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MG MGB Technical - Wiring battery isolator switch

How do I connect battery cables to an isolator swotch (the sort with a big red key that fits on the bulkhead behind the driver's seat). The problem is that the only battery cable lugs I can find have a 3/8" hole, while the switch screw terminals are about 5/8" dia. To make matters worse, the terminal nuts are extremely narrow, so if I just try wrapping the cable round them I think it would be pushed off when they are tightened. Does anyone know a supplier of decent sized lugs?
Steve Musgrave

Hi Steve.

There are suitable ring terminals out there.
Try electrical parts suppliers like RS or Farnell / CPC.

These connections need to be made properly, I would strongly advise against wrapping the wire around the post.

Don
Don

Hi again.

Try here :

http://www.vehicle-wiring-products.co.uk/VWPweb2000/BATTCON/BATTCON.html

Don
Don

And fit it in the earth side not the +'ve...
M
Michael Beswick

*Definitely* don't wrap the wire round the post, if the battery cable comes loose and shorts out ...

I got the terminals with my switch, from Stoneleigh show, which is on again this Sunday.

Whilst it is slightly more convenient having the disconnect switch in the earth cable instead of the live (which could be +ve or -ve depending on polarity :o)) for disconnecting the battery when working on live electrics elsewhere on the car, in order to have the switch on the heelboard (where you can reach it from the drivers seat in an emergency) it does mean the inconvenience of having cables snaking back and fore in what is already a limited space and purchase of extra cable.

Speaking of switching it off in an emergency, never turn it off with the engine running. Without a battery the alternator can go massively over-voltage and do all sorts of damage.
Paul Hunt 2

Terminals of varing size and capacity required can normaly be found at an electrical supply house as they are common in industrial equipment.
John H

If you wire it to the live, you can wire a small fuse in parallel, so that when you remove the switch, the clock, side lights, interior light and radio still work, but any attempt to start the car will blow it......I also wired an led in with it, to remind me to switch the key on before starting.

Mike
M Barnfather

Is it correct that if you connect the battery isolator switch to the negative, you can still run a small wire from the radio to ground to maintain the radio and clock memory provided that you install a small fuse on that ground? dave
David B

Wondering - why switch the earth side? I was told by a switch manufacturer to switch the hot side and it's been that way on my B for over 10 years with no problems. I repeated to them 'are you sure?' and they said yes. I always thought it was the earth side. Maybe it doesn't matter.
Sam Sullivan

Mike

You can also use an "auto-reset" automotive circuit breaker. That way you don't have to replace anything when you forgot that you had the switch off.

FWIW

Larry
Larry Hallanger

Steve:
Have a look at this link. I used a polyswitch (auto-reset) with my battery disconnect.

http://web.archive.org/web/20041028102023/www.mgcars.org.uk/electrical/switch.html
Phil O

Dave B - Yes! M
Michael Beswick

Dave B presuming your car is -'ve earth M
Michael Beswick

Correct Micheal , it is negative earth.

Dave
David B

Suddenly occured to me that as far as disconnecting the battery to safely work on other parts of the electrics it doesn't matter which leg the isolator is in. The only benefit of putting it in the earth/ground leg is if you want to work on the live post of the battery, otherwise it makes no difference.

Whichever leg it is in, you can either get a switch with a bypass fuse to power clock and interior lights, in which case you will be forever blowing it when you try to crank, or you can put the switch in the 12v leg and wire a separate in-line fuse from the 12v post to the clock, which is what I have done. That way I can open the door or hatch and confirm I have turned the isolator off (at my age one forgets these things).
Paul Hunt 2

The Hella switch instructions say to connect in the positve cable on a negative earth car.
Greg Bowman

Thanks a lot to all for the great info. I have now obtained the terminal lugs from super efficient and helpful Vehicle Wiring Products. I was suffering from Halforditis (defined as a chronic condition of a certain store either not having an essential item or theoretically stocking it but never making it available at any one time).
Steve Musgrave

David B Sorry brain fused! Running a fused wire to ground won't work! It has to run to the batt -'ve post or the side of the isolation switch that is connected to the -'ve. Assuming your radio is mounted in a way that connects it to the bodywork, then other electrical things will also "earth" thru this wire /fuse. eg interior light(s). If you forget to turn on the isloator then turning the ignition on will blow the fuse as the electric fuel pump comes on with the ign. Either major security or major pain. Paul's suggestion above is probably the best-he usually knows!! Michael
Michael Beswick

For a negative earth car, the reason you usually disconect the earth lead first when working on the electrics is because this prevents you from getting an electric shock AS YOU DISCONNECT.

Disconnecting the live terminal first would put you at risk from a short circuit (particularly in the cramped environment of the battery box) which will definitely hurt.

I have some experience in this field, having set fire to a MG Montego Turbo when disconnecting the live wire at the starter solenoid without disconnecting the battery...

Neil
Neil

The reason you always remove the ground cable first is to remove the risk of a short-circuit if you removed the the live terminal first i.e. solely one of safety, the polarity of the car is immaterial. There is the same risk of getting a shock with either cable first, and it is virtually zero.
Paul Hunt 2

This thread was discussed between 15/02/2006 and 21/02/2006

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