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MG MGA - New ignition switch won't turn off engine

Just acquired a 1960 MGA coupe modded with a 1970 B-engine. The ignition switch wouldn't shut off the engine and the only way, almost all of the time, to turn it off is to stall it out. Bought a new switch and installed it but had the same problem with the switch not cutting out the engine when switching the key to the off position. Fiddled with the wires to the ignition switch and changed the pins to which the live and return wires clip and found that this "fixed" the problem. I was now able to repeatedly start and turn off the engine without any problem.

However, I then noticed that the ignition light on the dashboard had begun misbehaving. It would come on and glow brightly at high engine revs and go out at low revs (opposite to normal behavior) - implying a grounding problem at the voltage regulator or a faulty voltage regulator. I fiddled with the ground wire on the voltage regulator and the ignition light returned to normal operation and behavior but now the ignition switch stopped working again!

So it appears that the switch only worked properly when the grounding wire on the voltage regulator was loose and when I tightened it back onto the snap-on pin to make contact, the switch stopped working again. I've tried different combinations of the two wires to the four pins on the back of the ignition switch and nothing works. I also unhooked one the two wires from the ignition switch while the engine was running and even this didn't cut off the engine! Is there an electrical short somewhere? Any ideas? What can I do to figure this out? Thanks for any help! It's perplexing and I hate having to stall the engine whenever I want to turn off the car.
F. Fitch

It's not the switch. Most likely a faulty regulator, or something miswired. Start here:
http://mgaguru.com/mgtech/electric/circ_c.htm
Disconnect all wires from the control box and check every one for continuity or short circuit. If the wires are good, reconnect and be sure the color codes and terminals are all hooked up right. If you still have the problem, replace the control box.
Barney Gaylord

Thanks Barney - I feared as much. Now to trace and follow all the wires and connections!
F. Fitch

This thread was discussed on 01/09/2006

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