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MG MGB Technical - Wiring Battery Cut off switch w/ courtesy lights

I've got a battery cut off switch wired to the positive terminal on my negative earth car. It's the type with the key. Could I put an inline fuse in to run my courtesy and trunk light when the switch is off, and if so, what amp fuse should I use? Thanks in advance.
Greg Bowman

I've read that a 1/4 amp would do the trick, but I could never find one of that amperage. I've got a battery cutoff switch wired to my negative terminal on a negative ground car with a 5 amp fuse, and it works beautifully, until I start the car, which always burns out the fuse. I wonder if wiring it to the positive terminal would make any difference. I haven't chased it down any further, since I've had other larger issues to deal with on my car. I know a number of people have done this, hopefully they can provide the appropriate advice.
SteveO

Greg,

I've got an ordinary (UK) domestic 5amp fuse in mine, it's sufficient to run the courtesy lights (I've one in each footwell, clock, and alarm which trips if doors or bonnet are opened.

I can also use the radio and lights with the key removed, but trying to start the engine blows it, simulating a flat battery (hopefully).

I'e also wired it with a flashing red neon on the dash, which switches off when the key is turned on (mainly to remind me that the power is off).

Mike
Michael barnfather

Michael, is you fuse wired between the positive terminal and the starter?
Greg Bowman

Greg,

No its across the terminals of the switch, effectively bypassing it, allowing up to 5 amps to be fed to the positive cable.

Mike
Michael barnfather

In most cases it is preferable to place the switch on the ground side, whichever type you may have. This is especially important in the later alternator equipped cars as it saves the solid state components in the rectifier pack and the voltage regulator from undue problems because of polarity.

You can then hook up a bypass wire with a 5 amp fuse of any sort you prefer inline which will keep the modern radio/clock happy, and even allow you run most low power stereos, but as said before, blows (when the switch is open/off) when someone tries to start the car. When the switch is in the "ON" position, both the main ground and the bypass ground are in parallel and you should not have a problem with the fuse blowing.
Bob Muenchausen

Bob, so you think I need to switch to the negative side of the battery?
Greg Bowman

Greg - What Bob is saying is that the switch and fuse need to be on the ground side of the battery to protect the electronic devices in the alternator. This will not change the way the fuse operates. one of the purposes of installing a cut out switch is to prevent someone from starting the car by hot wiring it and driving off, leaving you high and dry without a car. When you bypass the switch with a fuse, you are providing power to everything in the car that normally draws power from the battery, including the starter. Only those things that draw a small amount of current (less than the value of the fuse) will operate with the switch open and all the current being bypassed through the fuse. If you try to start the car, the current for the starter will also try to go through the fuse and with the 50 - 100 amps that is drawn by the starter, the fuse will blow. This is normal and desireable as it protects your car from being driven away by a unauthorized person. It also protects your car in the case of a short developing in one of the circuits when the car is unattended. cheers - Dave
David DuBois

Thanks everyone, but I'm afraid I don't understand about the harm to the alternator. The car was wired like this by the PO a long time ago. I thought some of these cutoff switches were specifically meant for the positive (ungrounded) side. BTW this is a Hella switch.
Greg Bowman

In support of the 'ground' terminal posse, most of today's cutoff switches seem to be made for the smaller (ie -ve) battery terminal too. So looks like they expect you to use it on the gnd/-ve side of things.

They don't help when trying to fit it to a +ve gnd car where the modern battery's +ve terminal is bigger... but nothing that a Big Hammer won't fix. :)
Will

I don't really understand how it would harm the alternator, unless you turned the switch whilst the engine was running.

My car has an alternator, and has shown no problems yet.
It is surely much more convenient to cut the main feed, and insert the switch in the body immediately behind the driver's seat, than to connect to the earth, which would presumably involve opening up the battery box every time you wanted to remove the switch.

My switch came with (I think) a resistor which bridged it, which I assume was something to do with the alternator, but as I have a LED across the switch I abandonned it.

Mike
Michael barnfather

I had been told that on some cars, alternator damage might occur because of the polarity chosen to cut out power and how that would interact with the diodes. Perhaps I am wrong, perhaps the info I picked up was wrong. I will stand corrected as no other electrical expert has chosen to say otherwise.

However, I think that the caution in most manuals to disconnect the earth/ground battery connection when working on a car's electrical system is probably as valid a reason for choosing that side to disconnect. I think that if anyone wants a good answer as to why the earth connection is chosen it is because nearly everything returns to the battery via this connection, and therefore, it serves to break the flow to the battery from any point on the car should some freak accident occur. This may have been the real reason for concern about the alternator. You could ask Paul Hunt or Rick Astley - they are the acknowledged masters in this area.
Bob Muenchausen

Chaps

Interesting stuff. The cut off switch should be on the power side of the battery - ie, not the earth lead.
A five amp fuse is more than enough and I have mine fixed under the battery cover. The cut off switch is in the 'usual' place behind the drivers seat.
Let me reinfirce this advice with a short story. A friend of mine had a mint TC with the cut off switch mounted under the dash on the earth side of the battery. He forgot and tried to start the car without turning it on. The pull start shorted out and created an earth which went red heat and seized on. The starter was whirring and he couldn't disengage the circuit. The subsequent fire did though. There have been instances of MGB's shorting through the choke cable - you know, difficult to start, funny smells from behind the dash - hot choke cable etc. In my view, to mount your cut off on earth side is asking for it.........
Regards
Dave
Dave Wellings

