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MG MGB Technical - Battery cut out switch/ regulator problem

Having recently rebuilt my MGB 73 I had decided to incorporate a couple of upgrades.One being installation of a battery cut out switch,fitting instructions suggested placing it between negative battery pole and ground.This was evidently not a generic accessory as anyone with a little knowledge wil realise the differculty obtaining space and lengths of ground cable to cut for a behind seat mount on outer battery case . Therefore selected positive side which lies immediately behind battery comp valance.Could this be giving me probems with my alternator 18ACR regulator.I normally disconnect always negative first when disconnecting battery.However now I am just flicking the cut out to positive line to starter /solenoid whenever working on anything electrical.Is this likely to give me inherent problems with as previosley mentioned alternator of which I have some issues.
As can be seen from the fitting instruction only one 12volt battery as opposed to my 2 :6v inline and they appear to bemaintaining direct feed from regulator /dynamo.My alternator/reg runs via start motor/solenoid to battery and is now isolated with the breaker switch activated?
Please any response welcome as I am having some isuues and need to isolate things that I definately do not have a problem with.

I.G.F. Provo

I'm about to do something similar with my 72 Roadster but will probably put the cut out switch on the driver's firewall (RHD) just above the starter solenoid. Obviously this is in the 12V feed but it is a pretty convenient place as you do not need to cut the main feed, just provide another short one from switch to solenoid.
However, I am also going to use a polyfuse across the switch terminals to provide power to the clock etc when the cut out is used and I was wondering if you had something similar? This would mean that you still had 12V at your alternator whenever you worked on it. Seems obvious but easily forgotten.
Apart from this I can't think of any reason why removing power completely using the switch is any different to disconnecting the battery at the 12V terminal.
Interestingly, Rick Astley's book, MGB Electrical Systems, also suggests putting the switch in the battery earth.
Richard
Richard Coombs

I don't think there is any problem with mounting the cut out on the positive cable. What you must not do is disconnect the battery with the engine running. That could easily fry the alternator. On one of my MGBs with twin 6v batteries I fitted the cut out in the linking cable.
Mike Howlett

The reason you disconnect the earth first is to avoid a spark at the battery and so avoid blowing the battery up, so you wont damage anything with a switch as you describe. A switch near the battery is buy far the best as it eliminates all of the possible points of a short.
Denis4

Thanks everyone for your comments,I understand the merit in all.However Denis4 seems to have arrived at my summit of logic and covered my underlying short circuit fear whilst allaying my hesitancy as to the wisdom of placing cut out in positive feed rather than recognised factory recommendation neg. cut
My original intention was to use cut out only for emergency such as fire etc therefore I really was looking to shut all chance of short out despite losing standby power to radio/sat nav etc.Which things would have a possible secondary feed anyway with normal ignition off.Good luck Richard with your 72, I must admit I am not quite sure why you would want to maintain power at the alternator at all whilst working on it?In closing, My main feed has the clamp positive end shortened by some 20cm/ this length then with clamp having been reused from back of switch to battery terminal;thus saving any further lengthening of current travel and more exposure and lastly no hunting for correct capacity cable extension.
Many thanks again everyone.
I.G.F. Provo

Negative or positive, 12v or earth, a cut-out switch makes no difference to the operation of circuits unless it is intermittently opening when it shouldn't i.e. when the engine is running, and that is the same no matter which cable it is in.

As far as working on anything but the battery goes it also makes no difference which cable it is in, but if you want to work on the 12v connector, or the link cable connectors on twin 6v batteries, it is marginally more convenient to have it in the earth.

However if you want to keep clocks, radios etc. powered when everything else is switched off you have to have it in the 12v side, with a bypass fuse coming off the live side of the switch feeding those items. I've done that on my V8 with a clock, and also my ZS (clock and stereo). I fitted it to those primarily to stop the alarm draining the battery needing replacement every 18 months or so, but the roadster has it purely for safety in case of a major short on one of the several unfused wires on MGBs.

I put mine in the 12v cable as I wanted the switch behind my RHD seat in both MGBs so it was convenient to put it in the 12v cable, and made sure I fitted rubber boots to the connectors and studs. However it is physically simpler to put it in the earth cable as all you have to do is shift the existing cable over from body to switch and provide a new cable from switch to body, and Halfords in the UK sell these in various lengths with the appropriate lugs on each end. Proved to be doubly convenient, as when I replaced the link cable on my 73 roadster it was annoying slightly shorter than the old one, which meant I had to turn the right-hand battery round, which meant I needed a longer 12v cable. But as I only had a short length between battery and switch it was easy to do. With the cut-off switch in the earth I would have been stuck between not fitting the new link cable or replacing the main cable up to the starter.
Paul Hunt

This thread was discussed between 04/09/2009 and 06/09/2009

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