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MG MGA - Battery cut-off switch

My MG has been converted to negative ground. I just got a battery cut-off switch. I believe that it should go on the positive terminal. Plese comment it this is correct.
Terry Marcus

Yes Terry, on your negative earth car, the cut off switch should be in the positive battery lead.

(On a positive earth set up it should be in the negative)

If you are doing some wiring on your car and want to test any of the circuits, I would connect a small 12 volt battery in place of the car battery. This is so that when you test any of circuits (lights etc), any wiring errors (short cicuits etc) wont burn out the wiring loom like I did on my MGA!

Colyn Firth

Thanks for the help. I will definitely use an small 12 v battery for circuit testing.
Terry Marcus

Hmmmm, maybe I just don't have my thinking cap on at the moment. What difference does it make which side of the battery you put the cut-off switch on? Aren't you interrupting the completion of the circuit and wouldn't it have essentially the same effect in either case?
Andy Bounsall

I'm with Andy on this. For the most part, it doesn't really matter. Now, there is one school of thought that it should be installed on the ground side. That means the negative for a negative ground car, and the positive for a positive ground car. This way, the exposed parts of the switch do not pose any hazard if you somehow accidentally hit one with a wrench or screwdriver. Another thing to consider is that the terminals of a battery are not the same size, and the switch is sized for the positive or negative. The Moss Motors catalog says of their switch, "Sized for negative posts and should be used only on negative ground cars".

-Steve Trovato
Steve Trovato

Thinking about it (and I hadnt before), I can see that it doesnt make much difference which battery lead is used to fit the cut off switch. I have only ever known of them being fitted on the live side and not the earth side.
The important point is to site the switch so that you can get to it really quickly in an emergency.
I would position it quite close to the battery which would then truly isolate the battery.
Mine is fitted under the dashboard which means that there must be approx 10 to 12 feet of "live" cable before the cut off switch which isnt ideal and is something I am going to change when I can.
Colyn Firth

Battery cut-off switch is very handy for routine electrical maintenance or for emergency disconnect. Gotta love it. Highly recommend a bulkhead switch, not one on the battery post, avoiding requirement to lift the battery cover to access the switch.

I recommend installing the switch in the ground cable. When switched off it will then isolate the battery from the frame. When wrenching on cable terminals on the battery you can touch the tools on the frame without causing short from hot terminal to ground. Otherwise always disconnect the ground cable first.

If the switch was in the hot cable, touching wrench from battery hot terminal to frame will give VERY undesirable results (melted wrench, burned hands, blown up battery).

I installed mine when I still had 2x6v batteries in the car. I put the switch low on the rear plywood bulkhead behind the drivers seat (LHD), accessible sitting in the car (with a bit of a stretch), and easily accessible from outside, no need to lift battery cover for access to the switch. Disconnect battery ground cable from frame and connect it to the switch. Install a standard bare engine grounding strap from switch to frame.

When changing to 1x12v battery, install battery in right side carrier using original hot cable, and buy/install a (cheap) longer insulated ground cable, from battery to switch, leaving switch in same location. If you already have 1x12v battery on passenger side (LHD), then you might install the switch behind the passenger seat.

However, many race sanctioning groups require the switch to be in the hot cable and on the outside of the car for easy outside access. Generally not relevant for a street car.
Barney Gaylord

My cutout is on the Ground Feed (negative Earth Car) and is fitted next to the battery cover behind the passengers seat. I have fitted a line fuse (5amp) to keep the radio presets live when the cut out is activated but will blow when the starter or any significant current is operated
P D Camp

Paul, just how may watts sound output does your radio have to be able to hear it above the special background music created by an MGA on the move?
If I had one fitted in my car I wouldnt be able to tell if it was switched on or off!
Colyn Firth

Radio is never used in motion ! just usefull when stopped for news etc -Its just an irritant if the presets are lost
P D Camp

Is yours fitted to the horizontal metal shelf alongside the battery "lid" (for want of a better name)? Mine was there but I had to remove it as, sod's law put it in the way of the new shock absorbers I have just fitted. Left a nice hole so I can watch the road while I drive!

I like Barney's idea of fitting to the wooden vertical rather than cutting more metal but there doesnt seem to be room behind the seat. (What size is your cut out switch Barney? - my switch protrudes 1.5" and I usually leave the key in which is another 1.5". I would buy a smaller one, but can't seem to source).
Graham M V


Put a wood spacer on the back of the bulkhead and move the switch back so it is even with the front surface. See image. You can hardly see the hole when the key is removed

My switch is on the earth connection behind the LH seat.


Jim Ferguson

I have the Lucas SSB103 switch. Permanently affixed round knob, no removable key. I cut the larger bole, screwed the flange to front of panel. The knob sticks out enough to get a good grip on it. I suppose you could drill the smaller hole and screw the flange to back of panel, but might have a tough time working the set screw to attach the knob. See picture. You might find prices from $46-USD to $186-USD (some of them are outrageous). Repros are likely to be cheaper than originals, although the white lettering and arrow may have different appearance.

Barney Gaylord

Thanks Jim and Barney. Very helpful as always. I think I will go with Jim's clever solution as I already have the switch. Thanks for your help, much appreciated
Graham M V

My switch is on the LH shelf behind the passengers seat (RH Drive Car)and just protudes 1 1/4 inch including the key . There is around 2 1/2 inches under the shelf and it isolates the ground wire (negative earth). It is one of the better quality heavy duty switches- there are a number of cheaper poor quality cut outs on the market. Was very usefull last year when I shorted a panel lamp wire and smoke appeared under the dash a quick shut off limited the damage to an easily repaired section of the panel wiring
P D Camp

There is a cheap version of the repro SSB103 on eBay for just under 21:

Here is a much better looking one (but at over twice the price):

This one is a better price (34) and made in the UK:
Neil McGurk


Good products but I prefer the type with removable keys. This allows the isolator to double up as an anti-theft option.

Steve Gyles

I agree with Steve. And Steve you will need that extra security once you come down South! We have to lock our front doors down here at night.

Paul, I was forgetting you have the coupe, so you have a lot more space behind the seats.
Graham M V


I keep my steering wheel removal socket in the car for that purpose, when leaving the car out.

Good call on locking doors. I need to find the keys to hand over to the new owners.

Steve Gyles

This thread was discussed between 13/06/2010 and 17/06/2010

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