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MG MGB Technical - Odd switch under centre consol

Help! I have discovered a switch under the ashtray on the centre consol. Have no idea what it is for.
It is a simple on/off switch - a bit like a toggle switch. Two wires lead from it. One wire is connected to the tunnel by way of a screw - perhaps the 'earth' wire. The second wire leads directly to one of the terminals on the coil in the engine bay. I have not tried the switch - I am renovating the interior (therefore I found the switch) - everything is out, including battery.
My initial thoughts were:
1.Wishfull thinking - overdrive!
2.Extra volt switch for more performance!!

Otherwise no idea what so ever...
Any suggestions?
/Moss
Moss

Kill switch?

Maybe?
TheMiz

It sound like a PO's attempt at installing an anti-theft switch. In the on position, it would ground the points and prevent the car from being started. RAY
rjm RAY

Thanks Ray, TheMiz...Ok - I'll put the battery back and try the switch. Perhaps a silly question but is it not dangerous? - the coil produces alot of volts...
/Moss
Moss

Moss. The coil's high tension lead can produce "alot of volts" but your switch is connected to the input lead, or the output lead, on the low tension circuit--battery voltage. But, when the switch is in the on position, and the ignition switched on, it creates a direct short to ground and the wire will become very hot. Not a great idea. If one wants to install a "kill" switch in the circuit, it would be better placed in line to the input side of the coil. When the switch was on, current could flow to the coil. When the switch was off, no current would flow and no spark would be produced. But, it is one more complexity to the system and I would recommend removing it.

Les
Les Bengtson

If the wire were on the 12v feed to the coil then it would indeed get hot - hot enough to burn. But it almost certainly is on the points side of the coil to short them out as an immobiliser as has been mentioned (more than an 'attempt' though, successful I'd say). As such it will only be carrying about 4 amps and so unless very thin won't get hot. The coil *will* overheat if left with the ignition on too long, but then it will do that anyway even without the switch as the points are usually closed, and hence the same current flowing, when the engine stops.
PaulH Solihull

Thanks for all the advice/comments -Can't see the point of it so will remove. Had the car sine June 2010 and I presume it has been turned off all this time as I have not noticed anything hot. The wiring looks very new and shows no signs of burning.
/Moss
Moss

The point is that it makes a rather neat immobiliser. It's drawback is that if the scrote looks at the coil he will see the extra wire ...

On 73 and later MGBs you can earth the pulse wire at the tach and so the same job, which will be a lot harder to find unless said scrote gets his test-lamp or meter out.

The switch is only carrying current when it is immobilising the engine, probably only when it is being cranked, and then only intermittently (think about it).
PaulH Solihull

The most common anti-theft device, that I've seen installed, is a power cutoff switch between the fuse box and the fuel pump. It's an easy installation and does not even require cutting any wires. Simply unplug the factory bullet connector, under the hood, and install the switch in the circuit. Just remember to turn it on before driving away. I forgot to do this, several years ago, and the car stalled a 1/4 mile down the road. It took me a minute to realize that I had run the carbs dry. A flick of the switch and I was on my way, a little embarrassed, but happy not to have to call a tow truck. RAY
rjm RAY

That's actually preferable (or in addition to) an 'instant' immobiliser precisely because it *would* let a scrote get onto the public highway in most cases. They would have to be a particularly cool customer to then implement a work-around.
PaulH Solihull

Paul,I know that I didn't look like a cool customer when it happened to me. I must have looked like a common thief while trying to get the car running again. Older but wiser. RAY
rjm RAY

It's quite scary how little notice people take of a car alarm. I remember being in a town in Hertfordshire some years ago when a car was being bump started in a car park with its alarm sounding. It was something like a modded Pug 205 and the crowd shoving it were a bunch of kids who certainly looked as if one of them owned it. It was a busy evening but no one, myself included, challenged them or called the police. The that really the police should be called was mentioned, but the fact that it looked like they owned it, and were in full view somehow stopped us. So, dont rely on anyone taking some notice of your car being worked on at the roadside.
Stan Best

Something to add...
Have investigated further - after deciding to remove this switch. But a problem has occured or rather my lack of knowledge of things electrical has created a problem.
Have found out that the wire from the switch to the coil continues from the coil to the alternator!
I have no idea what this means and it's packed in sticky blue insulating tape together with other wires going to the alternator.
What could be the purpose of the wire going to the coil be?
Any suggestions?
/Moss
Moss

Instant self destruct when someone tries to steal the car!

Sorry, but which connector on the alternator is it? I wondered if it was to put 12 volts on the other side of the coil. There would be power to the coil when the engine was not running but 12 volts either side on the coil would stop it. Sneaky and very hard to find.

Then I read your first post again and you said the switch was earthed. So I haven't a clue.
Mike

If you put 12v both sides of the coil you would also have to disconnect the wire to the points otherwise when the points were closed you would have a full short. Also the points are normally closed when the engine comes to a stop, so if the wire was connected to the brown at the you would have a full short all the time the car was parked. Setting fire to your car seems a bit of a drastic way of stopping someone stealing it :o)

If it were connected to the brown/yellow of the alternator it wouldn't stop anyone starting the car or driving it away, but would probably burn out the voltage regulator in the alternator.

I suggest the wire from the switch to the coil is one thing, and if there *is* another wire going from the same coil terminal to the alternator then there is something else going on inside that sticky mass of blue insulating tape.
PaulH Solihull

Mike, it might be me self destructing if I 'play' with wires at the present time due to my total lack of understanding of the wiring system.

Paul, yes, the same wire goes from the coil terminal to the alternator...I will of course be looking at the wiring diagram to try and fathom out what's what. I am reluctant to do anything at present due to the blue tape wrapped around wires. I cannot see what is what and to take the stuff off will create problems which I cannot solve...at the moment - I can already predict how it will be until I get more info/knowledge.

Thanks for the advice/comments...
Moss

Hmmm, very dodgy then. No point in looking at a wiring diagram as it is obviously a PO mod. The only thing you can do is tell us exactly which terminal on the coil and alternator it is connected to, i.e. what colour harness wires it is connected with, but even then it depends on said PO not having done anything to those wires, and is only out of interest. If nothing else I'd be disconnecting the connection between coil and alternator (who knows now what the PO had in mind?), and only then if the switch is shorting the points side of the coil to earth i.e. an immobiliser as first offered. If it's going to the live side then it should be disconnected altogether.
PaulH Solihull

I'll do that Paul - I'll take a pic so the wiring can be seen.
/Moss
Moss

This thread was discussed between 18/01/2011 and 29/01/2011

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