will still work as a safety and anti-theft device with the switch off.

If for preventing drains, and you have a clock and/or radio that needs a permanent supply, take that from the live side via a separate in-line fuse. On an MGB the easiest way to do that is to connect the live side of the switch via an in-line to the purple circuit at the boot or hatch light, then remove the purple fuse from the fusebox. However that means the courtesy lights will still work, I ran a wire forwards from the battery to the clock, so on mine I can check the master switch is off simply by opening the door.

I have read horror stories of turning the master switch off with the engine still running, but have managed to do that several times on the V8 with no ill-effects so-far. It also won't be a problem if you use a master switch with a bypass fuse, as the bypass fuse will still be connecting the battery to all circuits even with the master switch off, i.e. the master switch is only operating as a safety or anti-theft device by blowing the bypass fuse, you would need to remove that fuse as well if you want to disconnect the battery completely when working on the electrics.
Paul Hunt 2010

I bought the FIA type because I had been thinking about having a master switch for a while and one day I may move up from standard class motorsport into a class where the FIA switches are required.

Paul, I am interested that you put a switch in the live. I am sure that not all cars with the remote pull cut out have the switch in the battery earth as many of the remote pulls tend to come out of the air intake in front of the screen.

So how does the grounding of the altenator output via a resistor work?
Is the resistor built into the switch?
The output of the altenator is brown, so is the switch wired between a convenient brown wire and earth?
David Witham

I'm going to install the master switch with a polyswitch across it to provide power to radio/clock etc which behaves like a resettable fuse. Any attempt to start the car with the main switch removed will result in the polyswitch going high impedance for a few seconds before reverting to very low impedance as before. Obvious advantage is that you do not need to replace a blown fuse.
Richard Coombs

I have one in my car too fitted on the heel board behind the drivers (rhd) seat. Very handy and I also use mine as an anti theft device. It clips to my car key chain.

I also 'broke' the positive lead and needed to run a separate fused power lead to the positive terminal to power my alarm and, more importantly, my door lock actuators.

One issue, well maybe not issue, is my cooling fan is wired to run with the ignition off if needed. If the fan is running when I stop the car removing the key will also stop the fan.

The switch is just the simple two terminal one with the large, red key.
Simon Jansen

Hi,

Just a little aside about fuses across the kill switch terminals. I would suggest that a small 12V light bulb would do that trick of stopping the engine being started, but still provide a small current to power a clock and / or a radio.
Like Simon mine has an extra "keep alive", separately fused, wire, to operate electric door actuators.

Herb
Herb Adler

"Just a little aside about fuses across the kill switch terminals. I would suggest that a small 12V light bulb would do that"

Excellent!
Paul Hunt 2010

David - your comment about me putting a switch in the live confuses me somewhat. Competition masters do indeed have to be accessible, but I can imagine they would *have* to be in the main live feed. I was referring to safety/security switches inside the car which can be in either depending on the leanings of the owner.

I believe the resistor is switched onto the alternator output lead as it is disconnected, if not permanently connected to the alternator output lead, to reduce voltage spikes inside the alternator if the master is turned off with the engine running.
Paul Hunt 2010

This thread was discussed between 25/06/2010 and 26/06/2010

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