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MG MGB Technical - Wiring a battery cut off switch resistor

I am installing a battery cut-off switch in the central radio console of a '73 B Roadster. The switch has the facility to wire an alternator protection resistor in the circuit between the ignition switch and the coil and take the resistor to earth. I am trying to decide where is the best place to break into this circuit as I am trying to avoid running another cable to and from under the dash to under the bonnet. Does anyone have any suggestions please?
Graham Gilmore

There should be two pairs of contacts on the switch, one pair normally open and one pair normally closed - with the key in the OFF position.

The normally open contacts are used to interrupt the supply to the ignition coil. If you have the early type rev-counter where the ignition coil feed loops in and out, you can route that through the cut-off switch.

The normally closed contacts are used to ground the alternator output through the resistor. Ideally you want to take a feed from the brown circuit - the headlamp switch might be a convenient point.
Dave O'Neill 2

This is only necessary with the FIA switch with multiple contacts that cut the battery and coil circuits. The resistor is connected to the alternator when the battery and coil are disconnected to prevent the alternator voltage spiking if the switch is turned off with the engine running as a result of a racing 'incident'. I thought these FIA switches came with an integral resistor, and because one of the main battery switch terminals is connected to the 'load' which includes the alternator output, i.e. the one that *isn't* connected to the battery, it is this terminal that is grounded by the closed contacts when the switch is off to protect the alternator automatically.

With the bog-standard battery cut-off switch there is no need for this resistor as you simply don't turn the switch off with the engine still running. There are many dire warnings about turning this type off with the engine running, blowing bulbs and the alternator, but I must confess to having done it several times due to brain fart with no (apparent) ill-effects (so far!).

PaulH Solihull

Thanks for the responses guys. I have already wired up the resistor but my question is where is the most convenient place to braek into the ignition switch/coil circuit. I don't have the 'loop' type rev counter but there is a white/black line from the coil to the rev counter and that would appear to be a place to break it.
Graham Gilmore

I took another look at the wiring diagram and realised that my suggestion will only disable the rev counter. The break has got to be in the coil line if the engine is to be shut down.
Graham Gilmore

Quite. To cut the ignition the break must between the fusebox and the coil. The most convenient place to do that is at the coil terminal itself using a male and a female spade termination and two wires back to the switch, and easily reversible should one wish.

Paradoxically you can kill the engine by *earthing* that white/black wire at the tach on 73 and later MGBs, which makes a nifty immobiliser, but won't be adequate for a kill switch in case it's connection at the coil breaks.
PaulH Solihull

This thread was discussed between 11/11/2010 and 15/11/2010

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