Well, then, there is your answer. It makes sense to me now to break the positive side but still does not explain the admonition to break the earth lead when working on a car. You would think you would want the same protection even then and that the consensus of manufacturers would be for the maximum safety, even for their own service people. Hmm.
Bob Muenchausen

I was always under the impression that the reason you disconnect the earth lead is because there are hazards associates with disconnecting the positive lead on a negative earth car with out first disconnecting the earth lead i.e. If the earth lead is still connected and you put a ring spanner on the positive terminal clamp to release it and the spanner (Wrench) touches the body of car a great big spark occurs which is a bit of a fright, dangerous (could ignite hydrogen gas given of from battery, or fuel vapour) and can be painfull.
Dave - I don't fully understand how if the earth lead is disconnected close to the battery (not under the dash as in your storie) using a cutout switch the car could be earthed through the choke cable or anything else on the car. The current needs to 'flow' back to the negative terminal on the battery to close the circuit. I've heard of the engine earthing itself through the throttle or choke cable, but this only happens when the engine earth strap is bad or missing.

Regards

Matt
Matthew Crabb

Matt

Yes, an interesting point. Unfortunately (or fortunately) I wasn't there to see it happen. As you say, the route would have been power - battery to ignition switch to starter, then back along the starter pull cable to the dash, and there somehow it shorted across the cut off switch to complete the circuit. The point being that if power had been isolated instead of earth it could not have happened.
I'll tell you what, fires under the dash are scary. I've only had one when running on the road (without a master switch infortunately), and the natural reaction is to grab the offending wire in an attempt to break the circuit. Doing that gives you a lovely set of long thin blisters across your fingers, and a good deal of pain afterwards.
In terms of security, I take it we all have fuel pump cut out switches? I must admit to also having a fuel pump earth cut out, just in case some smart alec feeds power to the pump - which required isolating the fuel pump since it had a metal pipe direct to the tank (MGB).

Dave
Dave Wellings

Dave
I have always liked the idea of the fuel pump cut out switch for security. I like the idea (well I would rather my car is not stolen in the first place) of the thief driving off for about 1/4 of a mile or so on the fuel in the float chamber and then getting stranded in a public place where on exiting the car lots of people get a nice look at them. Not that most people would probably take any notice, a bit like car alarms.

As for a car fire, that one of my worst nightmares!

Cheers

Matt
Matthew Crabb

Regarding whether a cut-out switch should be in the ground or live side of the battery to protect electronics, logic dictates to me that it doesn't matter which side it is because a disconnected battery is a disconnected battery, live, ground, or both. Just MHO. Likewise running the engine with the switch off is likely to damage the alt regardless of which side the switch is.

I can't see Dave's TC problem occuring if a cut-out switch was fitted regardless of which side of the battery it was fitted. If the switch was off no current could flow from the battery, to get the pull-switch red hot or spin the starter, no matter where the short occurred.

Problems with melting choke, accelerator and heater cables occur when the *engine* ground strap is broken and you try to crank it.

When you want to remove or otherwise manually disconnect the battery i.e. when working on the car then you must always disconnect the *ground* strap first - and that is irrespective of whether it is a positive or negative ground car. If your spanner touches the car body whilst it is on the ground terminal nothing will happen. Once that is off if your spanner touches the body whilst on the 'hot' terminal again nothing will happen. But if you do that with the ground still connected you get a big arc and possibly a battery exploding in your face.

Thus it would seem to make more sense to have such a switch in the ground side, and as long as the switch were off you could disconnect either side of the battery first in safety. But personally I think I would always disconnect the ground side anyway - 'to be sure, to be sure' as the old joke has it. However it *is* more convenient to put it in the hot side where the cable passes behind the right-hand heel board - which is also convenient to rapidly switch it off in the event of a loom fire as a friend found on two occasions.

Paul Hunt

I called in at the local car accessory shop last night, the guy there does a lot of rallying,and he has two types of switch available.

One just has two terminals, and these are for connecting to the earth side, the other type( which I have) is for the live side, as it has additional contacts to by-pass the power.

he confirmed that I would probably damage the alternator if I were to turn the power off whilst the engine is running,

Mike
Michael barnfather

Paul

Just imagine you need to remove the battery.

You have a Cut out switch on the negative side of the battery to car body.

Take the switch out, now battery is disconnected from body work, but Youv'e left an item taking a high current switched on.

You start to remove negative terminal on battery but catch your spanner on the bodywork whilst it is still touching the battery terminal.

Steve
Steve Williams

OK Steve, so the ultimate answer is not to fit a cut-out switch at all ...

Completing a circuit for a 'high current' item is going to result in a tiny fraction of the current and spark than shorting out the battery. And disconnecting the current in such a case will create the small spark, not connecting it.
Paul Hunt

Paul

Not suggesting not using a cut-off switch, I've got one myself, but even a small spark can shock you when your not expecting it.

Just read that.

Can surprise you when not expecting it.

Steve

Steve Williams

This thread was discussed between 27/08/2002 and 30/08/2002

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